Functional roles of PIFs in Arabidopsis senescence

Abstract

Plant senescence constitutes an essential developmental process of life cycle that relocates nutrients from old or less efficient parts to newly developing or storage organs. Senescence-associated genes are activated by several transcription factors including ORE1. Phytochromes have been implicated to perceive environmental light conditions to inhibit senescence; however, a precise signaling mechanism remains elusive. Here I show that in Arabidopsis, two basic helix-loop-helix TFs, PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) and PIF5, promote senescence and that phytochrome B delays senescence by repressing PIFs activities. PIFs activate ORE1 expression by forming multiple coherent feed-forward loops involving EIN3 and Group A bZIP TFs. PIFs activate the expression of not only ORE1 but also EIN3 and bZIP genes including ABI5 and EEL through binding their promoters. The bZIPs and EIN3 also bind to ORE1 promoter and activate its expression. Such coherent feed-forward loops have been suggested to have the signaling process more robust, making PIF4 and PIF5 act as key regulators of senescence in response to the changing light conditions.

As an independent study, I also investigate the crosstalk of light and ethylene signaling pathways in young seedling. During the first several days after seed germination, Arabidopsis seedlings are highly sensitive to the environmental cues, and show a surprising flexibility in their developmental adaptation. I found that dark-grown seedling mutants of PIFs and EIN3 have a number of common phenotypes such as open apical hook/cotyledon and photobleaching. Based on the central transcription factor positions of PIFs and EIN3 in light and ethylene signaling, I hypothesized that the two different signaling pathways would share certain downstream target genes to regulate the similar phenotypes. Bioinformatic and genetic analysis revealed that HOOKLESS1 (HLS1) is a representative shared downstream gene which functions for the phenotypes. Th...