Magnesium chelatase involved in chlorophyll a degradation in the chlorophyll-protein complexes of photosystem I (PSI) and photosystem II (PSII) (PubMed:26732493, PubMed:27604697). Contributes to the degradation of PSI and PSII in the thylakoid membranes (PubMed:26732493, PubMed:27604697). Required to trigger chlorophyll degradation during natural and dark-induced leaf senescence (Probable) (PubMed:26732493). Mediates chlorophyll degradation during embryo degreening (PubMed:24043799, PubMed:26732493, PubMed:28873256). Recombinant SGR2 possesses high dechelating activity against chlorophyll a, very low activity against chlorophyllide a, and no activity against chlorophyll b (PubMed:27604697). chlorophyll a + 2 H(+) = Mg(2+) + pheophytin a Interacts with the light harvesting complex II (LHCII) (PubMed:24719469). Interacts with the chlorophyll catabolic enzyme (CCE) RCCR (PubMed:26732493, PubMed:24719469). 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. Induced during natural and dark-induced leaf senescence. 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.