Abstract
The heme-associated proteins (HAPs, also known as NUCLEAR FACTOR Y, SUBUNIT A/B/C (NF-YA/B/C)) have been reported to bind specifically to DNA fragments containing CCAAT-box, however, the physiological functions and direct targets of these HAP proteins remain unclear in plants. In our recent study, we found that AtHAP5A and AtXTH21 positively modulated freezing stress resistance, and chromatin immunoprecipitation (ChIP) assay and genetic evidence indicated that AtHAP5A might act in the upstream of AtXTH21 in freezing stress. Moreover, AtHAP5A and AtXTH21 had significant effects on inhibiting cold stress-induced reactive oxygen species (ROS) accumualtion and activating ABA-related genes’ expression. Thus, a possible model that depicting AtHAP5A-mediated cold stress responses was proposed in this study, and we highlighted that AtHAP5A modulates freezing stress resistance in Arabidopsis through binding to CCAAT motif of AtXTH21, which is independent of the CBF pathway.
Keywords: HAP5A, CCAAT motif, CBF, freezing stress, XTH21
Low temperature (cold stress) is one of the most serious environmental stresses that limits plant growth and crop production.1-5 To date, several plant hormones and genes especially a number of transcription factors confer improved cold stress resistance in plants.1-8 In Arabidopsis, INDUCER OF CBF EXPRESSION (ICE)-C-repeat (CRT)/ Dehydration responsive element (DRE) BINDING FACTORs (CBF/DREBs)- Cold-regulated (COR) genes mediated pathway plays essential roles during plant cold stress response.1-8 Briefly, ICE1/2, encoding basic helix-loop-helix (bHLH) transcription factors, directly bind to CANNTG box in the promoter regions of CBF/DREBs that interact with CRT/DRE of down-stream CORs.1-8 Additionally, ICE1 is negatively regulated by the ubiquitin E3 ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 1 (HOS1), and is positively regulated by the SMALL UBIQUITINRELATED MODIFIER (SUMO) E3 ligase SIZ1 (SAP and Miz) at the post-translational level.1-8 Recently, Jeon et al. (2010) showed that cytokinin receptors ARABIDOPSIS HISTIDINE KINASE2/3 (AHK2/3) and type-A ARABIDOPSIS RESPONSE REGULATOR (ARR) proteins mediated plant cold stress signaling through inhibiting ABA signaling.9 Shi et al. (2012) found that ethylene negatively modulated plant freezing stress responses through repressing CBF/DREBs and type-A ARRs in Arabidopsis.10 Hu et al. (2013) discovered that JA regulated cold resistance through functioning in the upstream signal of the ICE-CBF/DREB1 pathway.11 Since plant cold stress response is a complex signaling pathway, besides the above pathways, many unknown mechanisms that independent CBF pathays need to be further dissected.
Although several heme-associated proteins (HAPs, also known as NUCLEAR FACTOR Y, SUBUNIT A/B/C (NF-YA/B/C)) have been reported to bind specifically to DNA fragments containing CCAAT-box, the physiological functions and direct targets of these HAP proteins remain unclear in plants.12-15 Recently, Petroni et al. (2012) reviewed the possible involvement of plant HAPs in plant stress responses. However, no direct stress-related target of HAPs has been revealed, and the direct link between CCAAT element and plant-environment interaction is also unknown.15
In our recent study, we found that expression of HAP5A was induced by different abiotic stress treatments, and AtHAP5A as a transcription factor interacted with CCAAT motif in vivo.16 Through genome sequence analysis, CCAAT motif was found in promoters of many genes, and five CCAAT motifs were enriched in AtXTH21 promoter region.16 Based on the phenotype data of AtHAP5A modulated plants, we focused on the involvement of AtHAP5A in freezing stress.16 The direct link of expression between AtHAP5A and AtXTH21 under control and stress conditions made AtXTH21 as a potential target.16 Moreover, the genetic evidence suggested that AtXTH21 as a direct target of AtHAP5A was largely contributed to AtHAP5A-mediated freezing stress response.16 Notably, AtXTH21 might not be the only target of AtHAP5A, and the dissection of other targets of AtHAP5A will shed more light on the in vivo role of AtHAP5A (Fig. 1).16
Figure 1. A possible model that depicting AtHAP5A-mediated cold stress responses.
Based on our observations, a possible model that depicting AtHAP5A-mediated cold stress responses was proposed.16 When cold stress was applied, the expression of HAP5A was largely induced, and the activation of HAP5A increased the expressions of XTH21 through directly binding to the CCAAT elements of in the promoters of XTH21. Moreover, upregulation of XTH21 led to improved freezing stress resistance in Arabidopsis, via inhibiting cold stress-induced reactive oxygen species (ROS) accumulation, ABA-related genes’ expression, and other unknown pathways. Although ICE1/2-CBF/DREBs mediated cold stress responses were activated by cold stress treatment, the expressions of three core genes (CBF1/2/3) were not significantly affected by HAP5A and XTH21 expression, and no appropriate motif that is responsible for CBFs binding was found in the promoter of AtXTH21. Thus AtHAP5A and AtXTH21 might mediate cold stress resistance independent of the CBF pathway.
Conclusions
Taken together, our experiments highlight that AtHAP5A modulates freezing stress resistance in Arabidopsis through binding to CCAAT motif of AtXTH21, which is independent of the CBF pathway.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Acknowledgments
We thank Prof. Frantisek Baluska for the invitation and suggestion for this addendum. This research was supported by the National Natural Science Foundation of China (Grant No. 31370302) and “the Hundred Talents Program,” the Knowledge Innovative Key Program of Chinese Academy of Sciences (Grant No.54Y154761O01076 and Y329631O0263) to Zhulong Chan, and by the National Natural Science Foundation of China (Grant No. 31200194), Youth Innovation Promotion Association of of Chinese Academy of Sciences and the Outstanding Young Talent Program of Key Laboratory of Plant Germplasm Enhancement and Specialty Agriculture (Grant No. Y 452331O03) to Haitao Shi.
Glossary
Abbreviations:
- ABA
abscisic acid
- AHK
ARABIDOPSIS HISTIDINE KINASE
- ARR
ARABIDOPSIS RESPONSE REGULATOR
- bHLH
basic helix-loop-helix
- ChIP
chromatin immunoprecipitation
- CBF/DREBs
C-repeat/Dehydration responsive element BINDING FACTORs
- COR
Cold-regulated
- CRT
C-repeat
- DRE
Dehydration responsive element
- HAPs
heme-associated proteins
- HOS1
HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 1
- ICE
INDUCER OF CBF EXPRESSION
- NF-YA/B/C
NUCLEAR FACTOR Y, SUBUNIT A/B/C
- ROS
reactive oxygen species
- SUMO
SMALL UBIQUITINRELATED MODIFIER
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