Molecular characterization of banana NAC transcription factors and their interactions with ethylene signalling component EIL during fruit ripening

Abstract

The plant-specific NAC (NAM, ATAF1/2, and CUC2) transcription factors (TFs) play important roles in plant growth, development, and stress responses. However, the precise role of NAC TFs in relation to fruit ripening is poorly understood. In this study, six NAC genes, designated MaNAC1–MaNAC6, were isolated and characterized from banana fruit. Subcellular localization showed that MaNAC1–MaNAC5 proteins localized preferentially to the nucleus, while MaNAC6 was distributed throughout the entire cell. A transactivation assay in yeast demonstrated that MaNAC4 and MaNAC6, as well as their C-terminal regions, possessed trans-activation activity. Gene expression profiles in fruit with four different ripening characteristics, including natural, ethylene-induced, 1-methylcyclopropene (1-MCP)-delayed, and a combination of 1-MCP with ethylene treatment, revealed that the MaNAC genes were differentially expressed in peel and pulp during post-harvest ripening. MaNAC1 and MaNAC2 were apparently upregulated by ethylene in peel and pulp, consistent with the increase in ethylene production. In contrast, MaNAC3 in peel and pulp and MaNAC5 in peel were constitutively expressed, and transcripts of MaNAC4 in peel and pulp and MaNAC6 in peel decreased, while MaNAC5 or MaNAC6 in pulp increased slightly during fruit ripening. Furthermore, the MaNAC2 promoter was activated after ethylene application, further enhancing the involvement of MaNAC2 in fruit ripening. More importantly, yeast two-hybrid and bimolecular fluorescence complementation analyses confirmed that MaNAC1/2 physically interacted with a downstream component of ethylene signalling, ethylene insensitive 3 (EIN3)-like protein, termed MaEIL5, which was downregulated during ripening. Taken together, these results suggest that MaNACs such as MaNAC1/MaNAC2, may be involved in banana fruit ripening via interaction with ethylene signalling components.

Introduction

Fruit ripening is a complex and genetically programmed process that results in marked changes in colour, flavour, aroma, texture, and nutritional content of the flesh (Giovannoni, 2004). These changes are the result of the coordinated activation of multiple genetic and biochemical pathways, which are influenced by internal and external cues, including regulation by many critical transcription factors (TFs) (Martel et al., 2011). Numerous plant TFs such as apetalous (AP2), basic leucine zipper domain (bZIP), myeloblastosis oncogene (MYB), myelocytomatosis oncogene (MYC), WRKY, Mcm1-Agamous-Deficiens-Srf (MADS), heat-shock TF (HSF), and several classes of Cys2/His2-type zinc finger proteins, have been identified and characterized according to their DNA-binding motifs (Yamasaki et al., 2008). The NAC, named after NAM (no apical meristem), ATAF1,2, and CUC2 (cup-shaped cotyledon), constitutes one of the largest families of plant-specific TFs, with 106 members in Arabidopsis (Gong et al., 2004), 149 members in rice (Xiong et al., 2005), and 101 members in the soybean genome (Pinheiro et al., 2009). The distinguishing characteristic of NAC TFs is possession of a highly conserved N-terminal NAC domain, which is divided into five conserved subdomains (A–E), and a variable transcriptional regulation C terminus, which is able to activate or repress the transcription of multiple target genes (Duval et al., 2002; Hao et al., 2010).

NAC TFs have been implicated to contribute to diverse and vital physiological processes, including shoot apical meristem development (Hibara et al., 2003), floral morphogenesis (Sablowski and Meyerowitz, 1998), leaf formation and senescence (Berger et al., 2009; Balazadeh et al., 2011; Zhang and Gan, 2012), embryo development (Larsson et al., 2011), seed development (Verza et al., 2011), flavonoid biosynthesis (Morishita et al., 2009), lateral root development (He et al., 2005), grain nutrient remobilization (Waters et al., 2009), shoot branching determination (Mao et al., 2007), and secondary cell-wall biosynthesis (Zhao et al., 2010; Zhong et al., 2010). Moreover, some NAC TFs have been reported to participate in hormone signalling mediated by auxin (Xie et al., 2000), ethylene (He et al., 2005), abscisic acid (Fujita et al., 2004; Gao et al., 2010), gibberellic acid (Aida et al., 1997; Kim et al., 2008), salicylic acid (Delessert et al., 2005), methyl jasmonate (Bu et al., 2008; Yoshii et al., 2010), and cytokinin (Kim et al., 2007), as well as in the response to biotic and abiotic stresses such as fungus infection, drought, cold, high salinity, and mechanical wounding (He et al., 2005; Meng et al., 2009; Xia et al., 2010; Nakashima et al., 2012). These stress inducible NAC TFs have been shown to bind to cis-regulatory elements, termed NACRSs (NAC recognition sequences), upstream of other stress responsive genes and activate their expression (Tran et al., 2004). Furthermore, a few NAC TFs are differentially regulated by various pathways, including microRNA-mediated cleavage of mRNAs and ubiquitination-mediated proteolysis (Greve et al., 2003; Berger et al., 2009). Although the functions of NAC TFs in many plants have been studied extensively, the involvement of NAC TFs in relation to ripening of economic fruits has generally received less attention.

Banana (Musa acuminata, AAA group), a typical climacteric fruit, is one of the most important food crops after rice, maize, and wheat in tropical and subtropical countries. It comprises an important part in the diet of millions of people around the world. Commercially, it is a very prosperous crop in the world trade. The post-harvest physiology of banana fruit is characterized by a pre-climacteric phase, followed by a peak in ethylene production that orchestrates ripening-associated processes, including climacteric respiration, pulp softening, peel degreening, and the production of aroma compounds (Clendennen and May, 1997). However, the physiological climacteric attribute of banana fruit results in a short post-harvest life of 10–15 d at ambient temperature (Bapat et al., 2010). In addition, unlike many other climacteric fruits, the peel and pulp of banana fruit exhibit different ethylene biosynthesis patterns (Dominguez and Vendrell, 1993; Elitzur et al., 2010). For these reasons, it is imperative to gain a better understanding of the mechanism of fruit ripening, as this will aid in improving the quality and storage potential of this fruit. Some genes associated with ethylene biosynthesis and perception pathways have been identified in banana fruit, including 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, ACC oxidase, ethylene receptor, CTR1 orthologue, and ethylene insensitive 3-like (EIL) genes (Clendennen et al., 1997; Liu et al., 1999; Mbéguié-A-Mbeguié et al., 2008). However, there is little information about how ripening-related TFs regulate the ripening of banana fruit in relation to ethylene signalling. Recently, several MaMADS TFs were reported to be induced by ethylene, suggesting that the MADS TF may participate in banana fruit ripening (Liu et al., 2009; Elitzur et al., 2010). Moreover, current research into fruit ripening is likely to focus on the identification of new candidate genes, as well as understanding the interactions among the already characterized components (Kumar et al., 2012). NOR (non-ripening) is an NAC-domain TF whose mutation results in a non-ripening phenotype including ethylene synthesis, increased respiration, carotenoid accumulation, softening, and aroma volatile production (Martel et al., 2011). Furthermore, NAC is also ethylene inducible (He et al., 2005; Xia et al., 2010) and may represent a TF downstream of the ethylene insensitive 2 gene (EIN2) in parallel with EIN3 (Kim et al., 2009; Al-Daoud and Cameron, 2011). Thus, it is interesting to study the transcriptional regulatory network concerning NAC in relation to the ethylene signal during fruit ripening. In the present study, six NAC TFs were isolated and characterized from banana fruit, and the expression patterns of six MaNAC genes in the fruit of four different ripening characteristics, including natural, ethylene-induced, 1-methylcyclopropene (1-MCP, a competitive inhibitor of ethylene action)-delayed, and a combination of 1-MCP with ethylene treatment (1-MCP+ethylene), were analysed by real-time quantitative PCR. In addition, the promoter of one ripening-related NAC, MaNAC2, was isolated and its response to ethylene was also investigated. More importantly, the direct interactions between MaNAC1/MaNAC2 and MaEIL5, a downstream component of ethylene signalling, were detected by yeast two-hybrid and the bimolecular fluorescence complementation (BiFC) assays. Our results suggest that MaNAC genes may be involved in banana fruit ripening via interaction with ethylene signalling components.

