Rice Genetics IV - IRRI books - International Rice Research Institute

The effect of illumination on PPDK was investigated in rice and maize. PPDKactivity was much higher under illumination than in the dark (Fig. 5). However, largedifferences were observed between maize and rice in the extent of illumination. Thelight/dark activity ratios were 20 in maize, 6 in nontransformed rice, and 6–9 intransgenic rice. These findings indicate that endogenous rice PDRP can regulate theactivity of maize PPDK, although to a limited level, possibly because of the low levelof PDRP relative to PPDK in transgenic rice.Maize and rice both belong to Gramineae. Our recent study indicated that theintroduction of the intact gene from other C 4 gramineous plants could also lead tohigh-level expression of a C 4 enzyme in transgenic rice plants. The activity of thecytosolic form of AspAT in rice leaves was increased to reach 20-fold that ofnontransformants by introduction of the intact gene from Panicum miliaceum (M.Nomura and M. Matsuoka, unpublished observations). Panicum miliaceum is classifiedin the NAD-ME subtype of C 4 plants and maize in the NADP-ME subtype. Thus,the intact genes from C 4 gramineous plants, irrespective of the C 4 subtype, will probablylead to high-level expression of the C 4 enzymes in mesophyll cells of C 3 gramineousplants.This strategy, however, seems to have some limitation in that transgenes fromplants closely related phylogenetically have to be used to achieve high-level expressionof the C 4 enzyme in C 3 plants. The intact maize C 4 -specific PEPC gene was notexpressed at high levels in tobacco leaves because of incorrect transcription initiationPPDK activity (µmol mg –1 protein min –1 )0.80.60.40.20L D L D L D L DMaize Rice PD259 PD278Fig. 5. Light-dependent regulation of PPDK activity in leaves. Maizeand rice plants that had been grown in a growth chamber on theday/night cycle for 5 wk were illuminated for 8 h from the start ofthe daytime and then transferred to the dark for 10 h. The uppermostfully expanded leaves were harvested at the end of illumination(L) and after the subsequent dark incubation (D). The bars representstandard errors of three measurements. PD259 and PD278are transgenic rice lines.444 Matsuoka et al