The JA-responsive MYC2-BADH-like transcriptional regulatory module in Poncirus trifoliata contributes to cold tolerance by modulation of glycine betaine biosynthesis.

Glycine betaine (GB) is known to accumulate in plants exposed to cold, but the underlying molecular mechanisms and associated regulatory network remain unclear. Here, we demonstrated that PtrMYC2 of Poncirus trifoliata integrates the jasmonic acid (JA) signal to modulate cold-induced GB accumulation by directly regulating PtrBADH-l, a betaine aldehyde dehydrogenase (BADH)-like gene. PtrBADH-l was identified based on transcriptome and expression analysis in P. trifoliata. Overexpression and VIGS (virus-induced gene silencing)-mediated knockdown showed that PtrBADH-l plays a positive role in cold tolerance and GB synthesis. Yeast one-hybrid library screening using PtrBADH-l promoter as baits unraveled PtrMYC2 as an interacting candidate. PtrMYC2 was confirmed to directly bind to two G-box cis-acting elements within PtrBADH-l promoter and acts as a transcriptional activator. In addition, PtrMYC2 functions positively in cold tolerance through modulation of GB synthesis by regulating PtrBADH-l expression. Interestingly, we found that GB accumulation under cold stress was JA-dependent and that PtrMYC2 orchestrates JA-mediated PtrBADH-l upregulation and GB accumulation. This study sheds new light on the roles of MYC2 homolog in modulating GB synthesis. In particular, we propose a transcriptional regulatory module PtrMYC2-PtrBADH-l to advance the understanding of molecular mechanisms underlying the GB accumulation under cold stress.