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. 2016 Feb 23;11(3):e1139278. doi: 10.1080/15592324.2016.1139278

MYB-related transcription factors function as regulators of the circadian clock and anthocyanin biosynthesis in Arabidopsis

Nguyen Hoai Nguyen a,b, Hojoung Lee a,b
PMCID: PMC4883932  PMID: 26905954

ABSTRACT

In Arabidopsis, the MYB (myeloblastosis) gene family contains more than 190 members, which play a number of roles in plant growth and development. Based on their protein structure, this gene family was divided into several subclasses, including the MYB-related class. Currently, an MYB-related gene designated as MYB-like Domain (AtMYBD) has been shown to function as a positive regulator of anthocyanin biosynthesis in Arabidopsis. This gene was found to belong to the CCA1-like (circadian clock-associated 1) group, which represents several genes that are master regulators of the circadian clocks of plants. Here, we speculate that AtMYBD is able to regulate anthocyanin biosynthesis in Arabidopsis thaliana in a circadian clock-related manner.

Keywords: Anthocyanin; Arabidopsis; AtMYBD (AT1G70000), circadian clock; MYB-related

Circadian rhythms can be defined as endogenous biological oscillations over a period of approximately 24 h. These one-day-rhythms are controlled by a network of molecular components called a circadian clock, which is present in many organisms, including plants.1,2 In plants, several physiological processes have been found to be regulated by a circadian clock.1,3 In Arabidopsis, the circadian clock comprises 3 loops, a central loop and 2 side loops. Two MYB-related transcription factors, namely the CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL 1 (LHY1), were found to act as master regulators of the central loop.1,3-7 Among the 15 members belonging to the CCA1-like group in the MYB-related family, see ref.8, some other MYB-related transcription factors have been found to function in a similar manner in regards to the circadian clock. Transcription factors such as REVEILLE 2 (RVE2), RVE7, RVE8, and MYB HYPOCOTYL ELONGATION-RELATED (MYBH) (Table 1) act on the circadian clock in ways similar to that of CCA1 and LHY1.1,3,4,6,9-14 The RVE7/EARLY-PHYTOCHROME-RESPONSIVE1 (EPR1) was found to be regulated by PHYTOCHROME A and PHYTOCHROME B, and it functions as a component of the slave oscillator.9 Additionally, MYBH/KUA1 expression is regulated by the circadian clock and this MYBH/KUA1 transcription factor regulates cell expansion, hypocotyl elongation, and leaf senescence in the plant.14-16 Currently, we have found that AtMYBD expression is also controlled by diurnal regulation, see ref. 17, and this implies that the AtMYBD transcription factor may function to control downstream processes in a circadian clock-dependent manner.

Table 1.

List of genes belonging to the MYB-related subclass (CCA1-like group).

Locus ID Full name Short name Function Representative publication
AT1G01060 LATE ELONGATED HYPOCOTYL 1 LHY1 Circadian clock Kim et al. (2013)36;Adams et al. (2015)6
AT1G01520        
AT1G18330 EARLY-PHYTOCHROME-RESPONSIVE1, REVEILLE 7 EPR1RVE7 Circadian clock and auxin pathways Kuno et al. (2003)9;Rawat et al. (2009)10
AT1G70000 MYB-LIKE DOMAIN MYBD Anthocyanin biosynthesis Nguyen et al. (2015)17
AT1G74840        
AT2G46830 CIRCADIAN CLOCK ASSOCIATED 1 CCA1 Circadian clock Wang et al. (1997)35;Nagel et al. (2015)7
AT3G09600 LHY-CCA1-LIKE5/REVEILLE 8 LCL5/RVE8 Circadian clock by modulating the histone 3 acetylation Farinas and Mas (2011)34;Rawat et al. (2011)12
AT3G10590        
AT3G16350        
AT4G01280        
AT5G02840        
AT5G37260 CIRCADIAN 1/REVEILLE 2 CIR1/RVE2 Circadian clock and seed germination Zhang et al. (2007)13
AT5G47390 MYB HYPOCOTYL ELONGATION-RELATED/CHINESE FOR ENLARGE OR EXPAND MYBH/KUA1 Hypocotyl elongation; cell expansion and leaf senescence Kwon et al. (2013)15;Lu et al. (2014)14;Huang et al. (2015)16
AT5G52660        
AT5G56840        
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Flavonoid biosynthesis is regulated by both endogenous and environmental factors such as plant hormones, sucrose, and several stress conditions.18-23 In Arabidopsis, anthocyanin is only accumulated in the presence of light and this accumulation is promoted by different abiotic and biotic stress conditions such as cold, drought, and pathogen attack.24,25 Almost 2 decades ago, the expressions of the genes involved in anthocyanin biosynthesis, such as CHALCONE SYNTHASE (CHS), CHALCONE ISOMERASE (CHI), and DIHYDROFLAVONOL 4-REDUCTASE (DFR), were found to be regulated by a circadian rhythm.26 A number MYB transcription factors have been found to act as regulators of the flavonoid biosynthesis pathway. For example, MYB11, MYB12, MYB111, MYB75/PAP1, and AtMYBD work as positive regulators whereas MYB-LIKE 2 (MYBL2) functions as a negative regulator.17,19,27-30 Among them, MYB75/PAP1 is a master regulator of the anthocyanin biosynthesis pathway;19,31 however, direct evidence reflecting whether or not PAP1 can act on the circadian regulation of this biosynthesis pathway is still lacking.32

Currently, a study has revealed that 2 circadian components NIGHT LIGHT–INDUCIBLE AND CLOCK-REGULATED (LNK) and RVE8 collaborate to control the anthocyanin metabolic pathway.33 The MYB-related transcription factor RVE8 can bind directly to the promoter of TIMING OF CAB EXPRESSION1 (TOC1) and facilitate this gene expression by increasing the acetylation level of histone H3, whereas CCA1 inhibits TOC1 expression via the reduction of the histone acetylation level.34 In addition, the study by Perez-Garcia et al.33 revealed that RVE8 can directly associate with the promoters of genes involved in anthocyanin biosynthesis and regulate the expressions of these genes in response to diurnal oscillations. From these findings, RVE8 can be considered a circadian-related regulator of anthocyanin biosynthesis in plants. On the other hand, the MYBL2, which encodes a negative regulator of anthocyanin biosynthesis, was proposed to be regulated by circadian rhythm.29 Our recent study found that AtMYBD may also regulate MYBL2 expression in a diurnal manner leading to anthocyanin accumulation in the plant.17 Taken together, our results suggest that AtMYBD may also work as a regulator of anthocyanin accumulation in a circadian-dependent manner. This speculation still requires further studies to validate it.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Funding

This work was supported by a grant from the National Research Foundation of Korea (to Hojoung Lee, 2014; grant NRF-2014R1A1A3050272).

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