Materials and methods

Plant materials and treatments

Pre-climacteric banana (Musa acuminata, AAA group, cv. Carvendish) fruit at the 75–80% plump stage were obtained from a local commercial plantation near Guangzhou, south-eastern China. The hands were separated into individual fingers, and fruit of uniform weight, shape, and maturity, as well as being free of visual defects, were selected. The fruit were first surface sterilized by dipping in a 1% hypochloride solution for 1min and then immersed in 0.05% Sporgon (with 46% Prochloraz–Mn; Aventis, Valencia, Spain) for 3min to prevent fungal disease. They were then allowed to air dry at 25 °C for 2h and treated as follows.

The selected banana fruit were randomly divided into four groups of 150 fingers each for the following treatments. Fifteen unsealed polyethylene plastic bags (0.01mm in thickness) of ten fingers each were used for each treatment, and samples were taken based on the rate of ethylene production and fruit firmness change during ripening. Group 1 (control, non-conditioned for natural ripening) fruit were stored directly at 22 °C and 90% relative humidity for 25 days; samples were taken at 0, 1, 3, 5, 7, 15, 19, 21, and 25 days until the fruit ripened completely. Fruit of group 2 were treated with 100 µl l^–1 ethylene for 18h in a closed chamber and then stored at 22 °C for 7 days; samples were taken at 0, 1, 3, 5, and 7 days of storage. Fruit of group 3 were treated with 0.5 µl l^–1 1-MCP for 18h and then stored at 22 °C for 35 days; samples were taken at 0, 1, 3, 5, 7, 21, 25, 30, and 35 days of storage. Fruit of group 4 were treated with 100 µl l^–1 ethylene followed by 0.5 µl l^–1 1-MCP for 18h, and then stored at 22 °C for 30 days and samples were taken at 0, 1, 3, 5, 7, 15, 17, 19, 21, 25, 28, and 30 days of storage.

All of the samples were frozen in liquid nitrogen immediately after sampling, and stored at –80 °C for further use. All assessments were conducted using three biological replicates.

Fruit ripening evaluations

Fruit ripening was evaluated using the following two parameters: ethylene production and fruit firmness. Ethylene production was determined in three replicates each containing three fruit at the given sampling times. The three fruit were placed in a 2 l flask and capped with a rubber stopper for 2h at 22 °C. A 1ml sample of headspace gas was sampled and analysed for ethylene using gas chromatography (Model GC-17A; Shimadzu Co., Kyoto, Japan) fitted with a flame ionization detector and an activated alumina column (200 cm×0.3cm), with an injector temperature of 120 °C, column temperature of 60 °C, and detector temperature of 60 °C, according to the method of Wang et al. (2006).

Fruit firmness was measured using a penetrometer (Model Instron 5542; Instron Co., USA) equipped with a cylindrical flat-surfaced plunger (6mm diameter). A small slice of fruit skin was removed and firmness was then recorded from three different fruit, with three different points per fruit; means were expressed as newtons (N).

RNA extraction, gene isolation, and sequence analysis

Frozen tissues were ground in liquid nitrogen using a mortar and pestle. Total RNA was extracted using the hot borate method of Wan and Wilkins (1994). Potentially contaminating DNA was eliminated by treatment with DNAse I digestion using an RNAse-free kit (Promega Madison, WI, USA). The DNA-free total RNA was then used as template for RT-PCR. The first-strand cDNA of the product was subjected to PCR amplification.

Six NAC genes, termed MaNAC1–MaNAC6, were isolated from our transcriptome database obtained using a high-throughput Solexa/Illumina sequencing platform (Beijing Genomics Institute, Shenzhen, China) and the sequences were first verified by recloning and resequencing. Two of the six NAC genes, MaNAC3 and MaNAC4, were full-length sequences in the database, with complete start and stop codes, while full-length sequences of the other four NAC genes, including MaNAC1, MaNAC2, MaNAC5 and MaNAC 6, were obtained by 3’- or 5’-rapid amplification of cDNA ends (RACE) using a RACE kit (TaKaRa Biotechnology, Dalian, PR China) according to the manufacturer’s instructions. The specific primers used for RACE are provided in Supplementary Table S1 at JXB online.

Alignments were carried out on CLUSTALW version 1.83 and GeneDoc software, and a phylogenetic tree was constructed using the neighbour-joining method in the MEGA5 program and visualized by TreeView software. The theoretical isoelectric points (pIs) and mass values for mature peptides were calculated using the PeptideMass program (http://web.expasy.org/peptide_mass/).

Subcellular localization of MaNAC proteins

The coding sequences of MaNAC1–MaNAC6 without the stop codon were amplified by PCR (primers are listed in Supplementary Table S2 at JXB online) and subcloned into the pBI221-GFP vector, in frame with the green fluorescent protein (GFP) sequence, resulting in 35S::gene–GFP vectors under the control of the cauliflower mosaic virus (CaMV) 35S promoter. The fusion constructs and the control GFP vector were used for transient assays using a modified polyethylene glycol (PEG) transfection method with tobacco BY-2 suspension culture cell protoplasts as described previously (Abel and Theologis, 1994). GFP fluorescence was observed with a fluorescence microscope (Zeiss Axioskop 2 Plus). All transient expression assays were repeated at least three times.

