The Arabidopsis Myb genes MYR1 and MYR2 are redundant negative regulators of flowering time under decreased light intensity.

Changes in the duration, quality and intensity of light affect flowering time. Compared with the effects of light duration and quality, less is known about the effects of light intensity on flowering. Here we describe two paralogous single Myb domain genes, MYB-RELATED PROTEIN 1 (MYR1) and MYB-RELATED PROTEIN 2 (MYR2), and their roles as repressors of responses to decreased light intensity in Arabidopsis. Homozygous myr1 myr2 double mutants flowered early under low light intensities. Additionally, myr1 myr2 mutants exhibited increases in petiole length, leaf angle and apical dominance. Genetic analyses involving mutants in the long-day, gibberellin (GA) and phyB flowering pathways indicated that all aspects of the myr1 myr2 phenotype required GA biosynthesis. The early-flowering phenotype of myr1 myr2 also required FLOWERING LOCUS T, and myr1 myr2 mutants showed an epistatic interaction with the phyB-9 mutant. Over-expression of MYR1 or MYR2 produced GA-deficiency symptoms that were rescued by application of gibberellic acid (GA₃). Loss of MYR1 and MYR2 function was associated with a twofold increase in GA20ox2 expression and a 30% increase in GA₄ levels, while over-expression of MYR2 led to a threefold decrease in GA20ox2 expression and a 50% decrease in GA₄ levels. Considered together, these results suggest that the ability of MYR1 and MYR2 to repress flowering and organ elongation is at least partly due to their negative effect on levels of bioactive GA.