Arabidopsis Trichome Contains Two Plasma Membrane Domains with Different Lipid Compositions Which Attract Distinct EXO70 Subunits

Abstract

Plasma membrane (PM) lipid composition and domain organization are modulated by polarized exocytosis. Conversely, targeting of secretory vesicles at specific domains in the PM is carried out by exocyst complexes, which contain EXO70 subunits that play a significant role in the final recognition of the target membrane. As we have shown previously, a mature Arabidopsis trichome contains a basal domain with a thin cell wall and an apical domain with a thick secondary cell wall, which is developed in an EXO70H4-dependent manner. These domains are separated by a cell wall structure named the Ortmannian ring. Using phospholipid markers, we demonstrate that there are two distinct PM domains corresponding to these cell wall domains. The apical domain is enriched in phosphatidic acid (PA) and phosphatidylserine, with an undetectable amount of phosphatidylinositol 4,5-bisphosphate (PIP2), whereas the basal domain is PIP2-rich. While the apical domain recruits EXO70H4, the basal domain recruits EXO70A1, which corresponds to the lipid-binding capacities of these two paralogs. Loss of EXO70H4 results in a loss of the Ortmannian ring border and decreased apical PA accumulation, which causes the PA and PIP2 domains to merge together. Using transmission electron microscopy, we describe these accumulations as a unique anatomical feature of the apical cell wall—radially distributed rod-shaped membranous pockets, where both EXO70H4 and lipid markers are immobilized.

EXO70A2 is critical for the exocyst complex function in Arabidopsis pollen

“…Analogically to our observation that the artificial EXO70A2 expression in the sporophyte could complement the sporophytic defects of exo70a1 mutants, SEC15a, the main SEC15 isoform in pollen, could substitute for the function of SEC15b in the sporophyte, although SEC15a and SEC15b share mere 47% sequence identity at the protein level (Batystová et al, submitted). It is tempting to speculate that exocyst complexes with different subunit composition may bind vesicles with distinct secretory cargos and deliver them to special PM subdomains (due to different EXO70-membrane interactions, see Žárský et al, 2009;Sekereš et al, 2017;Kubátová et al 2019). In contrast, SEC10a and SEC10b represent a very recent duplication without any signs of expression or functional diversification (Vukašinović et al, 2014).…”

“…The exocyst, first described in yeasts and mammals, consists of eight subunits -Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70, and Exo84 (TerBush et al, 1996;Guo et al, 1999), with Sec3 and Exo70 generally believed to be responsible for targeting the complex to the PM, while the other subunits form the core of the complex (Boyd et al, 2004;He et al, 2007;Pleskot et al, 2015). Plant genomes encode all exocyst subunits (Cvrčková et al, 2001;Eliáš et al 2003) that form a functional complex (Hála et al, 2008;Fendrych et al, 2010) and engage in cellular processes requiring targeted secretion, including root hair and pollen tube elongation (Cole et al, 2005;Synek et al, 2006;Hála et al, 2008;Synek et al, 2017), cytokinesis (Fendrych et al, 2010;Rybak et al, 2014), secondary cell wall deposition in trichomes and during xylem development (Kulich et al, 2015;Kubátová et al, 2019;Vukašinović et al, 2017), localized deposition of seed coat pectins (Kulich et al, 2010), transport of PIN auxin carriers to the PM , and response to pathogens (Pečenková et al, 2011;Sabol et al, 2017). Viable mutants of A. thaliana defective in exocyst subunits (sec8-4, sec15b, exo70a1, exo84b) exhibit dwarfish growth with pleiotropic developmental defects (Cole et al, 2005;Synek et al, 2006;Hála et al, 2008;Fendrych et al, 2010;Batystová et al, submitted).…”

The importance of being edgy: cell geometric edges as an emerging polar domain in plant cells

“…Another striking example for morphologically and biochemically polarised cells are trichomes of Arabidopsis thaliana , which specify two distinct apical and basal domains with distinct cell wall compositions and structures. This cell wall polarisation was functionally linked to the establishment of apical and basal domains with distinct lipid signatures in the plasma membrane, which in turn recruit different subunits of the exocyst complex, which is involved in targeted secretion (Kubátová et al ., ). Cell polarity is also critical during the development of stomata in leaves, which involves a series of asymmetric cell divisions.…”

Plant Cell Wall Proteins and Development

“…Eight experimental articles, nine up-to-date review articles, as well as a concept article, have been published. We wish to thank all the authors for their great contribution to this unique collection of articles as well as the International Journal of Molecular Science supporting team.The content of this Special Issue embraces several topics, all of them stressing the roles of cell wall proteins: cell wall proteomics studies on monocot species [7,12]; the role of cell wall proteins during plant development [13][14][15] or in response to environmental stresses [16][17][18][19]; overviews on several cell wall protein families either from green microalgae [20] or from plants, i.e., fasciclin arabinogalactan proteins (FLAs) [21,22], membrane-bound class III peroxidases (Class III Prxs) [23], pectin methylesterases inhibitors [24], DUF642 (Domain of Unknown Function 642) proteins [25], and Proline-rich, Arabinogalactan proteins, conserved Cysteines (PAC) domain-proteins [26]; and the role of fasciclin arabinogalactan proteins (FLAs) in Ca 2+ signaling during plant morphogenesis [27,28].For two decades, cell wall proteomics has become a powerful experimental approach and has revealed the diversity of the cell wall protein families. Arabidopsis thaliana has been the most studied plant species, and almost half of its expected cell wall proteome has been described so far (see WallProtDB, http://www.polebio.lrsv.ups-tlse.fr/WallProtDB/).…”

“…Its composition can vary locally in the domains characterized by particular lipid compositions. Kubátová et al [15] show that two plasma membrane domains with a distinct lipid composition are located close to the Ortmannian ring, a cell wall domain-specific to trichomes. These plasma membrane domains are generated thanks to exocysts complex containing EXO70 subunits recognizing the target membrane.…”