Transcriptional activation analysis in yeast cells

The coding regions of MaNAC1–MaNAC6 were cloned into the pGBKT7 vector (Clontech, USA) to create the pGBKT7-MaNAC1 to -6 constructs, respectively. Following the protocol of the manufacturer, pGBKT7-MaNAC1 to -6, the positive control pGBKT7-53+pGADT7-T, and the negative control pGBKT7 plasmids were used to transform the AH109 yeast strain using the lithium acetate method. The transformed strains were streaked onto minimal medium without Trp (SD/–Trp) or SD/–Trp–His–Ade plates, and the transactivation activity of each protein was evaluated according to their growth status and the activity of α-galactosidase. For mapping the activation domain, different truncated derivatives were constructed and transformed into the yeast strain for transcription activation activity as described above. The primers used for transcriptional activation analysis are listed in Supplementary Table S3 at JXB online.

Quantitative real-time PCR analysis

Isolation of total RNA from the samples and synthesis of first-strand cDNA were performed as described above. The synthesized cDNA was diluted 1:40 with water, and 2 µl of the diluted cDNA was used as a template for quantitative real-time PCR analysis (qPCR). PCRs were performed in a total volume of 20 µl, containing 1 µl of each primer (10 µM; final concentration 200nM) and 10 µl of SYBR^® Green PCR Supermix (Bio-Rad Laboratories) on a Bio-Rad CFX96 Real-Time PCR System according to the manufacturer’s instructions. The qPCR program included an initial denaturation step at 94 °C for 5min, followed by 40 cycles of 10 s at 94 °C, 30 s at 60 °C, and 30 s at 72 °C. No-template controls for each primer pair were included in each run. The oligonucleotide primers for qPCR analysis were designed on the basis of the 3’-untranslated region using Primer 5.0 software. The sequences of all primers used for qPCR analysis are described in Supplementary Table S4 at JXB online. RPS2 (ribosomal protein 2) and CAC (clathrin adaptor complex) were selected as reference genes under different series samples according to our previous study on the selection of reliable reference genes for expression study by qPCR in banana fruit (L. Chen et al., 2011). Each assay using the gene-specific primers amplified a single product of the correct size with high PCR efficiency (90–110%). All qPCRs were normalized using the cycle threshold (C _t) value corresponding to the reference gene. The relative expression levels of the target gene were calculated using the formula 2^–ΔΔCt (Livak and Schmittgen, 2001). The values represent the mean of three biological replicates.

Promoter isolation and its activity assay

Genomic DNA was extracted from banana leaves using a DNeasy Plant Mini kit (Qiagen). Genomic DNA (2 µg for each) was digested separately with eight restriction enzymes (DraI, Ecl136II, EcoRV, HpaI, MscI, StuI, SspI, and ScaI). After enzyme digestion, all products were cleaned up using a DNA Clean and Concentration kit (Zymo Research) and then ligated to an adaptor (Clontech).

The 5’-upstream region of the MaNAC2 gene was isolated using a Genome Walker kit (Clontech) with nested PCR. The nested PCR analysis was performed with two sets of primers, including two adaptor primers that were obtained from the kit and two gene-specific primers that were designed by Primer 5.0, as listed in Supplementary Table S5 at JXB online. The amplification product was cloned into the pGEM-T Easy vector (Promega) and sequenced. Conserved cis-element motifs of the promoter were predicted using the PLACE (http://www.dna.affrc.go.jp/PLACE/signalscan.html) and Plant-CARE (http://bioinformatics.psb.ugent.be/webtools/plantcare/html) databases.

For the promoter activity assay, the MaNAC2 promoter region (~1.5kb) was amplified by PCR using the specific primers listed in Supplementary Table S6 at JXB online. The PCR product was inserted into the vector pUC-GFP by replacing the CaMV 35S promoter to generate the construct P_MaNAC2::GFP containing the GFP-coding region under the control of the MaNAC2 promoter. The construct P_MaNAC2::GFP and the positive control 35S::GFP were transfected into tobacco BY-2 protoplasts using a PEG method as described above. Transfected protoplasts were subjected to 0 (control) or 0.8 mmol l^–1 ethrel treatment and then incubated at 23 °C for 14h. The GFP signal was detected before and after ethylene treatment using a fluorescence microscope (Zeiss Axioskop 2 Plus).

Yeast two-hybrid (Y2H) assay

Y2H assays were performed using the Matchmaker™ Gold Yeast Two-Hybrid System (Clontech). The coding regions of MaNAC1–MaNAC6 and MaEIL5 were cloned into pGADT7 and pGBKT7 to fuse with the activation domain (AD) and DNA-binding domain (DBD), respectively, to create different baits and preys (primers are shown in Supplementary Table S7 at JXB online). MaEIL5 did not show any transcriptional activation activity in yeast cells (data not shown). Different pairs of bait and prey constructs were then co-transformed into yeast strain Gold Y2H using the lithium acetate method, and yeast cells were grown on a SD/–Leu/–Trp) according to the manufacturer’s protocol (Clontech) for 3 d. Transformed colonies were plated onto minimal medium quadruple dropout (SD medium with –Leu/–Trp/–His/–Ade) containing 125 µM Aureobasidin A and 4mg ml^–1 X-α-Gal at 30 °C to test for possible interactions between MaNAC1–MaNAC6 and MaEIL5 according to their growth status and the activity of α-galactosidase.

BiFC assay

To generate constructs for the BiFC assays, the full-length coding sequences of MaNAC1, MaNAC2 and MaEIL5 (without their stop codons) were subcloned into the pUC-pSPYNE or pUC-pSPYCE vector obtained from the laboratory of K. Harter and J. Kudla (Walter et al., 2004). Expression of target genes alone was used as negative controls. The resulting constructs were used for transient assays using a modified PEG transfection of tobacco BY-2 suspension culture cell protoplasts as described previously (Abel and Theologis, 1994). The transformed protoplasts were then grown in liquid MS medium containing 0.4M sucrose for 24–48h, and transfected cells were imaged using a fluorescence microscope with a yellow fluorescent protein (YFP) filter (Zeiss Axioskop 2 Plus). The primers used in the BiFC assay are listed in Supplementary Table S8 in JXB online.

Statistical analysis

The experiment was arranged in a completely randomized design. Each sample time point per treatment comprised three independent biological replicates. Data are plotted on figures as means ±standard error (SE).

Results

Isolation and sequence analysis of MaNAC genes from banana fruit

Six novel putative NAC full-length cDNAs were isolated from banana fruit and designated as MaNAC1–MaNAC6. MaNAC1–MaNAC6 were predicted to encode proteins of 283, 293, 284, 257, 302, and 278 aa, with calculated molecular weights of 31.3, 33.2, 31.9, 29.1, 33.8, and 31.1kDa and pI values of 8.71, 6.55, 9.20, 6.83, 7.60, and 5.43, respectively. Alignment of the full-length deduced proteins of the MaNACs clearly showed that the proteins contained the NAC conserved domain in their N-terminal regions, which was further divided into five subdomains (A–E) (Ooka et al., 2003) (Fig. 1). However, the C-terminal regions showed no significant similarity to that of any other members of NAC family (Fig. 1). In addition, although all the MaNACs proteins had the conserved NAC domain, the similarities among the six MaNACs varied from 20% (MaNAC1 and MaNAC4) to 66% (MaNAC2 and MaNAC4) (Supplementary Table S9 at JXB online).

