Pollen-Cysteine Rich Peptide CR3 Regulates Pollen Germination in Arabidopsis thaliana

Pollination is the first event in the plant reproduction, in which mature pollen grains adhere onto the stigma surface, hydrate for germination and then pollen tubes penetrate into the stigma. While many factors are thought to be involved in this process, cysteine-rich peptides (CRPs) are reported to play key roles such as S locus cysteine-rich protein (SCR) which is a male factor of self-incompatibility in Brassicaceae and LAT52 which is essential for pollen hydration and pollen tube growth in tomato. In this study, we focused on the pollen-expressed CRP, named CR3, and elucidated its function in the plant reproduction process, through various analyses. RT-PCR analyses showed that CR3 was expressed predominantly in the stamen. By the microscopic observation of GFP-fused CR3 transformants, GFP signals were detected within pollen grains and pollen tubes, not at the pollen surface, and this was further supported by in situ hybridization. The cr3 mutant had equivalent pollen fertility compared to that of WT. However, in vitro pollen tube observation indicated that cr3 exhibited lower percentage of pollen germination and higher percentage of pollen tube tip burst. These results suggest that CR3 acts as the key CRP for pollen germination at the inside of pollen grains.