Evolution and functional characterization of Populus salt stress-responsive calcineurin B-like protein-interacting protein kinases.

Identification of salt-responsive calcineurin B-like protein-interacting protein kinases (CIPKs) in Populus. Calcineurin B-like protein-interacting protein kinases (CIPKs) play vital roles in plant growth and abiotic stress responses. Currently, the regulatory mechanisms underlying these processes mediated by CIPK proteins are not completely understood in woody species. This study provided the first systematic analysis of 31 Populus CIPK genes and investigated their evolutionary relationships, gene structures, motif compositions, and salt stress responses. A total of 11 pairs of paralogous PtCIPK genes were identified, of which three pairs may be resulted from whole genome duplication, and two pairs that may be created by tandem duplications. RT-qPCR analysis revealed that 93.5% (29/31) genes showed altered expression levels in roots after salt treatment. Ectopic expression of PdCIPK21 or PdCIPK31 in Arabidopsis resulted in significant increases of seed germination, root elongation and fresh weight under salt stress conditions. Cytological observation revealed that PdCIPK21/31 overexpression lines showed increased number, lumen area and cell wall thickness of xylem vessels, and higher lignin content in stems compared with the wild type, with decreased sensitivity to long-term salt stress treatment. Our results suggest that PdCIPK21/31 serve as candidate genes for improving wood production and enhancing salt tolerance of tree species.