Fig. 1.

Amino acid sequence alignment of the MaNACs proteins with other plant NAC proteins. MaNACs were aligned with Arabidopsis ANAC019 (At1g52890.1), ANAC042 (At2g43000.1), ANAC043 (At2g46770.1), and ANAC073 (At4g28500.1), and rice ONAC022 (AK107090). Identical and similar amino acids are indicated by black and grey shading, respectively. Gaps were introduced to optimize alignment. The five highly conserved amino acid motifs (A–E) and the nuclear localization signal (NLS) are indicated by black and grey lines, respectively.

To examine the phylogenetic relationship among the NAC domain proteins in banana, Arabidopsis and rice, a phylogenetic tree was constructed based on their translated amino acid sequences. As shown in Fig. 2, the whole NAC proteins of Arabidopsis and rice could be classified into two major groups, I and II, which comprised 14 and four subgroups, respectively (Ooka et al., 2003). Based on the tree, the banana six NAC genes were clustered into four subfamilies. MaNAC2 and MaNAC4 belonged to ONAC022, and MaNAC5 and MaNAC6 belonged to NAC1, whereas MaNAC1 and MaNAC3 belonged to ONAC003 and NAM, respectively (Fig. 2). Collectively, these data suggested that MaNAC1–MaNAC6 may exhibit diverse functions.

Fig. 2.

Phylogenetic tree of NACs. Six banana MaNACs (black circles) were aligned with those of the Arabidopsis and rice NAC families as designated by Ooka et al. (2003). Multiple alignment was carried using CLUSTALW and the phylogenetic tree was constructed with MEGA5.0 using a bootstrap test of phylogeny with minimum evolution test and default parameters. The GenBank accession numbers of the Arabidopsis and rice NAC proteins are listed as Supplementary Data at JXB online.

Subcellular localization of the MaNAC proteins

NAC proteins are TFs that are usually located in the nucleus; however, some NAC proteins are also membrane associated (Seo et al., 2010; Hao et al., 2011). To validate the subcellular localizations of the MaNAC1–MaNAC6 proteins, the full-length coding sequences of MaNAC1–MaNAC6 were fused in frame with the GFP gene. Transient expression of these constructs in tobacco BY-2 protoplasts showed that the fluorescence of MaNAC1–MaNAC5 was localized exclusively in the nucleus (Fig. 3). By contrast, the fluorescence from the GFP control was distributed rather evenly within the cells. Interestingly, the fluorescence of MaNAC6 was not targeted exclusively to the nucleus, and the GFP signal was also found in the cytoplasm and cell membrane (Fig. 3), although it contained a nuclear localization sequence (NLS).

Fig. 3.

Subcellular localization of MaNACs in tobacco BY-2 protoplasts. Protoplasts were transiently transformed with MaNAC–GFP constructs or GFP vector using a modified PEG method. GFP fluorescence was observed with a fluorescence microscope. VirD2NLS-mCherry was included in each transfection to serve as a control for successful transfection, as well as for nuclear localization. Images were taken in a dark field for green fluorescence, while the outline of the cell and the merged were photographed in a bright field. Bars, 25 µm.

Transcriptional activation ability of MaNAC proteins in yeast

It has been reported that the N-terminal regions of NAC proteins show DNA-binding ability while the C-terminal regions function as a transcriptional activation domain (Olsen et al., 2005). To investigate the abilities of MaNAC1–MaNAC6 to activate transcription, a transient expression assay using a GAL4-responsive reporter system in yeast cells was performed. First, the full-length coding regions of MaNAC1–MaNAC6 were fused to the GAL4 DBD to generate pGBKT7-MaNAC1 to -6 fusion plasmids (DBD–MaNAC1 to -6) (Fig. 4A). The plasmids were then transformed into yeast strain AH109 and the transformants were assayed for their ability to activate transcription from the GAL4 upstream activation sequence and to promote yeast growth in medium lacking histidine. The transformants containing pGBKT7-53+pGADT7-T (DBD–P53+T-antigen) and pGBKT7 vectors (empty) were used as positive and negative controls, respectively. Fig. 4A showed that the transformed yeast cells harbouring DBD–MaNAC4, DBD–MaNAC6 and DBD–P53+T-antigen (positive control) grew well in SD medium lacking tryptophan, histidine, and adenine, and showed α-galactosidase activity, whereas cells containing DBD–MaNAC1, DBD–MaNAC2, DBD–MaNAC3, DBD–MaNAC5, or pGBKT7 (negative control) showed no α-galactosidase activity. To identify further the transcriptional activation domains of MaNAC4 and MaNAC6, the N- and C-terminal regions of MaNAC4 and MaNAC6 were fused to the GAL4 DBD (Fig. 4B) and transformed into yeast strain AH109. As shown in Fig. 4B, yeast cells harbouring DBD–MaNAC4-C and DBD–MaNAC6-C showed α-galactosidase activity. These data confirmed that MaNAC4 and MaNAC6 exhibited transcriptional activation ability in their C-terminal regions.

Fig. 4.

Transcriptional activation of MaNACs in yeast. (A) The coding regions of MaNAC1–MaNAC6 were cloned into the pGBKT7 (GAL4 DBD) vector to create the DBD–MaNAC1 to -6 constructs, respectively. (B) Truncation analysis of transcriptional activation of MaNAC4 and MaNAC6 for mapping activation domain. C- and N-terminal derivatives of MaNAC4 and MaNAC6 were fused with the pGBKT7 vector to create the DBD–MaNAC4 and -6 constructs, respectively. The numbers on the right indicate the last residues of the polypeptides. In (A) and (B), all of the constructs mentioned above, together with the positive control (p-53+T-antigen) and negative control (pGBKT7) were transformed into yeast strain AH109. Yeast clones transformed with different constructs were grown on SD plates without tryptophan or without tryptophan, histidine, and adenine but containing 125 µM Aureobasidin A for 3 d at 30 °C. Transcription activation was monitored by the detection of yeast growth and an α-galactosidase (α-Gal) assay.

