Drought tolerance of transgenic rice overexpressing maize C4-PEPC gene related to increased anthocyanin synthesis regulated by sucrose and calcium

In order to reveal the role of sucrose (Suc) in early drought response in plants, transgenic rice (Oryza sativa L.) plants overexpressing the maize (Zea mays L.) C4-phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) gene (C4-pepc) (PC) and their untransformed wild type (WT) were used under 12 % (m/v) polyethylene glycol 6000 to simulate drought conditions. The results showed that PC has higher relative water content, the increased Suc content, and anthocyanin accumulation than WT during PEG treatment. By spraying 1 % Suc and 1 % Suc non-metabolic analog, turanose, on these plants, Suc in PC leaves increased anthocyanin content and Ca2+ content. Further experiments using the Ca2+ chelator (EGTA), Ca2+ channel antagonist (ruthenium red), and abscisic acid inhibitor (nordihydroguaiaretic acid), showed that, in PC plants, Suc content is closely related to the expression of sucrose nonfermenting-1-related protein kinases 2 (SnRK2s) such as SAPK8, SAPK9, and SAPK10 via abscisic acid, and the SnRK3 such as SnRK3.1, SnRK 3.4, and SnRK3.21 via Ca2+ and calcineurin B-like as well. Furthermore, the target genes associated with anthocyanin synthesis phenylalanine ammonia lyase, chalcone isomerase, chalcone synthase, flavonoid-3-hydrogenase, flavonoid-3'-hydrogenase, dihydroflavonone reductase, and anthocyanin synthase, their regulated genes basic helix-loop-helix (bHLH) proteins OsB1 and OsB2, R2R3-MYB transcription factor OsC1, and some transcription factors (constitutively photomorphogenic 1, elongated hypocotyl 5, and purple acid phosphatase gene 2) in PC plants also increased via Suc and Ca2+ during the PEG treatment. Some Suc tranporter genes OsSUT1 and OsSUT5 in PC lines during PEG treatment further showed an enhancement for the function of the signal of Suc. Thereby, increasing anthocyanin biosynthesis via Suc and Ca2+ signaling cascade is one of the important mechanisms on drought tolerance in the PC lines.