The Calcium Sensor CBL2 and Its Interacting Kinase CIPK6 Are Involved in Plant Sugar Homeostasis via Interacting with Tonoplast Sugar Transporter TST2.

Calcineurin B-like protein (CBL) and CBL-interacting protein kinase (CIPK)-mediated calcium signaling has been widely reported to function in plant development and various stress responses, particularly in ion homeostasis. Sugars are the most important primary metabolites, and thus sugar homeostasis requires precise regulation. Here, we describe a CBL2-CIPK6-Tonoplast-Localized Sugar Transporter2 (TST2) molecular module in cotton () that regulates plant sugar homeostasis, in particular Glc homeostasis. GhCIPK6 is recruited to the tonoplast by GhCBL2 and interacts with the tonoplast-localized sugar transporter GhTST2. Overexpression of either , , or was sufficient to promote sugar accumulation in transgenic cotton, whereas RNAi-mediated knockdown of expression or CRISPR-Cas9-mediated knockout of resulted in significantly decreased Glc content. Moreover, mutation of or in -overexpressing cotton reinstated sugar contents comparable to wild-type plants. Heterologous expression of in Arabidopsis () also promoted Glc accumulation, whereas mutation of in -overexpressing Arabidopsis similarly reinstated wild-type sugar contents, thus indicating conservation of CBL2-CIPK6-TST2-mediated sugar homeostasis among different plant species. Our characterization of the molecular players behind plant sugar homeostasis may be exploited to improve sugar contents and abiotic stress resistance in plants.