Changes in fruit firmness and ethylene production in fruit with four different ripening characteristics

Changes in banana fruit firmness and ethylene production in fruit with four different ripening characteristics, comprising natural, ethylene-induced, 1-MCP-delayed, and a combination of 1-MCP with ethylene treatment, were shown in Fig. 5. The firmness of fruit ripened naturally began to decline at day 19, reaching a firmness of ~4 N at day 25, with ethylene production obviously increased at day 15, reaching a maximum at day 21 and then decreasing at day 25. The decline in fruit firmness and increase in ethylene production were substantially faster in ethylene-treated fruit, reaching the same ripening stage at day 7 as natural ripening at day 35, and ethylene production in ethylene-treated fruit began to be detected at 1 d of treatment, peaking at day 3. In contrast, treatment with 1-MCP delayed fruit softening and the increase in ethylene production, its firmness began to decline at day 28 reaching ~5 N at day 35, and ethylene production was detected at day 25 and reached a peak at day 30. However, application of ethylene to the 1-MCP-treated fruit promoted a decline in the firmness and an increase in ethylene evolution, the firmness of the fruit began to decline at day 21, reaching to ~4N at day 30, and ethylene production started to increase at day 25 and reached a peak at day 28. It should be noted that the higher ethylene production of 1-MCP+ethylene-treated fruit at day 1 might be the residual ethylene of the treatment.

Fig. 5.

Photograph of fruit with four different ripening characteristics, comprising natural (control), ethylene-induced, 1-MCP-delayed, and a combination of 1-MCP+ethylene treated ripening (A), and changes in fruit firmness and ethylene production (B) during ripening. Each value represents the mean ±SE of three replicates.

Expression of MaNAC and MaEIL genes in peel and pulp during fruit ripening

To understand the possible role of MaNAC1–MaNAC6 in banana fruit ripening, the expression patterns of MaNAC1–MaNAC6 in peel and pulp of fruit with the four different ripening characteristics described above were investigated by qPCR. As shown in Fig. 6, in peel tissues, among the six MaNAC genes, MaNAC1 and MaNAC2 were strongly induced at 1 d after ethylene treatment, with ~30- and ~350-fold increases at day 3, respectively. Their transcript levels in peel were low during early storage, but clearly began to increase with ethylene evolution. In contrast, MaNAC4 and MaNAC6 transcript levels decreased following ethylene treatment and ethylene evolution, and their transcript levels in peel decreased following the increase in ethylene production, with a more remarkable decrease for MaNAC6. In addition, MaNAC3 and MaNAC5 were expressed constitutively and their transcripts changed only slightly during ripening.

Fig. 6.

Expression of MaNAC and MaEIL genes in banana fruit peel with four different ripening characteristics: natural (control), ethylene-induced, 1-MCP-delayed, and a combination of 1-MCP and ethylene-treated ripening. The expression levels of each gene are expressed as a ratio relative to the harvest time (0 d of control), which was set at 1. Each value represents the mean ±SE of three replicates. The broken arrow and full arrow represent the time point of ethylene production beginning to increase and its peak for each treatment, respectively.

In pulp tissues, as shown in Fig. 7, expression of MaNAC1, MaNAC2, MaNAC3, and MaNAC4 showed a similar change in pattern to their expression in peel. MaNAC1 and MaNAC2 were ethylene induced and their transcript levels in natural, ethylene-induced, 1-MCP-delayed ripening, or a combination of 1-MCP with ethylene-treated fruit clearly increased with ethylene evolution. The MaNAC3 transcript level changed slightly and MaNAC4 transcript decreased following ethylene production increase during the ripening stage. Unlike the expression patterns in peel, expression of MaNAC5 and MaNAC6 was slightly enhanced after ethylene treatment, and their transcript levels in natural, 1-MCP-delayed ripening, or a combination of 1-MCP with ethylene-treated fruit also increased following the increase in ethylene production. These results indicated that MaNAC genes are differentially expressed in peel and pulp during post-harvest ripening, and that MaNAC1 and MaNAC2 may be more related to fruit ripening.

Fig. 7.

. Expression of MaNAC and MaEIL genes in banana fruit pulp with four different ripening characteristics: natural (control), ethylene-induced, 1-MCP-delayed, and a combination of 1-MCP and ethylene-treated ripening. The expression levels of each gene are expressed as a ratio relative to the harvest time (0 d of control), which was set at 1. Each value represents the mean ±SE of three replicates. The broken arrow and full arrow represent the time point of ethylene production beginning to increase and its peak for each treatment, respectively.

Five EIN-like genes, designated MaEIL1–MaEIL4, and AB266321 (termed MaEIL5 in the present work) have been isolated from banana fruit in a previous study, and it was shown that, in pulp tissue, MaEIL2 was a unique ripening- and ethylene-induced gene, while the MaEIL1, MaEIL4, and AB266321 (MaEIL5) genes were downregulated during ripening (Mbéguié-A-Mbeguié et al., 2008). As shown in Figs 6 and 7, similar to the previous report, MaEIL2 in peel and pulp was ethylene induced and its transcript level increased, while that of MaEIL5 in peel and pulp clearly decreased once ethylene evolution could be detected during ripening. MaEIL1and MaEIL4 transcript levels in pulp also decreased during ripening but the changes were not as obvious as that of MaEIL5 (data not shown).

The promoter activity of MaNAC2 is induced by ethylene

Gene expression profiles of the MaNAC genes during post-harvest fruit ripening suggested that MaNAC2 was ethylene inducible and might be involved in fruit ripening. To better establish the mechanisms by which MaNAC2 expression is modulated, a 1529bp upstream sequence from the start codon of MaNAC2 was isolated from the genome of Musa acuminata using a genome-walking PCR method. Analysis of the promoter using the PLACE and Plant-CARE databases, as illustrated in Table 1, revealed that 15 distinct putative cis-acting elements were identified in the promoter region. Besides the core cis-acting elements like the TATA-box and CAAT-box, other potential cis-regulatory elements involved in the activation of abiotic stress (cold, water, and heat) and hormone-responsive (GA and SA) genes were also found. Interestingly, two sites for the ethylene-responsive element, ATTTCAAA, with one nucleotide change found in the promoter at positions –11 (ATTACAAA) and –680 (ATTTCACAA) upstream of ATG, could probably govern expression in response to ethylene.

Table 1 .

