Differential biogenesis of photosystem‐II in mesophyll and bundle‐sheath cells of ‘malic’ enzyme NADP+‐type C4 plants

Abstract

We have investigated the photosystem‐II organization in differentiating‐bundle‐sheath cells of the three malate dehydrogenase (oxaloacetate decarboxylating) (NADP⁺)‐type C₄ species maize, Sorghum and Pennisetum. Using a set of nine different antisera raised against individual subunits of photosystem‐II, we demonstrate that photosystem‐II components constitute a substantial part of the thylakoid membranes of young bundle‐sheath chloroplasts. The abundance of subunits of the photosystem‐II core, i.e. the 47‐and 43‐kDa chlorophyll‐a‐binding proteins, polypeptides D1 and D2, cytochrome b₅₅₉, and the 34‐kDa polypeptide, varies with the developmental state of the plant. However, the levels of the 23‐kDa, 16‐kDa and 10‐kDa extrinsic polypeptides of the water‐oxidation complex are drastically reduced in bundle‐sheath chloroplasts of all three species analyzed, regardless of their state of differentiation. The reduction in protein abundance is also reflected at the transcript level: only traces of the nuclear‐encoded mRNAs are found in differentiating bundle‐sheath cells of Sorghum, suggesting that the transcription of these genes has been switched off. Our data are compatible with the idea that the water‐oxidation complex is a prime site for initiating or maintaining the process leading to photosystem‐II depletion during differentiation of bundle‐sheath cells.