Exocyst mutants suppress pollen tube growth and cell wall structural defects ofhydroxyproline O-arabinosyltransferasemutants

HYDROXYPROLINEO-ARABINOSYLTRANSFERASEs (HPATs) initiate a post-translational protein modification (Hyp-Ara) found abundantly on cell wall structural proteins. InArabidopsis thaliana,HPAT1andHPAT3are redundantly required for full pollen fertility. In addition to the lack of Hyp-Ara inhpat1/3pollen tubes (PTs), we also found broadly disrupted cell wall polymer distributions, particularly the conversion of the tip cell wall to a more shaft-like state. Mutant PTs were slow growing and prone to rupture and morphological irregularities. In a forward mutagenesis screen for suppressors of thehpat1/3low seed-set phenotype, we identified a missense mutation inexo70a2, a predicted member of the vesicle-tethering exocyst complex. The suppressed pollen had increased fertility, fewer morphological defects and partially rescued cell wall organization. A transcriptional null allele ofexo70a2also suppressed thehpat1/3fertility phenotype, as did mutants of core exocyst complex membersec15a, indicating that reduced exocyst function bypassed the PT requirement for Hyp-Ara. In a wild-type background,exo70a2reduced male transmission efficiency, lowered pollen germination frequency and slowed PT elongation. EXO70A2 also localized to the PT tip plasma membrane, consistent with a role in exocyst-mediated secretion. To monitor the trafficking of Hyp-Ara modified proteins, we generated an HPAT-targeted fluorescent secretion reporter. Reporter secretion was partially dependent onEXO70A2and was significantly increased inhpat1/3PTs compared with the wild type, but was reduced in the suppressedexo70a2 hpat1/3tubes.