Genome-wide identification of the RALF gene family in Camellia oleifera Abel. and the potentiality of CoRALF50 in pollen tube growth.

Rapid alkalinization factors (RALFs) play meaningful roles in the pollination and fertilization. However, the identification and study of Camellia RALFs is lacking, particularly the function of the RALF family in regulating self-incompatibility pollen tube growth. Herein, we identified 50 RALF genes from Camellia oleifera genome, and classified them into three groups: Clades Ⅰ, Ⅱ, and Ⅲ, with 11, 12, and 27 members being included, respectively. CoRALFs were unevenly distributed in chromosomes, and the segmental duplication events mainly facilitated their expansion. Gene structure and conserved motif analyses indicated that they were highly conserved. Cis-element analysis revealed that many light responsive elements, stress responsive elements and phytohormone responsive elements were found in CoRALFs promoters. Moreover, the expression analysis showed that pollen-specific CoRALF50 responded to the elongation and stagnation of self-incompatibility pollen tube in C. oleifera. Further experiments suggested that CoRALF50 could significantly down-regulate carbohydrate metabolism pathways in pollen tubes cultured in vitro, thereby causing abnormalities in ROS level, plasma membrane biosynthesis, and cell wall biosynthesis, which demonstrating the importance of CoRALF50 in regulating pollen tube growth. Altogether, this study extends information for exploring new functions of RALFs and provides new insights into the role of RALFs in self-incompatibility pollen tube growth.