Abstract
EXTENSINS (EXTs) are a 65-member subfamily of hydroxyproline-rich glycoproteins (HRGPs) of which 20 putatively form crosslinking networks in the cell wall. These 20 classical EXTs are involved at the start of new wall assembly as evidenced by a requirement for EXT3 during cytokinesis, and the ability of some EXTs to polymerize in vitro into dendritic patterns. EXT3 was previously shown to form pulcherosine (three Tyrosines) cross-links. Little direct data exists on the other 19 classical EXTs. Here, we describe the phenotypes of ext18 mutants and rescued progeny as well as associated expression profiles of all 20 classical EXT genes. We found that EXT18 is required for full male fertility, as well as for normal vegetative growth. EXT18 has potential to form crosslinking networks via di-iso-di-tyrosine (four Tyrosines) covalent bonds, and not via pulcherosine due to deficit of lone Tyrosines. This together with ext18 defective pollen grains and pollen tubes, and reduced plant size, suggests that EXT18-type EXTs are important contributors to wall integrity, in pollen and other rapidly extending walls. The data also show that a knockout of EXT18 had a pleiotropic affect on the expression of several EXTs, as did the reintroduction of the native EXT18 gene, thus supporting the thesis that transcription of groups of EXTs are co-regulated and work in different combinations to make distinctive inputs into wall assembly of different cell types. These insights contribute to basic knowledge of cell wall self-assembly in different cell types, and potentially enable biotechnological advances in biomass increase and plant fertility control.
Highlights
The outermost layer of young plant cells is a primary wall made up mainly of interpenetrating polymers of cellulose, hemicellulose, and pectins
We provide evidence in support of a role for EXT18 (At1g26250), a classical EXT, in pollen grain wall integrity and successful pollen tube growth, as well as in vegetative growth and biomass yield
To the nearest whole day, 57% of the wild-type progenitor (WT) population flowered in 27 days, whereas 54% of ext18 mutant population flowered at day 33 (FT50, defined as the days after germination (DAG) taken to flower by 50% of the plant population in this study)
Summary
The outermost layer of young plant cells is a primary wall made up mainly of interpenetrating polymers of cellulose, hemicellulose, and pectins. Walls contain proteins and glycoproteins with enzymatic and/or structural roles, most of unknown, or putative function based on sequence (Bacic et al, 1988; Fry, 2004; Albersheim et al, 2011) This dynamic matrix gives each cell, structure, shape, tensile strength, and protection, and it is intimately involved in plant size and architecture. The ever-changing composition of growing walls, and their recalcitrance to examination have presented challenges to identifying wall components and how they assemble and function (Burton et al, 2010) This is no less the case with hydroxyprolinerich glycoproteins (HRGPs), a large family in relatively low abundance when compared to other wall components, and defined by the presence of hydroxyproline, glycosylated amino acid residues and repetitive glycomodules (Kieliszewski and Lamport, 1994; Seifert and Roberts, 2007; Showalter et al, 2010). More recent evidence supports a role for HRGPs as a component of global calcium signaling in plants (Lamport and Varnai, 2013)
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