Novel insights into the function of Arabidopsis R2R3-MYB transcription factors regulating aliphatic glucosinolate biosynthesis

Arabidopsis transcription factors, MYB28, MYB29 and MYB76, positively regulate aliphatic glucosinolate (AGSL) biosynthesis. Mutual transcriptional regulation among these MYB genes makes it difficult to elucidate their individual function simply by analyzing knock-out mutants or ectopically overexpressing lines of these genes. In this study, we constructed transgenic lines expressing each MYB gene driven by its own promoter in the myb28myb29 background, where the expression of the endogenous MYB28, MYB29 and MYB76 was repressed with no AGSL accumulation. In leaves, transgenic MYB28 expression activated AGSL biosynthetic genes and restored accumulation of AGSLs with short side chains. Transgenic MYB29 expression activated the same biosynthetic pathway, but induction of the genes involved in side chain elongation was weaker than that by MYB28, resulting in a weaker recovery of AGSLs. Neither MYB28 nor MYB29 recovered long-chain AGSL accumulation. MYB76 was considered to require both MYB28 and MYB29 for its normal level of expression in leaves, and could not activate AGSL biosynthesis on its own. Interestingly, the accumulation in seeds of long- and short-chain AGSLs was restored by transgenic expression of MYB28 and MYB76, respectively. A sulfur stress experiment indicated that MYB28 expression was induced by sulfur deficiency, while the expression levels of MYB29 and MYB76 were positively correlated with sulfur concentration. This study illustrated how the individual MYBs work in regulating AGSL biosynthesis when expressed alone under normal transcriptional regulation.