PRJNA418881
Thylakoid-bound polysomes and a dynamin-related protein, FZL, mediate critical stages of the linear chloroplast biogenesis program in greening Arabidopsis cotyledons
Biogenesis of the complex 3D architecture of plant thylakoids remains an unsolved problem. We have analyzed this process in chloroplasts of germinating Arabidopsis cotyledons of wild type and two chloroplast mutants, rh3-4 and fzl plants by electron tomography, immunoelectron microscopy, immunoblot, and transcriptomic analyses of chloroplast proteins. Our study has identified a linear developmental sequence with five assembly stages: (1) tubulo-vesicular pro-thylakoids (24 HAI; hours after imbibition); (2) sheet-like pre-granal thylakoids that develop from the pro-thylakoids (36 HAI); (3) proliferation of pro-grana stacks with wide tubular connections to the originating pre-grana thylakoids (60 HAI); (4) structural differentiation of pro-grana stacks and expanded stroma thylakoids (84 HAI); (5) conversion of the pro-grana stacks into mature grana stacks (120 HAI). Development of the planar pre-granal thylakoids and of the pro-grana membrane stacks coincides with the appearance of thylakoid-bound polysomes and the assembly of photosystem II (PSII) complexes (36 HAI). This is followed at 60 HAI by the appearance of ATP synthase, cytochrome b6f and light harvesting complex II proteins. PSI proteins and the curvature-inducing CURT1A protein are the last to appear at 84 HAI. In rh3-4 plastids, in which ribosome biogenesis is delayed, pro-thylakoids accumulate until stromal ribosomes are produced, and pro-grana stacks develop after polysomes bind to the thylakoid membrane. Pro-grana stacks of fzl mutant form discrete membrane compartments with a spiral configuration instead of an organelle-wide membrane network, suggesting that FZL is involved in interconnecting thylakoid elements