Microtubule‐dependent targeting of the exocyst complex is necessary for xylem development in Arabidopsis

Abstract

Cortical microtubules (MTs) play a major role in the patterning of secondary cell wall (SCW) thickenings in tracheary elements (TEs) by determining the sites of SCW deposition. The EXO70A1 subunit of the exocyst secretory vesicle tethering complex was implicated to be important for TE development via the MT interaction. We investigated the subcellular localization of several exocyst subunits in the xylem of Arabidopsis thaliana and analyzed the functional significance of exocyst-mediated trafficking in TE development. Live cell imaging of fluorescently tagged exocyst subunits in TE using confocal microscopy and protein-protein interaction assays were performed to describe the role of the exocyst and its partners in TE development. In TEs, exocyst subunits were localized to the sites of SCW deposition in an MT-dependent manner. We propose that the mechanism of exocyst targeting to MTs involves the direct interaction of exocyst subunits with the COG2 protein. We demonstrated the importance of a functional exocyst subunit EXO84b for normal TE development and showed that the deposition of SCW constituents is partially compromised, possibly as a result of the mislocalization of secondary cellulose synthase in exocyst mutants. We conclude that the exocyst complex is an important factor bridging the pattern defined by cortical MTs with localized secretion of the SCW in developing TEs.

Analysis of Exocyst Subunit EXO70 Family Reveals Distinct Membrane Polar Domains in Tobacco Pollen Tubes

“…The small area of NtEXO70A1a localization corresponds to the previously described area of exocytosis maximum in the pollen tube (Bove et al, 2008;Idilli et al, 2013;Luo et al, 2016). This localization is in accordance with previous characterizations of Arabidopsis EXO70A1 as the isoform generally responsible for exocytosis in many plant tissues (Synek et al, 2006;Hála et al, 2008;Samuel et al, 2009;Kulich et al, 2010;Drdová et al, 2013;Fendrych et al, 2013;Zhang et al, 2016;Vukašinovi c et al, 2017). In accordance with the published observations, NtEXO70A1a localizes both to the cytoplasm and to the specific area of the PM.…”

Exocyst Subunit EXO70H4 Has a Specific Role in Callose Synthase Secretion and Silica Accumulation

“…Similar behavior of the exocyst subunits was observed during 276 xylogenesis, where EXO70A1-tagRFP first localized dispersedly to the PM and later on gradually 277 organized into a bundled pattern (Vukašinović et al, 2016). Such stabilization of the polarity was 278 observed previously in budding yeast (Brennwald and Rossi, 2007).…”

“…where EXO70A1 recruits the exocyst complex and is responsible for the development of the cell wall 73 thickenings (Oda et al 2015;Vukašinović et al 2016). Another example of the exocyst recruitment 74 was shown on EXO70B1, which is recruited to the plasma membrane by RIN4 (Sabol et al 2017).…”

Patterning of plant cell wall deposition by cortical microtubules

“…It is unclear if and how this barrier function of microtubules affects metaxylem pattern formation in Arabidopsis. Lacking the cortical microtubules that template secondary cell wall thickenings 52,56 , these local activated ROP11 GTP areas remain free of additional deposition of secondary cell wall material, resulting in the characteristic pitted pattern of the metaxylem 53 ( Fig. 1.4).…”

“…These master regulators were coupled with an inducible expression system enabling ectopic induction of xylem transdifferentiation in tobacco BY-2 cells, Arabidopsis plants and poplar trees 51,55 . This proved to be a powerful tool to advance the study of xylem formation both via microarray analysis 51,81,130 and experimental cell biology [54][55][56]131,132 . In this thesis, inducible ectopic expression of VND7 is used as an experimental system to study functional properties of the microtubule array which templates the patterned deposition of cell wall material in developing protoxylem vessel elements.…”