Two functionally interchangeable Vps9 isoforms mediate pollen tube penetration of style

Style penetration by pollen tubes is essential for reproductive success, a process requiring canonical Rab5s in Arabidopsis. However, functional loss of Arabidopsis Vps9a, the gene encoding for guanine nucleotide exchange factor (GEF) of Rab5s, did not affect male transmission, implying the presence of a compensation program or redundancy. By combining genetic, cytological, and molecular approaches, we report that Arabidopsis Vps9b is a pollen-preferential gene, redundantly mediating pollen tube penetration of style with Vps9a. Vps9b is functionally interchangeable with Vps9a, whose functional distinction results from distinct expression profiles. Functional loss of Vps9a and Vps9b results in the mis-targeting of Rab5-dependent tonoplast proteins, defective vacuolar biogenesis, disturbed distribution of post-Golgi vesicles, increased cellular turgor, cytosolic acidification, and disrupted organization of actin microfilaments (MF) in pollen tubes, which collectively lead to the failure of pollen tubes to grow through style.