Calcium/calmodulin modulates pollen germination and pollen tube growth and self-incompatibility response in Chinese cabbage (Brassica rapa L.)

Abstract

Self-incompatibility (SI) is a genetic mechanism to promote outcrossing and prevent self-fertilization in plants. Brassicaceae crops have a typical SI response that exhibits the inhibition of incompatible pollen germination and pollen tube growth. Calcium (Ca2+) is a necessary regulator of pollen germination and pollen tube growth. However, the signaling processes by which Ca2+ regulates the SI response are not clear in Chinese cabbage. In this study, exogenous CaCl2 and EGTA spraying was performed to alter the Ca2+ level of the stigma and detect the correlation between Ca2+ and SI response in Chinese cabbage. The decrease in Ca2+ levels in the stigma caused by EGTA treatment could promote pollen germination and break the SI response. Moreover, transmission electron microscopy showed that the accumulation and distribution of cytoplasmic Ca2+ in the stigma were related to the occurrence of the SI response. Additionally, eight BrCaM family genes were identified in Brassica rapa by genome-wide analysis. Expression analysis suggested that the expression of several BrCaM genes was coincident with Ca2+ level alterations and the SI response. These results facilitate understanding the putative roles of BrCaM genes in regulating pollen germination and pollen tube growth and provide valuable references for uncovering the molecular regulatory mechanism of the SI response in Brassicaceae crops.