Functional Characterization of Chinese Cabbage BrCML49 Gene in Pollen Germination and Pollen Tube Elongation and the Regulation of Ca²⁺-Mediated Self-Incompatibility Response

Self-incompatibility (SI) is a genetic mechanism that promotes out-breeding and maintains genetic diversity in plant species. The inhibition of pollen germination and pollen tube growth is one of the major characteristics of SI response. The involvement of Ca²⁺ signaling mediated by Ca²⁺ sensors in the SI response has been demonstrated to affect the germination of incompatible pollen and the normal elongation of pollen tubes. In this study, the calmodulin-like 49 (BrCML49) gene in Chinese cabbage (Brassica rapa L. ssp. pekinensis) was down-regulated expression during the SI process, indicating that BrCML49 may participate in the regulation of SI response. The transient suppression of BrCML49 gene induced the abnormal germination of pollen in vitro, which showed the putative positive roles of BrCML49 in regulating pollen germination. The inhibited germination of self- and cross-pollen in BrCML49-suppressed stigma was also observed, which revealed the activated SI response. After BrCML49 suppression, the increases in Ca²⁺ level in stigmatic papilla cells were consistent with the increases in ROS/H₂O₂ contents in the stigma, which may be essential for the occurrence of SI response. Furthermore, heterologous overexpression of BrCML49 in Arabidopsis validated the positive regulation of BrCML49 in pollen germination and pollen tube growth. In addition, BrCML49 overexpressed Arabidopsis exhibited an enhanced tolerance of pollen germination to high Ca²⁺ condition. These results will provide valuable information for understanding the roles of BrCML49 in regulating pollen germination and pollen tube growth during SI process in Chinese cabbage.