Chlorophyll Synthesis and Degradation Associated with Heat-Induced Leaf Senescence.

Heat stress is a major abiotic stress for the cool-season turfgrass species creeping bentgrass (Agostis stolonifera). Elevated temperatures during summer months often result in premature senescence in cool-season turfgrasses. One of the important symptoms of heat-induced premature leaf senescence is chlorosis of leaf tissue. This loss of chlorophyll not only causes a decrease in visual aesthetics but represents a perturbation of photosynthesis and energy metabolism. However, the mechanisms responsible for the decrease in chlorophyll levels which results in leaf yellowing are not well understood. The expression and activity of a number of key enzymes involved in chlorophyll metabolism were analyzed to gain insight into the regulation of chlorophyll loss during heat stress. Chlorophyll biosynthesis enzymes included porphobilinogen deaminase the enzyme responsible for generating tetrapyrolles for chlorophyll synthesis, magnesium-chelatase which is responsible for the insertion of magnesium-ions, and protochlorophyllide oxidoreductase a light-dependent enzyme which represents one of the final steps in chlorophyll production were investigated. Additionally chlorophyll degradation enzymes including chlorophyllase and pheophytinase were explored. Heat tolerant and sensitive plants differing in level of chlorophyll loss during elevated temperatures were compared using qPCR to explore gene expression levels of key enzymes, as well as enzyme activity assays comparing reaction rates of chlorophyll synthesis and degradation enzymes. Understanding how changes in chlorophyll synthesis and degradation enzyme are altered during heat stress will allow for a better understanding into mechanisms of chlorophyll loss and how to prevent it.