STAYGREEN-mediated chlorophyll a catabolism is critical for photosystem stability upon heat stress in ryegrass

Chlorophyll (Chl) loss is one of the most visible symptoms of heat-induced leaf senescence, especially for cool-season grass species. Suppression of the Chl a Me-dechelatase gene, SGR (also named as nye1), blocked the degradation of Chl a and resulted in the stay-green trait during leaf senescence. However, effect of Chl a catabolism on plant tolerance to long-term moderate heat stress (35-40{degrees}C) remains unclear. In this study, we suppressed the expression of Chl a catabolic gene, LpSGR, in both constitutive and inducible manners in perennial ryegrass. Constitutive suppression of LpSGR aggravated heat stress-induced chloroplast structure and photosystem damages, disrupted energy utilization/dissipation during photosynthesis, activated ROS generation with weakened ROS-scavenging enzyme activities. Transcriptome comparison among wildtype (WT) and transgenic RNAi plants under either the optimum or high temperature conditions also emphasized the effect of Chl a catabolism on expression of genes encoding photosynthesis system, ROS-generation and scavenging system, and heat shock transcription factors. Furthermore, making use of a modified ethanol-inducible system, we generated stable transgenic perennial ryegrass to suppress LpSGR in an inducible manner. Without ethanol induction, these transgenic lines exhibited the same growth and heat tolerance traits to WT, while under the induction of ethanol spray, the transgenic lines also showed compromised heat tolerance. Taken together, our data suggest that Chl a catabolism is critical for energy dissipation and electron transfer in photosynthesis, ROS-balancing and chloroplast membrane system stability upon long-term moderate heat stress.