Effect of regulating ATP on improving photosynthetic rate of transgenic rice with overexpressing maize C₄ pepc under drought stress

C₄-phosphoenolpyruvate carboxylase (PEPC) gene (C₄-pepc) plays a key role in C₄ photosynthesis by catalyzing initial fixation of CO₂ in C₄ plants. To determine whether adenosine triphosphate (ATP) limits photosynthetic rate of transgenic rice plant with over-expressing maize C4-pepc, the second upper leaves of non-transgenic rice (WT) and transgenic rice with over-expressing maizeC₄-pepc (PC) were sprayed with 2 mmol·L^-1 NaHSO₃, 100 umol·L^-1 3-(3 ,4 -dichlorophenyl)-1,1-dimethylurea (DCMU) and 10 umol·L^-1 oligomycin at 5 6 leaf blade seedling stage under 20% (m/v) polyethylene glycol 6000 (PEG-6000) treatment. Then the net photosynthetic rate (P_n), stomatal conductance (G_s), intercellular CO₂ concentration (C_i), PEPC activity, ATP content and ΦPS? of PC and WT rice plants measured in the next morning. The results showed that 2 mmol·L^-1 NaHSO₃ enhanced P_n of the upper leaves of PC and WT rice seedlings without PEG-6000 treatment. However, the 100 umol·L^-1 DCMU and 10 umol·L^-1 oligomycin decreased Pn of the upper leaves of PC and WT rice seedlings without PEG-6000 treatment. Treatment with 2 mmol·L^-1 NaHSO₃ increased Gs and C_i of WT rice leaves but decreased G_s and C_i of PC rice leaves. While treatment with 100 umol·L^-1 DCMU increased C_i of PC and WT rice leaves, it decreased Gs of PC and WT rice leaves. PEG-6000 treatment decreased Pn of the upper leaves in PC and WT plants under different treatments. However, PEG-6000 treatment combined with 2 mmol·L^-1 NaHSO₃ solution spray retarded the decrease in P_n. Then PEG-6000 treatment combined with DCMU and oligomycin sprays increased the rate of P_n decline. After 8 hours of 20% PEG-6000 treatment combined with different solutions, G_s of PC rice leaves remained unchanged while Pn of PC rice leaves changed obviously. The results further showed that ATP content, PEPC activity and ΦPS? content in rice leaves changed obviously under different treatments. While DCMU treatment accelerated the decrease in ATP content, PEPC activity and ΦPS? of PC and WT leaves, NaHSO₃ retarded the rates of decrease in these elements. Although oligomycin decreased ATP content in rice leaves, it had no effect on ΦPSII in rice leaves. It was therefore concluded that PC maintained high levels of PS? activity and P_n stability by inducing more ATP in rice leaves compared with WT plants under drought stress.