Exocyst mutants suppress pollen tube growth and cell wall structural defects of hydroxyproline O-arabinosyltransferase mutants.

HYDROXYPROLINE O-ARABINOSYLTRANSFERASEs (HPATs) initiate a post-translational protein modification (Hyp-Ara) found abundantly on cell wall structural proteins. In Arabidopsis thaliana HPAT1 and HPAT3 are redundantly required for full pollen fertility. In addition to the lack of Hyp-Ara in hpat1/3 pollen tubes, we also found broadly disrupted cell wall polymer distributions, particularly the conversion of the tip cell wall to a more shaft-like state. Mutant pollen tubes were slow growing and prone to rupture and morphological irregularities. In a forward mutagenesis screen for suppressors of the hpat1/3 low seed set phenotype, we identified a missense mutation in exo70a2, a predicted member of the vesicle-tethering exocyst complex. Suppressed pollen had increased fertility, fewer morphological defects and partially rescued cell wall organization. A transcriptional null allele of exo70a2 also suppressed the hpat1/3 fertility phenotype as did mutants of core exocyst complex member sec15a, indicating that reduced exocyst function bypassed the PT requirement for Hyp-Ara. In a wild-type background, exo70a2 reduced male transmission efficiency, lowered pollen germination frequency and slowed pollen tube elongation. EXO70A2 also localized to the PT tip plasma membrane, consistent with a role in exocyst-mediated secretion. To monitor trafficking of Hyp-Ara modified proteins, we generated an HPAT-targeted fluorescent secretion reporter. Reporter secretion was partially dependent on EXO70A2 and was significantly increased in hpat1/3 PTs compared to wild type, but reduced in the suppressed exo70a2 hpat1/3 tubes.