EXO70A1-Mediated Vesicle Trafficking Is Critical for Tracheary Element Development in Arabidopsis    

Abstract

Exocysts are highly conserved octameric complexes that play an essential role in the tethering of Golgi-derived vesicles to target membranes in eukaryotic organisms. Genes encoding the EXO70 subunit are highly duplicated in plants. Based on expression analyses, we proposed previously that individual EXO70 members may provide the exocyst with functional specificity to regulate cell type– or cargo-specific exocytosis, although direct evidence is not available. Here, we show that, as a gene expressed primarily during tracheary element (TE) development, EXO70A1 regulates vesicle trafficking in TE differentiation in Arabidopsis thaliana. Mutations of EXO70A1 led to aberrant xylem development, producing dwarfed and nearly sterile plants with very low fertility, reduced cell expansion, and decreased water potential and hydraulic transport. Grafting of a mutant shoot onto wild-type rootstock rescued most of these aboveground phenotypes, while grafting of a wild-type shoot to the mutant rootstock did not rescue the short root hair phenotype, consistent with the role of TEs in hydraulic transport from roots to shoots. Histological analyses revealed an altered pattern of secondary cell wall thickening and accumulation of large membrane-bound compartments specifically in developing TEs of the mutant. We thus propose that EXO70A1 functions in vesicle trafficking in TEs to regulate patterned secondary cell wall thickening.

The Fragile Fiber1 Kinesin Contributes to Cortical Microtubule-Mediated Trafficking of Cell Wall Components

“…We observed abnormal Golgi morphology and accumulation of enlarged vesicles in the fra1-5 mutant, which are characteristic symptoms of defective postGolgi trafficking and vesicle secretion (Synek et al, 2006;Feraru et al, 2012;Boutté et al, 2013;Li et al, 2013). Whether the accumulated vesicles in fra1-5 are destined to the plasma membrane and represent FRA1 cargo remains to be determined.…”

EXO70C2 Is a Key Regulatory Factor for Optimal Tip Growth of Pollen

“…The documented weakened cell wall at the tube apex 437 during the rapid growth is most likely the primary cause of the exo70C2 phenotype. Defects 438 in cell wall deposition in seed coats, trichomes, cell plates, and xylem development have been 439 well described for several Arabidopsis exocyst mutants (Fendrych et al, 2010;Kulich et al, 440 2010 andLi et al, 2013;Vukašinović et al, 2016) Another interpretation is that EXO70C2 acquired a negative regulatory function, 460 negatively affecting cell expansion under normal circumstances by e.g. interfering with 461 putative exocyst regulatory molecules such as small GTPases or lipids.…”

RNA silencing of exocyst genes in the stigma impairs the acceptance of compatible pollen in Arabidopsis

“…4C). The exo70A1 mutant pistils have also been previously described to have variable stigmatic papillar elongation and defects in tracheary element differentiation (Synek et al, 2006;Samuel et al, 2009;Li et al, 2013;Safavian et al, 2014). Late-stage 12 transgenic flower buds, just prior to anthesis, were carefully examined for stigmatic papillar elongation.…”

“…Several Arabidopsis subunit mutants display strong growth defects such as the sec3a mutant with an embryo-lethal phenotype (Zhang et al, 2013), sec6, sec8, and exo84b mutants with severely dwarfed phenotypes and defects in root growth (Fendrych et al, 2010;Wu et al, 2013;Cole et al, 2014), and exo70A1 with a milder dwarf phenotype (Synek et al, 2006). The Arabidopsis exo70A1 mutant has also been reported to have defects in root hair elongation, hypocotyl elongation, compatible pollen acceptance, seed coat deposition, and tracheary element differentiation (Synek et al, 2006;Samuel et al, 2009;Kulich et al, 2010;Li et al, 2013). Essential roles for other exocyst subunits include Arabidopsis SEC5a/SEC5b, SEC6, SEC8, and SEC15a/SEC15b in male gametophyte development and pollen tube growth (Cole et al, 2005;Hála et al, 2008;Wu et al, 2013), SEC8 in seed coat deposition (Kulich et al, 2010), SEC5a, SEC8, EXO70A1, and EXO84b in root meristem size and root cell elongation (Cole et al, 2014), and a maize (Zea mays) SEC3 homolog in root hair elongation (Wen et al, 2005).…”