Main regulatory motifs found within the promoter sequence of MaNAC2

Factor or site name	Site	Signal sequence	Function
MYB1AT	181(+); 147(+); 451(–); 900(–)	WAACA	MYB recognition site found in the promoters of the dehydration-responsive gene rd22 and many other genes
MYB2AT	1018(–)	TAACTG	Binding site for regulation of genes that are responsive to water stress
MYBCORE	162(+); 394(+); 633(+); 1018(+); 159 (–)	CNGTTR	Binding site for regulation of genes that are responsive to water stress
MYCCONSENSUSAT	375(+); 481(+); 499(+); 1239(+); 1523(+); 375(–); 481(–); 499(–); 1239(–); 1523(–)	CANNTG	Regulates the transcription of CBF/DREB1 genes in the cold
HSE	646(–); 972(+); 886(+); 973(+)	AAAAAATTTC	Cis-acting element involved in heat stress responsiveness
GARE	603(+)	TCTGTTG	Gibberellin-responsive element
LTRE1HVBLT49	1491(–)	CCGAAA	Cis-acting element involved in low-temperature responsiveness
LTRECOREATCOR15	1154(–)	CCGAC	Core of low temperature responsive element (LTRE) of cor15a gene
GAREAT	866(+)	TAACAAR	Gibberellin-responsive element
P-box	670(–)	GCCTTTTGAGT	Gibberellin-responsive element
MBS	989(–)	TAACTG	MYB-binding site involved in drought inducibility
TC-rich repeats	444(+); 970(–);850(–)	ATTTTCTCCA	Cis-acting element involved in defense and stress responsiveness
TCA element	1223(+); 1226(–)	CAGAAAAGGA	Cis-acting element involved in salicylic acid responsiveness
WBOXNTERF3	425(–);1290(–);1275(–);1335(–)	TGACY	May be involved in activation of ERF3 gene by wounding
WRKY71OS	77(+); 359(+); 695(+); 104(–); 426(–);1276(–); 1281(–); 1291(–); 1336(–)	TGAC	Binding site of rice WRKY71, a transcriptional repressor of the gibberellin signaling pathway

To analyse MaNAC2 promoter activity in response to ethylene, we conducted a transient protoplast assay using a GFP reporter construct containing the MaNAC2 promoter (MaNAC2pro::GFP). As shown in Fig. 8, in tobacco BY-2 protoplast transfected with MaNAC2pro::GFP, GFP signals were not detected before ethylene treatment but were observed after ethylene treatment. As a positive control, a GFP signal was constitutively observed before or after ethylene treatment in protoplasts transfected with the CaMV 35S promoter-driven construct (35S::GFP), and no GFP signal was detected in protoplasts that were not transfected. Similar results were also obtained in Arabidopsis mesophyll protoplasts (Supplementary Fig. S1 at JXB online). The results that ethylene induced the MaNAC2 promoter activity further support the suggestion that MaNAC2 is ethylene inducible and might be involved in fruit ripening.

Fig. 8.

MaNAC2 promoter activity in response to ethylene. GFP reporter constructs containing the MaNAC2 promoter (MaNAC2pro::GFP) and the CaMV 35S promoter (35S::GFP, positive control) were transiently transformed into tobacco BY-2 protoplasts using a modified PEG method and test for ethylene induction. Non-transformed protoplasts were used as a negative control. After incubation for 12h, GFP fluorescence was observed by fluorescence microscopy. Bar, 25 µm. The experiment was repeated at least three times.

MaNAC1 and MaNAC2 physically interact with MaEIL5

Several reports have suggested that NAC TFs are involved in ethylene signalling pathways through interaction with ethylene signalling components, such as EIN2 (He et al., 2005; Kim et al., 2009; Al-Daoud and Cameron, 2011). Based on the expression characteristics of the MaNAC and MaEIL genes during fruit ripening, it is thought that possible interactions between MaNAC and MaEIL proteins might exist.

In order to investigate whether MaNACs physically interact with MaEILs, the Matchmaker™ Gold Yeast Two-Hybrid System was used to investigate the interactions between MaNACs and MaEILs. MaNAC1–MaNAC6 and MaEIL5 coding sequences were subcloned into pGADT7 and pGBKT7 vectors for a Y2H assay, as MaNAC4 and MaNAC6 showed transactivation activity in yeast when fused with the DBD (Fig. 4). As shown in Fig. 9A, yeast cells co-transformed the positive control (pGBKT7-53+pGADT7-T) and MaNAC1 or MaNAC2 with MaEIL5, could grow on selective medium (synthetic medium lacking tryptophan, leucine, histidine, and adenine) supplement with the toxic drug Aureobasidin A, and turned blue in the presence of the chromagenic substrate X-α-Gal. However, yeast cells harbouring MaNAC3, MaNAC4, MaNAC5, or MaNAC6 with MaEIL5, and the negative controls, could not grow on the selective medium and did not turn blue under the same conditions (Fig. 9A). Interactions between MaNAC1 or MaNAC2 and MaEIL5 were also observed, when MaNAC1 or MaNAC2 and MaEIL5 coding sequences were subcloned into pGBKT7 and pGADT7, respectively (Fig. 9B). Moreover, the C terminus of MaNAC1 or MaNAC2 was sufficient for the interaction with MaEIL5 (Fig. 9C). In addition, we also examined the interactions between MaNAC1–MaNAC6 and MaEIL2 but detected no interactions among them (data not shown).

Fig. 9.

. Physical interactions between MaNAC proteins and MaEIL5 detected in Y2H assays. (A) The coding regions of MaNAC1–MaNAC6 were cloned into the pGADT7 vector to create the AD–MaNAC1 to -6 constructs, while the coding region of MaEIL5 was cloned into the pGBKT7 vector to create the DBD–MaEIL5 construct. Gold Y2H yeast strains were co-transformed with DBD–MaEIL5 and AD–MaNAC1 to -6, respectively. (B) The coding regions of MaNAC1/2 were cloned into the pGBKT7 vector to create the DBD–MaNAC1 and -2 constructs, while the coding region of MaEIL5 was cloned into the pGADT7 vector to create the AD–MaEIL5 construct. Gold Y2H yeast strains were co-transformed with DBD–MaNAC1 or -2 and AD–MaEIL5, respectively. (C) The N and C termini of MaNAC1 and MaNAC2 were tested for interaction with MaEIL5. Gold Y2H yeast strains were co-transformed with DBD–MaNAC1 or -2 derivatives and AD–MaEIL5, respectively. In (A), (B) and (C), the ability of yeast cells to grow on synthetic medium lacking tryptophan, leucine, histidine, and adenine but containing 125 µM Aureobasidin A, and to turn blue in the presence of the chromagenic substrate X-α-Gal, was scored as a positive interaction. Yeast cells transformed with pGBKT7-53+pGADT7-T, DBD–MaEIL5+pGADT7-T, or pGBKT7-Lam+pGADT7-T were included as positive or negative controls, respectively.

To further confirm the interactions between MaNAC1/MaNAC2 and MaEIL5 observed in the Y2H assays, we performed BiFC assays in tobacco BY-2 protoplasts. MaNAC1 or MaNAC2 tagged with pSPYNE (split YFP N-terminal fragment expression) and MaEIL5 tagged with pSPYCE (split YFP C-terminal fragment expression) were transiently co-expressed in tobacco BY-2 protoplasts following PEG transfection. As shown in Fig. 10A, robust YFP fluorescent signal was detected in the nucleus of BY-2 cells expressing MaNAC1-pSPYNE or MaNAC2-pSPYNE and MaEIL5-pSPYCE, while no YFP fluorescent signal was observed either in the cells expressing MaNAC1/MaNAC2-pSPYNE with only the pSPYCE or in those expressing MaEIL5-pSPYCE with only the pSPYNE (Fig. 10A). Similar results were also observed when MaNAC1/MaNAC2-pSPYCE was co-transfected with MaEIL5-pSPYNE (Fig. 10B). The BiFC results not only demonstrated the in vivo interaction among the three proteins tested but also showed the specific localization of the interacting proteins in the nucleus, which was consistent with the subcellular localization of MaNAC1, MaNAC2, and MaEIL5 in the nuclear compartment (Fig. 3 and Supplementary Fig. S2 at JXB online).

