Abstract
GRAS, termed after gibberellic acid insensitive (GAI), RGA (repressor of GA1), and SCR (scarecrow), is a plant-specific transcription factor crucial for plant development and stress response. However, understanding of the functions played by the GRAS members and their target genes in citrus is limited. In this study, we identified a cold stress-responsive GRAS gene from Poncirus trifoliata, designated as PtrPAT1, by yeast one-hybrid library screening using the promoter of PtrBADH-l, a betaine aldehyde dehydrogenase (BADH)-like gene. PtrPAT1, belonging to the PAT1 subfamily, was localized in the nucleus and plasma membrane, exhibited transactivation activity and showed a remarkable upregulation under cold stress. Overexpression of PtrPAT1 elevated BADH activity, increased glycine betaine (GB) accumulation, and conferred enhanced cold tolerance in transgenic tobacco plants compared with wild type, while downregulation in trifoliate orange by virus-induced gene silencing (VIGS) resulted in opposite trends. Furthermore, the activities of two antioxidant enzymes, including peroxidase (POD) and superoxide dismutase (SOD), were significantly increased in the overexpression plants, but remarkably decreased in the VIGS line, consistent with accumulation patterns of the reactive oxygen species (ROSs). PtrPAT1 was demonstrated to interact with and activate the PtrBADH-l promoter through the putative PAT1-binding motif with the core sequence of TTTCATGT, indicating that PtrBADH-l is a target gene of PtrPAT1. Taken together, these results demonstrate that PtrPAT1 positively affects cold tolerance through the regulation of GB biosynthesis by modulating PtrBADH-l expression.