Monogalactosyldiacylglycerol Facilitates Synthesis of Photoactive Protochlorophyllide in Etioplasts

Abstract

Cotyledon cells of dark-germinated angiosperms develop etioplasts that are plastids containing unique internal membranes called prolamellar bodies (PLBs). Protochlorophyllide (Pchlide), a precursor of chlorophyll, accumulates in PLBs and forms a ternary complex with NADPH and light-dependent NADPH:protochlorophyllide oxidoreductase (LPOR), which allows for the rapid formation of chlorophyll after illumination while avoiding photodamage. PLBs are 3D lattice structures formed by the lipid bilayer rich in monogalactosyldiacylglycerol (MGDG). Although MGDG was found to be required for the formation and function of the thylakoid membrane in chloroplasts in various plants, the roles of MGDG in PLB formation and etioplast development are largely unknown. To analyze the roles of MGDG in etioplast development, we suppressed MGD1 encoding the major isoform of MGDG synthase by using a dexamethasone-inducible artificial microRNA in etiolated Arabidopsis (Arabidopsis thaliana) seedlings. Strong MGD1 suppression caused a 36% loss of MGDG in etiolated seedlings, together with a 41% decrease in total Pchlide content. The loss of MGDG perturbed etioplast membrane structures and impaired the formation of the photoactive Pchlide-LPOR-NADPH complex and its oligomerization, without affecting LPOR accumulation. The MGD1 suppression also impaired the formation of Pchlide from protoporphyrin IX via multiple enzymatic reactions in etioplast membranes, which suggests that MGDG is required for the membrane-associated processes in the Pchlide biosynthesis pathway. Suppressing MGD1 at several germination stages revealed that MGDG biosynthesis at an early germination stage is particularly important for Pchlide accumulation. MGDG biosynthesis may provide a lipid matrix for Pchlide biosynthesis and the formation of Pchlide-LPOR complexes as an initial step of etioplast development.