The chloroplast-localised metalloprotease EGY1 and Mg-dechelatase SGR1 control chloroplast development and senescence by modulating photosynthetic functions

The chloroplast plays an essential role in the production of biochemical energy and metabolism, as well as in plant development, including senescence. We describe a mutant with unusual progression of senescence and identify it as a new allele of ethylene-dependent gravitropism-deficient and yellow-green 1 (egy1-5). EGY1 encodes a chloroplast-localized metalloprotease that regulates chlorophyll metabolism. The single egy1-5 mutant exhibits various defects in chloroplast function, including abnormal accumulation of photosynthetic complexes and defects in photosynthetic parameters, as well as altered responses to the chemicals lincomycin and methyl viologen. Furthermore, chlorophyll fluorescence could not be detected in egy1 guard cells. We performed a suppressor mutant screen in the egy1 background to identify novel regulators of senescence and chloroplast development. We found two suppressors that restored green leaves, and both suppressors had mutations in STAY-GREEN 1 (SGR1). SGR1 encodes the Mg2+ dechelatase that catalyses the initial step in chlorophyll breakdown. The defects of the egy1 mutant were either fully or partially suppressed in egy1 sgr1. This allowed us to isolate the function of EGY1, independent of the pleiotropic effects of chlorophyll degradation. Our findings suggest that chlorophyll homeostasis regulates the overall development and function of the chloroplast as well as plant senescence.