Phospholipase C6 Regulates HydroxyprolineO-arabinosyltransferase-mediated Pollen Tube Growth inArabidopsis

Abstract

Hydroxyproline O-arabinosylation is a highly-conserved and plant-specific post-translational modification found on extensins and other structural proteins in the cell wall, and is catalyzed by Hydroxyproline O-arabinosyltransferases (HPATs). In Arabidopsis, loss of HPAT1 and HPAT3 (hpat1/3) causes reorganization of components in the pollen tube (PT) cell wall, which compromises cell wall structural integrity and decreases PT growth and fertility. We have previously shown that reduced secretion (caused by loss-of-function mutations in secretory genes EXO70A2, SEC15A, and SEC1A) suppressed cell wall defects and strongly rescued poor growth and fertility in hpat1/3 PTs. Here, we show that a missense mutation in PHOSPHOLIPASE C6 (PLC6) also rescues hpat1/3 PT growth and fertility. Transgenic insertion mutations that disrupt PLC6 expression did not improve hpat1/3 pollen fertility, and did not affect PT growth or fertility in the wild type background. This data suggests that our missense allele (plc6-4) does not function like a true loss-of-function allele, and that PLC6 is not required for wild type PT growth. However, in the absence of hpat1/3, plc6-4 PTs have defects in transmission and polarized growth, as indicated by meandering growth paths and a resulting crooked appearance. plc6-4 PT elongation and straightness are more sensitive to elevated levels of calcium than wild type. This may be due the nature of the plc6-4 mutation, which causes an E569K amino acid substitution in the lipid-binding C2 domain. The 569 position is located among conserved residues that bind calcium. The resulting charge inversion caused by the E569K substitution may disrupt the lipid binding and phospholipase activities of PLC6. Here, we show that PLC6 influences polarized PT growth and HPAT-mediated PT growth and fertility, and future studies are necessary to better understand the relationship between calcium and PLC6 in PT growth.