Differential expression pattern of C₄ bundle sheath expression genes in rice, a C₃ plant

NADP-malic enzyme (NADP-ME) and phosphoenolpyruvate carboxykinase (PCK) are specifically expressed in bundle sheath cells (BSCs) in NADP-ME-type and PCK-type C₄ plants, respectively. Unlike the high activities of these enzymes in the green leaves of C₄ plants, their low activities have been detected in the leaves of C₃ plants. In order to elucidate the differences in the gene expression system between C₃ and C ₄ plants, we have produced chimeric constructs with the β-glucuronidase (GUS) reporter gene under the control of the maize NADP-Me (ZmMe) or Zoysia japonica Pck (ZjPck) promoter and introduced these constructs into rice. In leaves of transgenic rice, the ZmMe promoter directed GUS expression not only in mesophyll cells (MCs) but also in BSCs and vascular cells, whereas the ZjPck promoter directed GUS expression only in BSCs and vascular cells. Neither the ZjPck nor ZmMe promoters induced GUS expression due to light. In rice leaves, the endogenous NADP-Me (OsMe1) was expressed in MCs, BSCs and vascular cells, whereas the rice Pck (OsPck1) was expressed only in BSCs and vascular cells. Taken together, the results obtained from transgenic rice demonstrate that the expression pattern of ZmMe or ZjPck in transgenic rice was reflected by that of its counterpart gene in rice.