Michel Vincentz - The circadian clock-regulated transcription factor bZIP63 modulates starch degradation

Efficient management of energy resources is crucial for plants to complete their life cycle. In Arabidopsis thaliana, the SUCROSE NON-FERMENTING RELATED KINASE 1 (SnRK1) mitigates the adverse effects of energy supply deficit imposed by unfavorable environmental conditions by triggering gene expression programs that favors energy saving and catabolism to reallocate energy resources. The phosphorylation of the basic LEUCINE ZIPPER 63 (bZIP63) transcription factor by SnRK1 has an important role in this process. bZIP63 is also essential for circadian clock entrainment by sugars, suggesting that this transcription factor has a role in maintaining
the energy homeostasis during the day. We will present data that show that bZIP63 expression is regulated by the circadian clock and energy status. This dual control, by setting the phase and amplitude of daily rhythms of bZIP63 transcript levels, is likely to be instrumental in establishing the diel expression pattern of bZIP63, which in turn binds to the promoter and modulates the expression of target genes including of STARCH EXCESS 1 / GLUCAN WATER DIKINASE (SEX1/GWD1). The induction of SEX1 and PHOSPHOGLUCAN WATER DIKINASE (PWD) expression in two bZIP63 mutants was correlated with a faster consumption of starch during the night. This alteration may explain the impaired growth and biomass accumulation of bZIP63
mutants. bZIP63 may therefore integrate circadian clock and energy status signals to fine tune starch degradation and, more generally, the energy supply for optimal plant growth through the diel cycle.