Plant immune and defense responses to microorganisms and insects: molecular signaling, crosstalk, and adaptive plasticity

Plants encounter a diverse array of pathogens and herbivorous insects, necessitating a well-coordinated response for survival and growth. Recent research has elucidated a complex network of molecular processes, including signal perception, hormonal interactions, transcriptional changes, and metabolic modifications, that govern plant responses to these biotic challenges. Plants recognize microbial and insect-derived signals through specific receptor systems that trigger cascades of signal transduction, which ultimately converge on shared defense mechanisms. Additionally, plant hormones such as salicylic acid, jasmonic acid, and ethylene play a critical role in regulating these responses, enabling plants to defend against multiple pathogens and pests while maintaining overall fitness. The initiation of defense responses results in extensive alterations in gene expression and epigenetic regulation, as well as a redirection of primary metabolism toward the synthesis of specific secondary metabolites, including phenolics, terpenoids, and alkaloids, which are essential for defense against both pathogens and insects. Notably, the findings indicate considerable overlap between defense mechanisms activated by microbial and insect threats, underscoring the importance of integrated signaling over distinct defense pathways. This review aims to synthesize current knowledge of the molecular and metabolic foundations of plant responses to biotic stressors, with an emphasis on common regulatory components and adaptable defense strategies that enable effective, sustainable plant defense.