AtPIG-S, a predicted Glycosylphosphatidylinositol Transamidase subunit, is critical for pollen tube growth in Arabidopsis

Abstract

Background Glycosylphosphatidylinositol (GPI) addition is one of the several post-translational modifications to proteins that increase their affinity for membranes. In eukaryotes, the GPI transamidase complex (GPI-T) catalyzes the attachment of pre-assembled GPI anchors to GPI-anchored proteins (GAPs) through a transamidation reaction. A mutation inAtGPI8(gpi8-2), the putative catalytic subunit of GPI-T in Arabidopsis,is transmitted normally through the female gametophyte (FG), indicating the FG tolerates loss of GPI transamidation. In contrast,gpi8-2almost completely abolishes male gametophyte (MG) function. Still, the unexpected finding thatgpi8-2FGs function normally requires further investigation. Additionally, specific developmental defects in the MG caused by loss of GPI transamidation remain poorly characterized. Results Here we investigated the effect of loss ofAtPIG-S,another GPI-T subunit, in both gametophytes. Likegpi8-2, we showed that a mutation inAtPIG-S(pigs-1) disrupted synergid localization of LORELEI (LRE), a putative GAP critical for pollen tube reception by the FG. Still,pigs-1is transmitted normally through the FG. Conversely,pigs-1severely impaired male gametophyte (MG) function during pollen tube emergence and growth in the pistil. ApPIGS:GFP-PIGStransgene complemented these MG defects and enabled generation ofpigs-1/pigs-1seedlings. However, thepPIGS:GFP-PIGStransgene seemingly failed to rescue the function of AtPIG-S in the sporophyte, aspigs-1/pigs-1, pPIGS:GFP-PIGSseedlings died soon after germination. Conclusions Characterization ofpigs-1provided further evidence that the FG tolerates loss of GPI transamidation more than the MG and that the MG compared to the FG may be a better haploid system to study the role of GPI-anchoring.Pigs-1pollen develops normally and thus represent a tool in which GPI anchor biosynthesis and transamidation of GAPs have been uncoupled, offering a potential way to study free GPI in plant development. While previously reported male fertility defects of GPI biosynthesis mutants could have been due either to loss of GPI or GAPs lacking the GPI anchor, our results clarified that the loss of mature GAPs underlie male fertility defects of GPI-deficient pollen grains, aspigs-1is defective only in the downstream transamidation step.