Microbial proteins as elicitors of plant resistance to pathogens and their potential for eco-friendly crop protection in sustainable agriculture (review)

To combat plant diseases, modern agriculture has a large arsenal of xenobiotic pesticides that are toxic to microorganisms. However, the hazardous effects of such pesticides or their degradation products on the environment and human health urgently require the search for new harmless and environmentally safe means of plant pathogen control. In this regard, the attention of researchers is attracted by the phenomenon of natural plant resistance, including the active plant immunity and natural substances that can induce plant defense mechanisms (J.D. Jones et al., 2006; M. Albert, 2013; L. Wiesel et al., 2014; E.J. Andersen et al., 2018; D.F. Klessig et al., 2018). Various microorganisms, pathogenic or nonpathogenic to plants can serve as sources of such substances, including proteins and peptides. When interacting with them, microbial proteins play the role of nonspecific resistance elicitors recognized by plants as conserved microbial patterns (MAMPs or PAMPs) that induce the first line of active plant defense (basic resistance, or PTI) (C. Zipfel, 2009; M.A. Newman, 2013; J. Guo, Y. Cheng, 2022).. Other microbial proteins play the role of effectors involved in the development of the disease, and, if recognized by host plants, can also activate defense responses as elicitors of race-specific resistance (B.P. Thomma et al., 2011; W. Zhang et al., 2022; B.C. Remick et al, 2023). The perception of microbial protein elicitors by plant receptors causes rapid responses and can lead to the development of prolong systemic resistance in plants (T. Boller, G. Felix, 2009; J.B. Joshi et al., 2022; S. Wang et al., 2023). Studying the properties and mechanisms of action of microbial proteins is new and fast-paced research cluster, which results create the basis for one of the most eco-friendly avenue in the field of plant protection and can lead to the development of novel effective biocontrol agents for sustainable agriculture. Over the past few decades, in nonpathogenic and plant pathogenic fungi, oomycetes, bacteria, and viruses, including those affecting agricultural crops, a number of elicitor proteins have been identified that belong to the MAMP/PAMP type, as well as effectors that induce specific immunity (ETI). The review below summarizes and analyzes information on the most important advances in the identification and studying of elicitor proteins produced by various bacteria, fungi, oomycetes, and viruses (D. Qutob et al., 2003; M. Tarallo et al., 2022; Q. Xu et al., 2022). If the corresponding information is known, the peculiarities of the elicitor structure and their mechanisms of action, namely, defense responses of various plants induced by the corresponding elicitors, are briefly described. We also tried to illustrate the diversity of microorganism species able to produce elicitor proteins, which trigger the mechanisms of both specific and nonspecific resistance. The examples of protein and peptide elicitors, for which both basic and novel data are presented, are described in more details. Such bacterial elicitors include flagellin, harpins (similar and differing effects of harpins and flagellins are described), elongation factor Tu, cold shock proteins; elicitors produced by mycelial fungi include effectors of Cladosporium fulvum , elicitors of pathogenic and nonpathogenic Fusarium fungi, and recently discovered МАМРs/PAMPs and ETI-inducing proteins. The review also includes information on the oomycetal elicitors, microbial enzymes possessing eliciting properties, glycoproteins and peptidoglycans, and vector proteins of viruses (Y. Jin et al., 2021; L. Cai et al., 2023). In addition, the prospects for practical application of microbial elicitor proteins are described in a separate section by the example of commercial preparations based on bacterial and fungal protein elicitors, which have been developed in Russia and China and have proved their protective efficiency under field conditions (Dzhavakhiya et al., 2003; W.P. Liu