Fig. 10.

BiFC visualization of the MaNAC1/MaNAC2 and MaEIL5 interaction in transiently co-expressed tobacco BY-2 protoplasts. (A) MaNAC1/ MaNAC2 and MaEIL5 proteins were fused with the N (YN) and C (YC) termini of YFP, respectively. (B) MaNAC1/MaNAC2 and MaEIL5 proteins were fused with the C (YC) and N (YN) terminus of YFP, respectively. Expression of MaNAC1/MaNAC2 or MaEIL5 alone was used as a negative control. VirD2NLS-mCherry was included in each transfection to serve as a control for successful transfection, as well as for nuclear localization. YFP fluorescence is yellow; the merged image is a digital merge of bright field and fluorescent images. Bar, 25 µm.

Discussion

Identification of banana MaNAC genes

NAC is a family of plant-specific TFs that have been identified in various species (Berger et al., 2009; Meng et al., 2009; Zhong et al., 2010). There are approximately 100 NAC TFs in both the Arabidopsis and rice genomes (Gong et al., 2004; Xiong et al., 2005). From our transcriptome database, there are at least six putative proteins that exhibit a significant sequence identity with Arabidopsis AtNAC and rice OsNAC homologues, further confirming the notion that NAC proteins are encoded by a large multigene family (Ooka et al., 2003). The predicted MaNAC paralogues were 20–66% identical to each other at the amino acid level (Supplementary Table S9). Alignment of the six MaNACs proteins showed that they shared a highly conserved N-terminal DBD, termed the NAC domain (Ooka et al., 2003), which contains five consensus subdomains (Fig. 1). The NAC domain may function as a potential NLS (Duval et al., 2002; Hegedus et al., 2003; Ernst et al., 2004) and most isolated NAC proteins are localized in the nucleus (Pinheiro et al., 2009; Hao et al., 2011). Similarly, the subcellular localizations of the MaNACs indicated that MaNAC1–MaNAC5 were all localized in the nucleus, while MaNAC6 was distributed throughout the whole cell, including nucleus, cytoplasm, and cell membrane (Fig. 3). Thirteen members of the membrane-associated type of NAC proteins have been found in Arabidopsis (Kim et al., 2007). These NACs, which are larger in molecular weight than non-membrane-associated NAC TFs (~320 residues), have residue numbers ranging from 335 to 652, because of their C-terminal extensions, which contain the transmembrane motif (Morishita et al., 2009). In eukaryotic cells, molecular mechanisms that regulate the localization of TFs have been investigated extensively, one of which is cytoplasmic sequestration caused by the presence of a membrane anchor (Lee and Hannink, 2003). Following specific cues, membrane-bound precursors are proteolytically cleaved, allowing nuclear localization of the TF (Morishita et al., 2009). One Arabidopsis NAC protein, AtNAC78, has been located in the nucleus and cytoplasm in onion epidermal cells when fused with its full-length sequence (including the transmembrane motif), but was located in the nucleus when the putative transmembrane motif was deleted (Morishita et al., 2009). However, the same is not true for MaNAC6, which includes a similar C-terminal extension to the other five MaNACs (Fig. 1). Thus, the mechanism and role of MaNAC6 localization within whole cells need to be elucidated further. In addition, the C-terminal regions of MaNAC1–MaNAC6 proteins are highly variable (Fig. 1). It has been reported that the non-conserved C-terminal regions of several NAC proteins, but not NAC domains, function as transcriptional activation domains (Hegedus et al., 2003; Fujita et al., 2004; Kim et al., 2006), which may confer the regulation diversities of transcriptional activation activity. Our data also showed that, among the six MaNAC proteins, only MaNAC4 and MaNAC6 exhibited transcriptional activation ability in their C-terminal regions, and the NAC domain in their N-terminals did not activate reporter genes in yeast cells (Fig. 4). In addition, it should be pointed out that as the largest TF family in plants, other NAC genes may exist in banana fruit and their functions also need to be elucidated.

Arabidopsis thaliana and Oryza sativa NAC proteins have been classified previously into two groups based on similarities in the NAC domain structures (Ooka et al., 2003). Group I is divided into 14 subgroups, namely TERN, ONAC022, SENU5, NAP, AtNAC3, ATAF, OsNAC3, NAC2, ANAC011, TIP, OsNAC8, OsNAC7, NAC1, and NAM, while the remaining four subgroups, comprising ANAC011, ONAC003, ONAC001, and ANAC063, constitute group II (Ooka et al., 2003). Although many reported NAC TFs are known for their diverse functions, the functions of the majority of members in this family are still unclear. Generally, NAC genes with the same function show a tendency to fall into one subgroup. For example, Arabidopsis ANAC072 (RD26), ANAC019, and ANAC055, which have been found to be involved in the response to various environmental stresses, are clustered into subgroup AtNAC3 (Tran et al., 2004; Fujita et al., 2004). Subgroup NAM encompasses NAM and CUC2, which have been shown to function in shoot apical meristem formation and development (Souer et al., 1996; Aida et al., 1997). SND1 (ANAC012), together with NST1 (ANAC043) and VND6 (ANAC101), are grouped into subgroup OsNAC7, which has been proposed to regulate secondary wall synthesis (Zhong et al., 2010). Additionally, the membrane-associated NAC proteins that harbour a transmembrane motif with a predicted α-helix in the far C-terminal region have been placed phylogenetically into subgroups ANAC001, TIP, and NAC2 (Kim et al., 2006). The members of these subgroups are highly expressed in vegetative organs that are more susceptible to abiotic stress conditions (Kim et al., 2006). In the present work, the six banana MaNAC genes fell into four subgroups, in which MaNAC1 and MaNAC3 belonged to ONAC003 and NAM, respectively, while MaNAC2 and MaNAC4 belonged to ONAC022, and MaNAC5 and MaNAC6 belonged to NAC1. These results suggest that MaNAC genes may have diverse functions in banana.

