Multifunctional protein involved in chlorophyll synthesis, plastid-to-nucleus retrograde signaling and abscisic acid (ABA) perception. In chlorophyll synthesis, catalyzes the insertion of magnesium ion into protoporphyrin IX to yield Mg-protoporphyrin IX. The reaction takes place in two steps, with an ATP-dependent activation followed by an ATP-dependent chelation step. In addition to its function in the Mg-chelatase enzyme, is required for the plastid-to-nucleus retrograde signaling. The plastid-to-nucleus signal plays an important role in the coordinated expression of both nuclear- and chloroplast-localized genes that encode photosynthesis-related proteins. Has a role in mediating ABA signaling in stomatal guard cells and during seed germination. Binds ABA and is a positive regulator of ABA signaling. ATP + H2O + Mg(2+) + protoporphyrin IX = ADP + 3 H(+) + Mg-protoporphyrin IX + phosphate Porphyrin-containing compound metabolism; chlorophyll biosynthesis. The magnesium chelatase complex is a heterotrimer consisting of subunits CHLI, CHLD, AND CHLH. Interacts with GUN4, WRKY18, WRKY40, WRKY60, CHLI1 and CHLD. Predominantly associated with the chloroplast envelope. Spans the chloroplast envelope and is its N- and C-termini are exposed to the cytosol (PubMed:20543028). A number of isoforms are produced. According to EST sequences. Widely expressed. By red, far-red and blue light. Down-regulated by white light. Pale-green phenotype with low survival rates during de-etiolation and severe embryonic lethality when homozygous. Chli1 and chli2 double mutants are albino. Plants silencing CHLH show significant ABA-insensitive phenotypes in seed germination, post-germination growth arrest by ABA and ABA-induced promotion of stomatal closure and inhibition of stomatal opening. In contrast, plants over-expressing ABAR display ABA-hypersensitive phenotypes (PubMed:17051210). Belongs to the Mg-chelatase subunit H family.