The promoter for C4-type mitochondrial aspartate aminotransferase does not direct bundle sheath-specific expression in transgenic rice plants

For NAD-malic enzyme (NAD-ME)-type C4 photosynthesis, two types of aspartate aminotransferase (AAT) are involved. We examined the expression pattern of the Panicum miliaceum mitochondrial Aat gene (PmAat) and P. miliaceum cytosolic Aat gene (PcAat) in transgenic rice plants, which were specifically expressed in bundle sheath cells (BSCs) and mesophyll cells (MCs), respectively. Expression of a beta-glucuronidase (GUS) reporter gene under the control of the PcAat promoter was regulated in an organ-preferential and light-dependent manner in the transgenic rice plants. However, the PmAat promoter drove the GUS expression in all organs we tested without light dependency, and this non-preferential expression pattern was also observed in transgenic rice with introduction of the intact PmAat gene. The expression patterns of the rice counterpart Aat genes to PmAat or PcAat showed that the rice mitochondrial Aat (RmAat1) gene was expressed in all organs tested in a light-independent manner, while expression of the rice cytosolic Aat (RcAat1) gene showed an organ-preferential and light-dependent pattern. Taking these results together, we can generalize that the regulatory system of BSC-specific or light-dependent expression of mitochondrial Aat is not shared between P. miliaceum (C4) and rice (C3) and that the expression of the C4 genes introduced into rice mimics that of their counterpart genes in rice.