Possible roles of MaNAC TFs in relation to banana fruit ripening

The regulation of fruit ripening has been well studied, and ripening is known to be influenced by hormones, light, temperature, and developmental gene regulation (Klee and Giovannoni, 2011). However, critical TFs involving in fruit ripening were identified only recently, even in tomato fruit (Martel et al., 2011; Qin et al., 2012). One of the tomato MADS-box TFs, RIPENING INHIBITOR (RIN), exhibited elevated expression at the onset of ripening (Vrebalov et al., 2002). Moreover, recently, it has been confirmed that RIN interacts with the promoters of genes involved in the major pathways associated with observed and well-studied ripening phenotypes and phenomena, including the transcriptional control network involved in overall ripening regulation, ethylene biosynthesis, ethylene perception, the downstream ethylene response, cell-wall metabolism, carotenoid biosynthesis, and aroma formation (Martel et al., 2011; Qin et al., 2012). MADS-box TFs have also been suggested to be involved in the ripening of other fruit, such as strawberry (Seymour et al., 2011), apple (Yao et al., 1999), banana (Elitzur et al., 2010), and avocado (Hershkovitz et al., 2011). Interestingly, although banana MaMADS2 showed a high sequence similarity to tomato LeMADS-RIN, complementation analysis demonstrated that it could not complement a ripening rin mutant in tomato (Elitzur et al., 2010). In addition, other TFs, such as FaSPT, which encodes a basic helix–loop–helix TF, and FaASR, a TF involved in abscisic acid signal transduction, are related to strawberry ripening (Tisza et al., 2010; J.Y. Chen et al., 2011).

Little information about plant-specific NAC TFs associated with fruit ripening is available. In the present work, gene expression profiles in fruit with four different ripening characteristics – natural, ethylene-induced, 1-MCP-delayed, and 1-MCP and ethylene-treated ripening – revealed that banana MaNAC genes were differentially expressed in peel and pulp during ripening. MaNAC1 and MaNAC2 were apparently upregulated by ethylene in peel and pulp, consistent with the increase in ethylene production (Figs 6 and 7). In contrast, MaNAC3 in peel and pulp and MaNAC5 in peel were expressed constitutively, and transcripts of MaNAC4 in peel and pulp and MaNAC6 in peel decreased, while MaNAC5 or MaNAC6 in pulp slightly increased during fruit ripening (Figs 6 and 7). These results suggest that the transcription complexes created during ripening are constantly changing and MaNAC1/MaNAC2 expression may be more related to fruit ripening. Moreover, the MaNAC2 promoter, when transiently expressed in protoplasts, was activated after ethylene application, further confirming that MaNAC2 is ethylene inducible and may be involved in fruit ripening (Fig. 8 and Supplementary Fig. S1). Other plant NAC TFs, including Arabidopsis AtNAC2 (He et al., 2005), pepper CaNAC1 (Oh et al., 2005), rice OsNAC19 (Lin et al., 2007), and wheat TaNAC4 (Xia et al., 2010), are also ethylene inducible, supporting the notion that NAC TFs might be a common downstream component of the ethylene signalling pathway. In tomato, NOR (non-ripening) is a NAC-domain TF and fruit carrying its mutation, nor, cannot ripen even after exogenous ethylene application, suggesting an important role of NOR in regulating fruit ripening (Cara and Giovannoni, 2008). Thus, it will be interesting to investigate whether MaNAC1/MaNAC2 is able to complement ripening in a tomato nor mutant.

Interactions between MaNAC1/MaNAC2 and MaEIL5

NAC TFs have been found not only to bind to DNA but also to interact with other proteins, including several NAC proteins interacting with a RING protein (Greve et al., 2003), a TCP-domain TF (Weir et al., 2004), and a SNF1-related kinase (Kleinow et al., 2009). Moreover, some NAC proteins can interact with other NACs or with themselves to function by forming homodimers and/or heterodimers in plants, such as Arabidopsis NAC1 and ANA019 (Xie et al., 2000; Ernst et al., 2004), Brassica BnNAC14 with BnNAC3, BnNAC5-8, BnNAC5-11, and BnNAC485 (Hegedus et al., 2003), and rice OsNAC5 with OsNAC6, OsNAC9, OsNAC10, and itself (Jeong et al., 2009). Several groups have reported that NAC may represent a TF downstream of EIN2 but in parallel with EIN3, which is also a TF downstream of EIN2 (He et al., 2005; Kim et al., 2009; Al-Daoud and Cameron, 2011). In the present study, banana MaNAC1/MaNAC2 physically interacted with an EIN3-like protein, MaEIL5, in Y2H and BiFC assays (Figs 9 and 10). EIN3 and EIN3-like (EIL) proteins are key TFs in ethylene signalling, and play important roles in regulating fruit ripening (Mbéguié-A-Mbeguié et al., 2008; Yin et al., 2010). Our results, together with previous reports, clearly suggest that NAC TFs are involved in ethylene signalling pathways through interaction with ethylene signalling components. However, it should be pointed out that the MaEIL5 transcript level decreased following ethylene evolution during ripening (Figs 6 and 7). Generally, EILs act as positive regulators of ethylene response, but levels of EIL genes show no increase during fruit ripening, implying that changes in EIL mRNA levels may be not necessary to induce an ethylene response (Tieman et al., 2001). At the same time, EILs may be regulated at the post-transcriptional or post-translational level, as demonstrated by Potuschak et al. (2003), who showed that EBF1/EBF2 physically interacted with EIN3/EIL1 and probably targeted these proteins for degradation. Accordingly, it is imperative to study the protein modifications of EILs during banana fruit ripening in the future. In addition, it will be interesting to find why only MaNAC1/MaNAC2, which showed no transcription activation ability in yeast cells, could interact with MaEIL5, while MaNAC4/MaNAC6 with transcription activation ability, could not interact with MaEIL5 (Figs 4 and 9), and why none of the six MaNACs interacted with MaEIL2 (data not shown), whose transcript level increased during fruit ripening (Figs 6 and 7).

In summary, six banana fruit MaNAC genes were isolated and characterized. Gene expression profiles in fruit with four different ripening characteristics revealed that MaNAC genes are expressed differentially in peel and pulp during ripening. More importantly, MaNAC1/MaNAC2 physically interacted with MaEIL5. Taken together, these results suggest that MaNACs such as MaNAC1 and MaNAC2 may be involved in banana fruit ripening via interaction with ethylene signalling components. To the best of our knowledge, this is the first report that NACs interact physically with EILs, and our findings reveal a novel mechanism of NAC TFs participating in ethylene signalling.

© 2011 The Authors.

Glossary

Abbreviations:

Abbreviations	Expansion
ACC	1-aminocyclopropane-1-carboxylic acid
AD	activation domain
BiFC	bimolecular fluorescence complementation
CaMV	cauliflower mosaic virus
DBD	DNA-binding domain
EIN3	ethylene insensitive 3
EIL	ethylene insensitive 3-like
GFP	green fluorescent protein
1-MCP	1-methylcyclopropene
MADS	Mcm1-Agamous-Deficiens-S
NAC	NAM ATAF1/2 and CUC2
PEG	polyethylene glycol
NLS	nuclear localization signal
pI	isoelectric point
qPCR	quantitative real-time PCR
TF	transcription factor
Y2H	yeast two-hybrid
YFP	yellow fluorescent protein