Dr. Cora MacAlister
The Environmental and Plant Biology Colloquium Series presents Dr. Cora MacAlister discussing “Getting to the point: the importance of hydroxyproline O-arabinosylation in pollen tube growth and fertility” on Oct. 19 from 11:50 a.m. to 12:45 p.m. in Porter Hall 104.
MacAlister is Assistant Professor of Molecular, Cellular and Developmental Biology.
Dr. Allan Showalter is host for this event.
Abstract: Sexual reproduction in flowering plants relies on pollen tubes to deliver their sperm cargo to receptive ovules. The tip-growing pollen tubes elongate quickly and over considerable distances and must be able to reorient their growth in response to guidance cues as they pass through the pistil. They rely on their highly polarized and compositionally unique cell wall for this elongation. The importance of several polysaccharides, including callose and pectins for maintaining cell wall structure is well established. We have recently shown that glycoproteins also play a critical role in pollen tube growth. The hydroxyproline-rich glycoproteins (HRGPs) form a large and diverse superfamily whose members are modified by different types and degrees of glycosylation. We have found that hydroxyproline O-arabinosylation, the stepwise addition of short linear chains of arabinose sugars onto hydroxyproline residues, is required for full pollen fertility. Pollen from Arabidopsis mutants of the enzymes that initiate this sugar chain fail to fertilize ~90% of their available ovules, leading to poor seed yield. The hydroxyproline o-arabinosyltransferase (hpat) mutant pollen tubes show reduced total elongation in vitro and disrupted pollen tube morphology. Despite this strong effect on male fertility, female fertility and vegetative growth are normal, suggesting that the pollen tube is particularly sensitive to the loss of this modification. Using a forward genetic screen, we have isolated several suppressor mutants that improve seed set in the hpat mutant background. Cloning the causative mutations via genome sequencing has identified several novel components of the pollen tube secretion pathway, suggesting that altering the rate of secretion is able to compensate for the hpat defect.
Both the modification itself and the HPAT enzymes that produce it are plant-specific, yet deeply conserved across all plants and green algae. A recent gene duplication in the tomato lineage has produced two highly similar HPATs that are expressed only in the male tissues, unlike other members of this gene family which are broadly expressed. Despite high sequence conservation and overlapping expression patterns, we have found that these two have non-redundant roles in male fertility, with one functioning primarily in early stages of pollen grain development and hydration, and the other in pollen tube elongation. Thus indicating a broadly conserved function for this modification in angiosperm reproduction.
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