Abstract
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 Transporter 2 (TST2) molecular module in cotton (Gossypium hirsutum) that regulates plant sugar homeostasis, in particular glucose homeostasis. GhCIPK6 is recruited to the tonoplast by GhCBL2 and interacts with the tonoplast-localized sugar transporter GhTST2. Overexpression of either GhCBL2, GhCIPK6, or GhTST2 was sufficient to promote sugar accumulation in transgenic cotton, whereas RNAi-mediated knock-down of GhCIPK6 expression or CRISPR-Cas9 mediated knock-out of GhTST2 resulted in significantly decreased glucose content. Moreover, mutation of GhCBL2 or GhTST2 in GhCIPK6-overexpressing cotton reinstated sugar contents comparable to wild-type plants. Heterologous expression of GhCIPK6 in Arabidopsis also promoted glucose accumulation, whereas mutation of AtTST1/2 in GhCIPK6-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.