Rab5 GTPases mediate the targeting of ROP signalling to establish polarity for pollen germination

The establishment of polarity is an evolutionarily recurring theme in all eukaryotes. Pollen germination in angiosperms establishes polarity, which is critical for reproduction and a barrier for inter-species crosses. Arabidopsis Rho of plants (ROPs) and their guanine nucleotide exchange factor 8 (RopGEF8) are essential for pollen germination through their polar distribution at the plasma membrane and asymmetric activation of downstream activities. Events leading to the polar plasma membrane association of RopGEF8 and ROPs during pollen germination are yet to be uncovered. We report here that canonical and plant-unique Rab5 GTPases, regulators of vesicular trafficking, synergistically regulate pollen germination. Intriguingly, the role of Rab5s in this process is independent of their own GEFs. Instead, Rab5 GTPases directly interact with RopGEF8 in an activity-independent way and mediate its polar plasma membrane targeting from endosomes. Our finding that Rab5 GTPases influence ROP signalling through RopGEF8 suggests an evolutionary innovation by plants. By using the combination of both canonical and plant-unique Rab5 GTPases, ROP signalling can be regulated in a more delicate manner. Consequently, various internal and external inputs are interpreted to ensure the proper timing of pollen germination.