The Ca²⁺ -regulated protein kinase CIPK1 integrates plant responses to phosphate deficiency in Arabidopsis thaliana.

Phosphate (Pi) deficiency severely restricts plant growth and development, as Pi is an essential macronutrient. Calcium (Ca²⁺ ) is a ubiquitous second messenger in plants; calcineurin B-like proteins (CBL) and CBL-interacting protein kinases (CIPK) are signalling pathways that act as an important Ca²⁺ signalling network which integrates plants to fine tune the response to stress; however, whether CIPK are involved in Pi deficiency stress remains largely unknown. In this study, we carried out a reverse genetic strategy to screen T-DNA insertion mutants of CIPK isoforms under Pi deficiency in Arabidopsis thaliana. Then Pi content, transcription of phosphate starvation-induced (PSI) genes, acid phosphatase activity and hydrogen peroxide were determined in the wild-type (WT) and cipk1 mutant, respectively. The phenotype of CIPK1 complementation lines was analysed. The cipk1 mutant had a more sensitive phenotype, with lower root elongation and root length, and decreased Pi content compared with the WT under Pi deficiency. Moreover, CIPK1 mutation caused phosphate starvation-induced (PSI) genes to be significantly induced under Pi deficiency. Histological staining demonstrated that the cipk1 mutant had increased acid phosphatase activity and hydrogen peroxide concentration under Pi deficiency. By using the yeast two-hybrid system, we further demonstrated the interaction between CIPK1 and the WRKY transcription factors, WRKY6 and WRKY42. Overall, we demonstrate that CIPK1 is involved in the Pi deficiency signalling pathway in A. thaliana, revealing the important role of Ca²⁺ in the Pi nutrition signalling pathway, and potentially providing a theoretical foundation for molecular breeding of crops with better Pi utilization efficiency. © 2020 German Society for Plant Sciences and The Royal Botanical Society of the Netherlands.

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