Magnesium chelatase involved in chlorophyll a degradation in the chlorophyll-protein complexes of photosystem I (PSI) and photosystem II (PSII) (PubMed:27604697). Contributes to the degradation of PSI and PSII in the thylakoid membranes (PubMed:27604697). Required to trigger chlorophyll degradation during natural and dark-induced leaf senescence (Probable) (PubMed:17468209). Mediates chlorophyll degradation during embryo degreening (PubMed:24043799, PubMed:28873256). Recombinant SGR1 possesses high dechelating activity against chlorophyll a, very low activity against chlorophyllide a, and no activity against chlorophyll b (PubMed:27604697). Magnesium dechelation of chlorophyll a by SGR1 activates chlorophyll b degradation by inducing the expression of NYC1, an enzyme involved in chlorophyll b degradation (PubMed:29425814). chlorophyll a + 2 H(+) = Mg(2+) + pheophytin a Interacts with HCAR, the chlorophyll catabolic enzymes (CCEs) NYC1, PAO and RCCR, and the LHCII complex. Part of a SGR1-CCE-LHCII complex, which acts in chlorophyll breakdown. Expressed in roots, leaves, seeds, flowers, buds, petals, sepals and siliques. Constitutively expressed at low level during leaf development, but up-regulated during seed maturation and senescence, when the leaf color changes from green to yellow. Up-regulated by dark treatment (PubMed:17468209). Induced during natural and dark-induced leaf senescence (PubMed:24719469). RNAi-mediated knockout of the protein results in a stay-green leaf phenotype. No effect on seed degreening; probably due to redundancy with SGR1. Sgr1 and sgr2 double mutant has an embryo stay-green phenotype. Belongs to the staygreen family.