Balance of chlorophyll synthesis and thylakoid protein quality control influences chloroplast development in <i>var2</i> mutants

Filamentous temperature-sensitive H (FtsH) is a major protease for thylakoid protein quality control in photosynthetic organisms. Mutations of the AtFtsH2 subunit in Arabidopsis result in yellow variegated2 (var2) mutants. Genetic screening of ENHANCER OF VARIEGATION (EVR) loci provides new insights into the role of FtsH in chloroplast development beyond the degradation of photodamaged D1 protein. Here, using Arabidopsis, we uncover a novel function of EVR3, previously reported as ETHYLENE-DEPENDENT GRAVITROPISM-DEFICIENT AND YELLOW-GREEN1 (EGY1), in regulating chlorophyll biosynthesis by association with the H subunit of magnesium chelatase (CHLH). Additionally, we identify a new var2 enhancer mutant, evr4-1, caused by a missense mutation in CHLH. The evr4-1 mutant shows a significant decline in accumulation of light-harvesting complexes rather than in photosystem II core proteins, while evr3 evr4 double mutants exhibit synthetic lethal phenotypes accompanied by a drastic reduction in the accumulation of chlorophyll and light-harvesting complexes. Furthermore, disruption of the thylakoid protein sorting pathway mediated by the chloroplast Signal Recognition Particle 54 kDa protein mitigates the chloroplast development defect in var2-4 evr4-1. Our findings underscore the critical role of thylakoid FtsH for thylakoid protein quality control when chlorophyll biosynthesis is disrupted.