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

Small secreted proteins play an important role in plant development, as well as in reactions to changes in the environment. In Arabidopsis thaliana, they are predominantly members of highly expanded families, such as the pathogenesis-related (PR) 1-like protein family, whose most studied member PR1 is involved in plant defense responses by a so far unknown mechanism, or Clavata3/Endosperm Surrounding Region (CLE) protein family, whose members' functions in the development are well described. Our survey of the existing literature for the two families showed a lack of details on their localization, trafficking, and exocytosis. Therefore, in order to uncover the modes of their secretion, we tested the hypothesis that a direct link between the secreted cargoes and the secretion regulators such as Rab GTPases, SNAREs, and exocyst subunits could be established using in silico co-expression and clustering approaches. We employed several independent techniques to uncover that only weak co-expression links could be found for limited numbers of secreted cargoes and regulators. We propose that there might be particular spatio-temporal requirements for PR1 and CLE proteins to be synthesized and secreted, and efforts to experimentally cover these discrepancies should be invested along with functional studies.

• Keywords
• CLE, PR1, SNARE, co-expression, exocyst, secretion,
• MeSH
• Arabidopsis * genetics   metabolism   MeSH
• Cytoplasm metabolism   MeSH
• Exocytosis physiology   MeSH
• Arabidopsis Proteins * genetics   metabolism   MeSH
• SNARE Proteins metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Arabidopsis Proteins * MeSH
• SNARE Proteins MeSH

Localized delivery of plasma-membrane and cell-wall components is a crucial process for plant cell growth. One of the regulators of secretory-vesicle targeting is the exocyst tethering complex. The exocyst mediates first interaction between transport vesicles and the target membrane before their fusion is performed by SNARE proteins. In land plants, genes encoding the EXO70 exocyst subunit underwent an extreme proliferation with 23 paralogs present in the Arabidopsis (Arabidopsis thaliana) genome. These paralogs often acquired specialized functions during evolution. Here, we analyzed functional divergence of selected EXO70 paralogs in Arabidopsis. Performing a systematic cross-complementation analysis of exo70a1 and exo70b1 mutants, we found that EXO70A1 was functionally substituted only by its closest paralog, EXO70A2. In contrast, none of the EXO70 isoforms tested were able to substitute EXO70B1, including its closest relative, EXO70B2, pointing to a unique function of this isoform. The presented results document a high degree of functional specialization within the EXO70 gene family in land plants.

• Keywords
• Arabidopsis, EXO70, EXO70A1, EXO70B1, exocyst complex, polar exocytosis,
• MeSH
• Arabidopsis genetics   growth & development   metabolism   MeSH
• Cell Membrane metabolism   MeSH
• Exocytosis MeSH
• Arabidopsis Proteins genetics   metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Transport Vesicles metabolism   MeSH
• Vesicular Transport Proteins genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Arabidopsis Proteins MeSH
• Vesicular Transport Proteins MeSH