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Exocyst component of 70-kDa (EXO70) proteins are constituents of the exocyst complex implicated in vesicle tethering during exocytosis. MILDEW RESISTANCE LOCUS O (MLO) proteins are plant-specific calcium channels and some MLO isoforms enable fungal powdery mildew pathogenesis. We here detected an unexpected phenotypic overlap of Arabidopsis thaliana exo70H4 and mlo2 mlo6 mlo12 triple mutant plants regarding the biogenesis of leaf trichome secondary cell walls. Biochemical and Fourier transform infrared spectroscopic analyses corroborated deficiencies in the composition of trichome cell walls in these mutants. Transgenic lines expressing fluorophore-tagged EXO70H4 and MLO exhibited extensive colocalization of these proteins. Furthermore, mCherry-EXO70H4 mislocalized in trichomes of the mlo triple mutant and, vice versa, MLO6-GFP mislocalized in trichomes of the exo70H4 mutant. Expression of GFP-marked PMR4 callose synthase, a known cargo of EXO70H4-dependent exocytosis, revealed reduced cell wall delivery of GFP-PMR4 in trichomes of mlo triple mutant plants. In vivo protein–protein interaction assays in plant and yeast cells uncovered isoform-preferential interactions between EXO70.2 subfamily members and MLO proteins. Finally, exo70H4 and mlo6 mutants, 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. Shared trichome- and defense-related mutant phenotypes, subcellular colocalization, and isoform-preferential protein–protein interaction indicate interplay of EXO70 and MLO proteins in focal secretion. [ABSTRACT FROM AUTHOR]

Although angiosperm plants generally react to immunity elicitors like chitin or chitosan by the cell wall callose deposition, this response in particular cell types, especially upon chitosan treatment, is not fully understood. Here we show that the growing root hairs (RHs) of Arabidopsis can respond to a mild (0.001%) chitosan treatment by the callose deposition and by a deceleration of the RH growth. We demonstrate that the glucan synthase‐like 5/PMR4 is vital for chitosan‐induced callose deposition but not for RH growth inhibition. Upon the higher chitosan concentration (0.01%) treatment, RHs do not deposit callose, while growth inhibition is prominent. To understand the molecular and cellular mechanisms underpinning the responses to two chitosan treatments, we analysed early Ca2+ and defence‐related signalling, gene expression, cell wall and RH cellular endomembrane modifications. Chitosan‐induced callose deposition is also present in the several other plant species, including functionally analogous and evolutionarily only distantly related RH‐like structures such as rhizoids of bryophytes. Our results point to the RH callose deposition as a conserved strategy of soil‐anchoring plant cells to cope with mild biotic stress. However, high chitosan concentration prominently disturbs RH intracellular dynamics, tip‐localised endomembrane compartments, growth and viability, precluding callose deposition. [ABSTRACT FROM AUTHOR]