Biological Control Microorganisms that Induce Plant Defense Responses

Some plant-beneficial microbes, including bacteria and fungi, can induce plant defense, enabling plants to resist pathogen infections. Successful defense elicitation depends on compatible host-microbe interactions at multiple stages. The initial interaction begins with plant root exudates, which contain chemical cues that attract beneficial microbes by enhancing their motility, biofilm formation, and expression of symbiosis- and immunity-related genes. In turn, these microbes produce a diverse array of immune elicitors-such as proteins, carbohydrates, lipids, and volatile compounds-that are perceived by plants through various mechanisms. Some elicitors are recognized by membrane-bound pattern recognition receptors, whereas others interact with the plant plasma membrane or cytoplasmic targets such as MYB72 and LOX3. These interactions can either trigger local pattern-triggered immunity, characterized by reactive oxygen species production and activation of the mitogen-activated protein kinase signaling pathway, or generate long-distance signals such as oxylipins that induce systemic resistance in distal tissues. A central outcome of these interactions is induced systemic resistance, which primes plants for a heightened immune state, enabling faster and stronger defense responses upon a subsequent pathogen challenge. In some cases, beneficial microbes can also trigger salicylic acid-mediated systemic acquired resistance, particularly enhancing resistance against biotrophic pathogens. Furthermore, beneficial microbes must balance immune activation and immune evasion by suppressing microbe-associated molecular pattern-triggered immune responses and avoiding the formation of hyperbiofilm, which allows them to establish a long-term symbiotic relationship with the host.