Galactolipids Are Essential for Internal Membrane Transformation during Etioplast-to-Chloroplast Differentiation

Etioplasts developed in angiosperm cotyledon cells in darkness rapidly differentiate into chloroplasts with illumination. This process involves dynamic transformation of internal membrane structures from the prolamellar bodies (PLBs) and prothylakoids (PTs) in etioplasts to thylakoid membranes in chloroplasts. Although two galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are predominant lipid constituents of membranes in both etioplasts and chloroplasts, their roles in the structural and functional transformation of internal membranes during etioplast-to-chloroplast differentiation are unknown. We previously reported that a 36% loss of MGDG by an artificial microRNA targeting major MGDG synthase (amiR-MGD1) only slightly affected PLB structures but strongly impaired PT formation and protochlorophyllide biosynthesis. Meanwhile, strong DGDG deficiency in a DGDG synthase mutant (dgd1) disordered the PLB lattice structure in addition to impaired PT development and protochlorophyllide biosynthesis. In this study, thylakoid biogenesis after PLB disassembly with illumination was strongly perturbed by amiR-MGD1. The amiR-MGD1 expression impaired the accumulation of Chl and the major light-harvesting complex II protein (LHCB1), which may inhibit rapid transformation from disassembled PLBs to the thylakoid membrane. As did amiR-MGD1 expression, dgd1 mutation impaired the accumulation of Chl and LHCB1 during etioplast-to-chloroplast differentiation. Furthermore, unlike in amiR-MGD1 seedlings, in dgd1 seedlings, disassembly of PLBs after illumination was retarded. Because DGDG but not MGDG prefers to form the bilayer lipid phase in membranes, the MGDG-to-DGDG ratio may strongly affect the transformation of PLBs to the thylakoid membrane during etioplast-to-chloroplast differentiation.