Arabidopsis RPG1 is important for primexine deposition and functions redundantly with RPG2 for plant fertility at the late reproductive stage.

Arabidopsis Ruptured Pollen Grain-1 (RPG1/Sweet8) is a member of the MtN3/saliva protein family that functions as a sugar transporter. The rpg1 mutant shows defective exine pattern formation. In this study, transmission electron microscopy (TEM) observations showed that much less primexine was deposited in rpg1 tetrads. Furthermore, microspore membrane undulation was abnormal, and sporopollenin accumulation was also defective. This suggests that a reduced primexine deposition in rpg1 leads to abnormal membrane undulation that affects exine pattern formation. Chemical staining revealed thinning of the callose wall of rpg1, as well as significantly reduced expression of Callose synthase-5 (CalS5) in rpg1. The fertility of the rpg1 mutant could be partly restored at late reproductive stages, potentially complemented in part by RPG2, another member of the MtN3/saliva family, which is expressed in the anther during microsporogenesis. The double mutant, rpg1rpg2, was almost sterile and was not restored during late reproduction. These results suggest that RPG1 and RPG2 are involved in primexine deposition and therefore pollen wall pattern formation.