This review on the plant EXO70 family of exocyst subunits provides a new perspective on the diverse functions of the three subfamilies implying EXO70.2’s role in autophagy, unconventional secretion, and defence.

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.

EXO70 proteins are essential constituents of the octameric exocyst complex implicated in vesicle tethering during exocytosis, while MLO proteins are plant-specific calcium channels of which some isoforms play a key role during fungal powdery mildew pathogenesis. We here detected by a variety of histochemical staining procedures an unexpected phenotypic overlap ofA. thaliana exo70H4andmlo2 mlo6 mlo12triple mutant plants regarding the biogenesis of leaf trichome secondary cell walls. Biochemical and Fourier transform infrared spectroscopic analyses of isolated trichomes corroborated deficiencies in the composition of trichome cell walls inexo70H4andmlo2 mlo6 mlo12mutants. Transgenic lines expressing fluorophore-tagged EXO70H4 and MLO variants exhibited extensive co-localization of these proteins at the trichome plasma membrane and cell wall. Furthermore, mCherry-EXO70H4 mislocalized in trichomes of themlotriple mutant and,vice versa, MLO6-GFP exhibited aberrant subcellular localization in trichomes of theexo70H4mutant. Transgenic expression of GFP-marked PMR4 callose synthase, a previously identified cargo of EXO70H4 dependent exocytosis, revealed reduced cell wall delivery of GFP-PMR4 in mlo triple mutant plants. In vivo protein-protein interaction assays uncovered isoform-preferential physical interaction between EXO70 and MLO proteins. Finally,exo70H4andmlomutants, when combined, showed synergistically enhanced resistance to powdery mildew attack. Taken together, our data point to an isoform-specific interplay of EXO70 and MLO proteins in the modulation of trichome cell wall biogenesis and powdery mildew susceptibility, possibly by (co-)regulating focal secretion of cell wall-related cargo.