Differential accumulation of the 10-, 16- and 23-kDa peripheral components of the water-splitting complex of photosystem II in mesophyll and bundle-sheath chloroplasts of the dicotyledonous C4 plant Flaveria trinervia (Spreng.) C. Mohr

Abstract

The differential expression of PSII genes was investigated in mesophyll and bundle-sheath cells of Flaveria trinervia, a dicotyledonous C4 plant of the NADP-malic enzyme type. A comprehensive immunoblot analysis showed that three extrinsic proteins of the water-splitting complex (10, 16 and 23 kDa) are selectively depleted in mature bundle-sheath chloroplasts. In contrast, the reaction-centre core remained virtually unaffected as inferred from the abundance of the 47-kDa chlorophyll-a-binding protein, the D1 and D2 polypeptides, cytochrome b559 and the 34-kDa polypeptide. The selective depletion of the 10-, 16- and 23-kDa polypeptides in bundle-sheath chloroplasts was paralleled by a diminished PSII capacity. On the basis of oxygen evolution in the presence of the artifical electron acceptor 2,5-dimethyl-p-benzoquinone, bundle-sheath chloroplasts maintained up to 23 % of the PSII capacity shown by mesophyll chloroplasts. However, the levels of the 10-, 16- and 23-kDa proteins and, concomitantly, PSII activity varied to some degree and appeared to be correlated with environmental factors caused by seasonal changes. The selective depletion of the three members of the water-splitting complex was not reflected at the transcript level. The corresponding mRNAs were detectable in considerable amounts in bundle-sheath cells, indicating that the depletion of these proteins is regulated by post-transcriptional events. These findings reinforce the view that the peripheral proteins of the water-splitting complex are a focal point for controlling PSII activity in bundle-sheath chloroplasts of both mono- and dicotyledonous C4 plants of the NADP-malic enzyme subtype.