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.

• Keywords
• arabidopsis, cell wall, defence, gene expression, signalling,
• MeSH
• Arabidopsis * growth & development   drug effects   metabolism   physiology   MeSH
• Cell Membrane metabolism   MeSH
• Cell Wall * metabolism   MeSH
• Chitosan * pharmacology   MeSH
• Glucans * metabolism   MeSH
• Glucosyltransferases metabolism   MeSH
• Plant Roots * growth & development   metabolism   drug effects   MeSH
• Arabidopsis Proteins * metabolism   genetics   MeSH
• Gene Expression Regulation, Plant drug effects   MeSH
• Calcium metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• callose MeSH Browser
• Chitosan * MeSH
• Glucans * MeSH
• Glucosyltransferases MeSH
• Arabidopsis Proteins * MeSH
• Calcium MeSH

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.

• MeSH
• Arabidopsis * metabolism   MeSH
• Cell Wall metabolism   MeSH
• Plant Diseases microbiology   MeSH
• Disease Resistance genetics   MeSH
• Protein Isoforms genetics   metabolism   MeSH
• Arabidopsis Proteins * genetics   metabolism   MeSH
• Plant Proteins metabolism   MeSH
• Trichomes genetics   metabolism   MeSH
• Vesicular Transport Proteins metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• EXO70H4 protein, Arabidopsis MeSH Browser
• Protein Isoforms MeSH
• Arabidopsis Proteins * MeSH
• Plant Proteins MeSH
• Vesicular Transport Proteins MeSH

The pathogenesis-related 1 (PR1) proteins are members of the cross-kingdom conserved CAP superfamily (from Cysteine-rich secretory protein, Antigen 5, and PR1 proteins). PR1 mRNA expression is frequently used for biotic stress monitoring in plants; however, the molecular mechanisms of its cellular processing, localization, and function are still unknown. To analyse the localization and immunity features of Arabidopsis thaliana PR1, we employed transient expression in Nicotiana benthamiana of the tagged full-length PR1 construct, and also disrupted variants with C-terminal truncations or mutations. We found that en route from the endoplasmic reticulum, the PR1 protein transits via the multivesicular body and undergoes partial proteolytic processing, dependent on an intact C-terminal motif. Importantly, only nonmutated or processing-mimicking variants of PR1 are secreted to the apoplast. The C-terminal proteolytic cleavage releases a protein fragment that acts as a modulator of plant defence responses, including localized cell death control. However, other parts of PR1 also have immunity potential unrelated to cell death. The described modes of the PR1 contribution to immunity were found to be tissue-localized and host plant ontogenesis dependent.

• Keywords
• extracellular proteins, multivesicular bodies, pathogenesis-related 1, plant immunity, vesicular trafficking,
• MeSH
• Arabidopsis * metabolism   MeSH
• Endoplasmic Reticulum metabolism   MeSH
• Stress, Physiological MeSH
• Plant Immunity genetics   MeSH
• Arabidopsis Proteins * metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Nicotiana genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Arabidopsis Proteins * MeSH