Transcriptional regulation of chlorophyll-catabolic genes associated with exogenous chemical effects and genotypic variations in heat-induced leaf senescence for perennial ryegrass

Abstract

Heat-induced leaf senescence is hallmarked with accelerated chlorophyll (Chl) catabolism. The involvement of Chl catabolic genes (CCGs) in heat-induced leaf senescence is not well characterized. The objectives of this study were to investigate transcriptional changes in CCGs associated with exogenous chemical effects and genotypic variations during heat-induced leaf senescence in perennial ryegrass (Lolium perenne L.), and to characterize its underlying mechanisms. Perennial ryegrass plants of ‘Buena Vista’ and ‘Pinnacle’ differing in heat tolerance were exposed to heat stress at 35/30 °C (day/night) and control condition at 25/20 °C (day/night) in growth chambers and foliar sprayed with the following chemicals: salicylic acid (SA), 6-benzylaminopurine (6-BA; a cytokinin), calcium dichloride (CaCl2), aminoethoxyvinylglycine (AVG; an ethylene biosynthesis inhibitor), sodium nitroprusside (SNP, a nitric oxide donor), and urea (N, nitrogen source). Higher Chl content, photochemical efficiency and membrane stability were found in ‘Buena Vista’ than ‘Pinnacle’ whereas lower expression levels of three CCGs (LpNYC1, LpNOL, and LpPPH) were detected in the former genotype under heat stress. Exogenous application of SA, 6-BA, CaCl2, AVG, SNP, and N suppressed heat-induced Chl decline and expression of CCGs in heat sensitive genotype ‘Pinnacle’ under heat stress. Heat-induced oxidative damage was alleviated by these compounds, as indicated by significantly lower reactive oxygen species (ROS) content and higher activities of ROS scavenging enzymes. The expression levels of CCGs were negatively correlated to ROS level, including O2− and H2O2, while positively correlated to activities of ROS scavenging enzymes, such as CAT and SOD. The current results suggest that the transcript levels of LpNYC1, LpNOL, and LpPPH were negatively correlated with heat-induced leaf senescence, and down-regulation of CCGs genes could suppress heat-induced leaf senescence and oxidative damages, conferring heat tolerance in perennial ryegrass.