Rice Genetics IV - IRRI books - International Rice Research Institute

(Hudspeth et al 1992). Not only incorrect initiation and termination of transcriptionbut also incorrect splicing could occur when genes from monocots are introducedinto dicots (see Goodall and Filipowicz 1991). Thus, phylogenetic distance may hamperthe expression of genes from C 4 plants in the leaves of C 3 plants.Unlike the C 4 enzymes located in mesophyll cells of C 4 plants, those located inbundle sheath cells can be expressed at high levels in mesophyll cells of C 3 plants byintroducing a chimeric gene containing full-length cDNA for the C 4 enzyme fused tothe Cab promoter, which directs mesophyll cell-specific expression in C 3 plants(Sakamoto et al 1991). The expression of the maize C 4 -specific NADP-ME cDNAunder the control of the rice Cab promoter increased the activity of NADP-ME in riceleaves to 30- or 70-fold that of nontransformants (Takeuchi et al 2000, Tsuchida et al2000). The level of the NADP-ME protein also increased several percentage pointscompared with that of total leaf-soluble protein (Takeuchi et al 2000, Tsuchida et al2000). Such high-level expression was unique to cDNA for the C 4 -specific NADP-ME, and the expression of cDNA for the C 3 -specific isoform increased the activityonly several fold (Lipka et al 1999, Tsuchida et al 2000). Recently, expression of theintact gene for C 4 enzymes located in bundle sheath cells of C 4 plants in C 3 plants hasalso been addressed. When the intact gene for the mitochondrial AspAT of Panicummiliaceum, which is located in bundle sheath cells, was introduced into rice, highAspAT activity was detected in vascular tissues and bundle sheath cells of transgenicrice plants (M. Nomura and M. Matsuoka, unpublished observations). Similar resultswere observed with the PEP-CK gene from Zoysia japonica and β-glucuronidaseactivity under the control of the PEP-CK promoter, which was selectively detected invascular tissues and bundle sheath cells (M. Nomura and M. Matsuoka, unpublishedinformation). These results demonstrate that the C 4 -specific genes for the bundle sheathcell enzymes can retain their property of cell-specific expression even in a C 3 plant,rice, and therefore suggest that C 3 plants have a regulatory mechanism for gene expressionof the bundle sheath cell-specific C 4 genes at their correct site. This fact isinteresting in terms of the evolutionary aspect of C 4 plants, but also indicates that thestrategy to introduce intact C 4 -specific genes is not applicable to building the C 4 pathwaysolely in mesophyll cells of C 3 plants.Physiological effects of overproduction of C 4enzymes in C 3plantsTo examine the effects of high-level expression of maize PEPC on photosyntheticcharacteristics, O 2 sensitivity of photosynthetic CO 2 assimilation of transgenic riceplants was investigated by measuring CO 2 assimilation rates. O 2 inhibition of photosyntheticCO 2 assimilation was negatively correlated to the activity of PEPC intransgenic rice plants. This reduced O 2 inhibition was explained by direct CO 2 fixationby the maize PEPC in transgenic rice (Ku et al 1999). However, photosyntheticCO 2 assimilation of transgenic rice decreased with increasing PEPC activity at both O 2concentrations (21% and 2%). In addition, the slope of the regression line was steeperin 2% O 2 than in 21% O 2 . Thus, the observed reduction in O 2 inhibition might resultfrom decreased CO 2 assimilation in 2% O 2 in transgenic rice. Since the photosyntheticHigh-level expression of C 4photosynthetic genes in transgenic rice 445