Improved short-term drought response of transgenic rice over-expressing maize C phosphoenolpyruvate carboxylase via calcium signal cascade.

To understand the link between long-term drought tolerance and short-term drought responses in plants, transgenic rice (Oryza sativa L.) plants over-expressing the maize Cpepc gene encoding phosphoenolpyruvate carboxylase (PC) and wild-type (WT) rice plants were subjected to PEG 6000 treatments to simulate drought stress. Compared with WT, PC had the higher survival rate and net photosynthetic rate after 16days of drought treatment, and had higher relative water content in leaves after 2h of drought treatment as well, conferring drought tolerance. WT accumulated higher amounts of malondialdehyde, superoxide radicals, and HO than PC under the 2-h PEG 6000 treatment, indicating greater damages in WT. Results from pretreatments with a Ca chelator and/or antagonist showed that the regulation of the early drought response in PC was Ca-dependent. The NO and HO levels in PC lines were also up-regulated via Ca signals, indicating that Ca in PC lines also reacted upstream of NO and HO. 2-h drought treatment increased the transcripts of CPK9 and CPK4 in PC via positive up-regulation of Ca. The transcripts of NAC6 [NACs (NAM, ATAF1, ATAF2, and CUC2)] and bZIP60 (basic leucine zipper, bZIP) were up-regulated, but those of DREB2B (dehydration-responsive element-binding protein, DREB) were down-regulated, both via Ca signals in PC. PEPC activity, expressions of C-pepc, and the antioxidant enzyme activities in PC lines were up-regulated via Ca. These results indicated that Ca signals in PC lines can up-regulate the NAC6 and bZIP60 and the downstream targets for early drought responses, conferring drought tolerance for the long term.