The CtrCBL1/CtrCIPK6 Complex of Citrus Phosphorylates CtrBBX32 to Regulate CtrSTP1-Mediated Sugar Accumulation and Cold Tolerance.

Sugar accumulation is a crucial adaptive strategy for plant cold tolerance, yet its regulatory mechanisms in the roots under cold conditions remain largely unclear. Here, a cold-inducible sugar transporter protein (CtrSTP1), which is highly expressed in the roots of trifoliate orange (Citrus trifoliata L.), is identified. CtrSTP1 functions positively in cold tolerance by facilitating sugar accumulation in the roots. Two transcription factors, CtrBBX32 and CtrZAT10, bind to the CtrSTP1 promoter, with CtrBBX32 acting as transcriptional repressor and CtrZAT10 as activator. CtrBBX32 also suppresses CtrZAT10 expression. Accordingly, CtrBBX32 and CtrZAT10 act as negative and positive regulators, respectively, of cold tolerance by modulating CtrSTP1 expression. Moreover, CtrCBL1 and CtrCIPK6 form a complex to phosphorylate CtrBBX32 and promote its protein degradation, leading to relief of CtrBBX32-mediated repression on CtrZAT10 and CtrSTP1. Further transgenic evidence confirms that CtrCIPK6 plays a positive role in cold tolerance through modulating the sugar pathway. Taken together, these findings uncover a novel regulatory module composed of CtrCBL1/CtrCIPK6-CtrBBX32 that controls cold-induced sugar accumulation by directly or indirectly (via CtrZAT10) modulating the expression of the root-localized CtrSTP1. The work provides valuable knowledge to advance the understanding of sugar transport and homeostasis in the roots when plants are exposed to adverse environments.