Numerous harmful microorganisms and insect pests have the ability to cause plant infections or damage, which is mostly controlled by toxic chemical agents. These chemical compounds and their derivatives exhibit hazardous effects on habitats and human life too. Hence, there's a need to develop novel, more effective and safe bio-control agents. A variety of microbes such as viruses, bacteria, and fungi possess a great potential to fight against phytopathogens and thus can be used as bio-control agents instead of harmful chemical compounds. These naturally occurring microorganisms are applied to the plants in order to control phytopathogens. Moreover, practicing them appropriately for agriculture management can be a way towards a sustainable approach. The MBCAs follow various modes of action and act as elicitors where they induce a signal to activate plant defense mechanisms against a variety of pathogens. MBCAs control phytopathogens and help in disease suppression through the production of enzymes, antimicrobial compounds, antagonist activity involving hyper-parasitism, induced resistance, competitive inhibition, etc. Efficient recognition of pathogens and prompt defensive response are key factors of induced resistance in plants. This resistance phenomenon is pertaining to a complex cascade that involves an increased amount of defensive proteins, salicylic acid (SA), or induction of signaling pathways dependent on plant hormones. Although, there's a dearth of information about the exact mechanism of plant-induced resistance, the studies conducted at the physiological, biochemical and genetic levels. These studies tried to explain a series of plant defensive responses triggered by bio-control agents that may enhance the defensive capacity of plants. Several natural and recombinant microorganisms are commercially available as bio-control agents that mainly include strains of Bacillus, Pseudomonads and Trichoderma. However, the complete understanding of microbial bio-control agents and their interactions at cellular and molecular levels will facilitate the screening of effective and eco-friendly bio-agents, thereby increasing the scope of MBCAs. This article is a comprehensive review that highlights the importance of microbial agents as elicitors in the activation and regulation of plant defense mechanisms in response to a variety of pathogens.
Keywords: ABA, Abscisic acid; BABA, β-Aminobutyric acid; BTH, Benzothiadiazole; CKRI, Cross kingdom RNA interference; DAMPs, Damage-associated molecular patterns; Defense mechanism; ET, Ethylene; ETI, Effector-triggered immunity; Elicitors; Fe, Iron; GSH, Glutathione; HAMP, Herbivore-associated molecular patterns; HG, Heptaglucan; HIR, Herbivore induced resistance; HRs, Hormonal receptors; ISR, Induced systemic resistance; ISS, Induced systemic susceptibility; Induced resistance; JA, Jasmonic acid; LAR, Local acquired resistance; LPS, Lipopolysaccharides; MAMPs, Microbe-associated molecular patterns; MBCAs, Microbial biological control agents; Microbiological bio-control agent; N, Nitrogen; NO, Nitric oxide; P, Phosphorous; PAMPs, Pathogen-associated molecular patterns; PGP, Plant growth promotion; PGPB, Plant growth promoting bacteria; PGPF, Plant growth promoting fungi; PGPR, Plant growth promoting rhizobacteria; PRPs, Pathogenesis-related proteins; PRRs, Pattern recognition receptors; PTI, Pattern triggered immunity; Plant defense; Plant disease; RLKs, Receptor-like-kinases; RLPs, Receptor-like-proteins; ROS, Reactive oxygen species; SA, Salicylic acid; SAR, Systemic acquired resistance; TFs, Transcription factors; TMV, Tobacco mosaic virus; VOCs, Volatile organic compounds.
© 2021 The Author(s).