Polarised subcellular activation of ROPs by specific ROPGEFs drives pollen germination in Arabidopsis thaliana

During plant fertilisation, excess male gametes compete for a limited number of female gametes. The dormant male gametophyte, encapsulated in the pollen grain, consists of two sperm cells enclosed in a vegetative cell. After reaching the stigma of a compatible flower, quick and efficient germination of the vegetative cell to a tip-growing pollen tube is crucial to ensure fertilisation success. RHO OF PLANTS (ROP) signalling and their activating ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS (ROPGEFs) are essential for initiating polar growth processes in multiple cell types. However, which ROPGEFs activate pollen germination is unknown. We investigated the role of ROPGEFs in initiating pollen germination and the required cell polarity establishment. Of the five pollen-expressed ROPGEFs, we found that GEF8, GEF9, and GEF12 are required for pollen germination and male fertilisation success, as gef8;gef9;gef12 triple mutants showed almost complete loss of pollen germination in vitro and had a reduced allele transmission rate. Live cell imaging and spatiotemporal analysis of subcellular protein distribution showed that GEF8 and GEF9, but not GEF12, displayed transient polar protein accumulations at the future site of pollen germination minutes before pollen germination, demonstrating specific roles for GEF8 and GEF9 during the initiation of pollen germination. Furthermore, this novel GEF accumulation appears in a biphasic temporal manner and can shift its location. We showed that the C-terminal domain of GEF8 and GEF9 confers this protein accumulation and demonstrated that GEFs locally activate ROPs and alter Ca2+ signalling, which is required for pollen tube germination. We demonstrated that GEFs do not act redundantly during pollen germination and described for the first time a polar domain with spatiotemporal flexibility, which is crucial for the de novo establishment of a polar growth domain within a cell and, thus, for pollen function and fertilisation success.