Membrane protein provision controls prothylakoid biogenesis in tobacco etioplasts.

The cytochrome b559 heterodimer is a conserved component of photosystem II whose physiological role in photosynthetic electron transfer is enigmatic. A particularly puzzling aspect of cytochrome b559 has been its presence in etiolated seedlings, where photosystem II is absent. Whether or not the cytochrome has a specific function in etioplasts is unknown. Here, we have attempted to address the function of cytochrome b559 by generating transplastomic tobacco (Nicotiana tabacum) plants that overexpress psbE and psbF, the plastid genes encoding the two cytochrome b559 apoproteins. We show that strong overaccumulation of the PsbE apoprotein can be achieved in etioplasts by suitable manipulations of the promoter and the translation signals, while the cytochrome b559 level is only moderately elevated. The surplus PsbE protein causes striking ultrastructural alterations in etioplasts; most notably, it causes a condensed prolamellar body and a massive proliferation of prothylakoids, with multiple membrane layers coiled into spiral-like structures. Analysis of plastid lipids revealed that increased PsbE biosynthesis strongly stimulated plastid lipid biosynthesis, suggesting that membrane protein abundance controls prothylakoid membrane biogenesis. Our data provide evidence for a structural role of PsbE in prolamellar body formation and prothylakoid biogenesis, and indicate that thylakoid membrane protein abundance regulates lipid biosynthesis in etioplasts.