Antagonistic Roles of PhyA and PhyB in Far-Red Light-Dependent Leaf Senescence in Arabidopsis thaliana

Abstract

Leaf senescence is regulated by diverse developmental and environmental factors to maximize plant fitness. The red to far-red light ratio (R:FR) detected by plant phytochromes is reduced under vegetation shade, thus initiating leaf senescence. However, the role of phytochromes in promoting leaf senescence under FR-enriched conditions is not fully understood. In this study, we investigated the role of phyA and phyB in regulating leaf senescence under FR in Arabidopsis thaliana (Arabidopsis). FR enrichment and intermittent FR pulses promoted the senescence of Arabidopsis leaves. Additionally, phyA and phyB mutants showed enhanced and repressed senescence phenotypes in FR, respectively, indicating that phyA and phyB antagonistically regulate FR-dependent leaf senescence. Transcriptomic analysis using phyA and phyB mutants in FR identified differentially expressed genes (DEGs) involved in leaf senescence-related processes, such as responses to light, phytohormones, and temperature, photosynthesis, and defense, showing opposite expression patterns in phyA and phyB mutants. These contrasting expression profiles of DEGs support the antagonism between phyA and phyB in FR-dependent leaf senescence. Among the genes showing antagonistic regulation, we confirmed that the expression of WRKY6, which encodes a senescence-associated transcription factor, was negatively and positively regulated by phyA and phyB, respectively. The wrky6 mutant showed a repressed senescence phenotype compared with the wild type in FR, indicating that WRKY6 plays a positive role in FR-dependent leaf senescence. Our results imply that antagonism between phyA and phyB is involved in fine-tuning leaf senescence under varying FR conditions in Arabidopsis.

Regulation of Photomorphogenic Development by Plant Phytochromes

“…The first part is the light absorption and conformational changes, the second part is the interaction of phytochromes with various downstream components and signaling initiation, the third part is the regulation of signaling via ubiquitin/26S proteasome-mediated proteolysis and signal integration, and the final part is the regulatory gene expression of light-responsive genes. In genome-wide expression data, approximately 2,500 genes which is~10% of Arabidopsis genome were regulated by phytochrome under prolonged light exposure, where~80% of the total light-responsive genes were induced, with~20% being repressed [98]. Most of these genes are involved in the plant transition from heterotrophic to autotrophic life, which includes photosynthesis, hormone pathways, and metabolic pathways.…”

“…When phytochromes sense FR light, the signal is transferred to the leaf senescence pathway via WRKY6 that binds directly to the promoter of SIRK (senescence-induced receptor-like protein kinase) and induces the gene expression. Thus, WRKY6 acts as a positive regulator for leaf senescence, and regulates dark-induced senescence by upregulating SAG expression [98]. Another report indicates a possible role of HY5 in the leaf senescence pathway by demonstrating that WRKY6 and SAG29 are the putative targets of HY5 [99].…”

“…This helps maintain a positive carbon balance and represses leaf senescence under prolonged shade conditions [96]. Furthermore, a recent study reported that phyA and phyB work antagonistically to regulate FR enhanced senescence by WRKY6 [97,98]. Under FR light, phyA inhibited leaf senescence by repressing SAG gene expression, whereas phyB promoted leaf senescence.…”

A WRKY transcription factor, TaWRKY42-B, facilitates initiation of leaf senescence by promoting jasmonic acid biosynthesis

“…Among those senescence-related WRKYs, AtWRKY53 has been intensively studied for its critical role in promoting leaf senescence initiation [40][41][42]. AtWRKY6 plays a role in GA-and light-related senescence processes [43][44][45]. AtWRKY57 coordinates JA and Aux signaling for fine tuning leaf senescence [46].…”

Regulation of Early Plant Development by Red and Blue Light: A Comparative Analysis Between Arabidopsis thaliana and Solanum lycopersicum

“…However, most of these studies have focused on one crop species, one wavelength, or only on one developmental trait. Moreover, photoreceptor function and downstream pathways have been studied extensively in Arabidopsis thaliana (Arabidopsis) (Wang et al, 2016;Lim et al, 2018;Schumacher et al, 2018), but only a small fraction of these pathways have been investigated in commercial crops. In contrast, many lightinduced physiological traits have been studied in different crops (Kaiser et al, 2019;Pennisi et al, 2019;Song et al, 2019) but not in Arabidopsis.…”