Phosphorylation of the transcriptional regulator MYB44 by mitogen activated protein kinase regulates Arabidopsis seed germination.

The phytohormones abscisic acid (ABA) and gibberellic acid (GA) have antagonistic roles in the control of seed germination and seedling development. We report here that the transcriptional regulator MYB44 has a role in the control of seed germination in Arabidopsis thaliana. High levels of the MYB44 transcript are found in dry seeds but the transcript levels decrease during germination. The decrease in transcript level during germination is inhibited by the GA biosynthesis inhibitor paclobutrazol (PAC). MYB44 is phosphorylated by both recombinant and native forms of MPK3 and MPK6 at Ser(53) and Ser(145). Transgenic overexpression of MYB44 results in increased sensitivity of seed germination to ABA or PAC treatment. The PAC-insensitive germination phenotype of the myb44 mutant is complemented by overexpression of wild type MYB44 but not by overexpression of a mutant protein that lacks the MPK-target serines indicating that phosphorylation of MYB44 by MPKs is required for its biological function.