Genome-Wide Identification of BrCAX Genes and Functional Analysis of BrCAX1 Involved in Ca(2+) Transport and Ca(2+) Deficiency-Induced Tip-Burn in Chinese Cabbage (Brassica rapa L. ssp. pekinensis)

Calcium (Ca(2+)) plays essential roles in plant growth and development. Ca(2+) deficiency causes a physiological disorder of tip-burn in Brassiceae crops and is involved in the regulation of cellular Ca(2+) homeostasis. Although the functions of Ca(2+)/H(+) exchanger antiporters (CAXs) in mediating transmembrane transport of Ca(2+) have been extensively characterized in multiple plant species, the potential roles of BrCAX genes remain unclear in Chinese cabbage. In this study, eight genes of the BrCAX family were genome-widely identified in Chinese cabbage. These BrCAX proteins contained conserved Na_Ca_ex domain and belonged to five members of the CAX family. Molecular evolutionary analysis and sequence alignment revealed the evolutionary conservation of BrCAX family genes. Expression profiling demonstrated that eight BrCAX genes exhibited differential expression in different tissues and under heat stress. Furthermore, Ca(2+) deficiency treatment induced the typical symptoms of tip-burn in Chinese cabbage seedlings and a significant decrease in total Ca(2+) content in both roots and leaves. The expression changes in BrCAX genes were related to the response to Ca(2+) deficiency-induced tip-burn of Chinese cabbage. Specially, BrCAX1-1 and BrCAX1-2 genes were highly expressed gene members of the BrCAX family in the leaves and were significantly differentially expressed under Ca(2+) deficiency stress. Moreover, overexpression of BrCAX1-1 and BrCAX1-2 genes in yeast and Chinese cabbage cotyledons exhibited a higher Ca(2+) tolerance, indicating the Ca(2+) transport capacity of BrCAX1-1 and BrCAX1-2. In addition, suppression expression of BrCAX1-1 and BrCAX1-2 genes reduced cytosolic Ca(2+) levels in the root tips of Chinese cabbage. These results provide references for functional studies of BrCAX genes and to investigate the regulatory mechanisms underlying Ca(2+) deficiency disorder in Brassiceae vegetables.