Figure 9.

A Working Model Depicting that MYB30 Acts to Promote PIF4 and PIF5 Protein Accumulation under Prolonged R Light Irradiation.

In the dark, MYB30 is degraded via the ubiquitin/26S proteasome-mediated pathway, and PIF4 and PIF5 promote hypocotyl elongation by inducing the expression of growth-promoting genes. Upon short R light exposure, PIF4 and PIF5 are rapidly degraded through the ubiquitin/26S proteasome-mediated pathway, and MYB30 abundance is induced by phyA and phyB. The interaction between MYB30 and the Pfr forms of phyA and phyB may stabilize MYB30 in the light. Under prolonged R light exposure, MYB30 promotes PIF4 and PIF5 protein accumulation by directly binding to their promoters to induce their expression, and by inhibiting the interaction of PIF4 and PIF5 with the Pfr form of phyB. Also, MYB30 acts in parallel with PIFs to induce the expression of growth-promoting genes.