Characterization of the expression of the photosystem II-oxygen evolving complex in C4 species of flaveria

Abstract

We have determined the levels of photosystem II activity and polypeptide abundance in whole leaves and isolated bundle sheath and mesophyll cells of C4, "C4-like," and C3 species of the genus Flaveria (Asteraceae). On a chlorophyll basis, the whole leaf levels of the D1, D2, and 34-kilodalton photosystem II polypeptides were similar for each Flaveria species. Photosystem II activity varied twofold, but was not correlated with photosynthetic type (C3 or C4). The bundle sheath cell levels of photosystem II activity and associated polypeptides in C4-like and C4 Flaveria species were approximately one-half those observed in mesophyll cells but equivalent to those in bundle sheath cells of the C3 species, Flaveria cronquistii. Analyses of the steady-state levels of transcripts encoding photosystem II polypeptides indicated that there were no differences in transcript abundance between mesophyll and bundle sheath cells of the C4 Flaveria species. This pattern was in contrast to the three- to tenfold higher levels of transcripts encoding photosystem II polypeptides in mesophyll versus bundle sheath cells of maize. It is apparent that the higher mesophyll cell to bundle sheath ratio of photosystem II polypeptides in C4- and C4-like species of Flaveria is the result of higher levels of photosystem II expression in mesophyll cells rather than lower levels of expression in bundle sheath cells.