Developmental Phytohormones: Key Players in Host-Microbe Interactions

Developmental phytohormones regulate many physiological responses in plants, including growth and immunity in response to pathogens. Disease resistance mediated by developmental phytohormones such as cytokinin (CK), gibberellin (GA), and auxin (Aux), is the result of complex interactions between these phytohormones with the salicylic acid (SA)-jasmonate (JA)-ethylene (ET) backbone of immune signaling circuitry. Based on the available findings concerning phytohormone mediated plant resistance, this review attempts to develop the understanding of the defense roles of CKs, GAs, and Auxs, and their interplay with pathogen targeted defense hormones such as SA, JA and ET in plants, during biotic stress. Several phytohormone pathway genes and proteins have been reported to influence biotic stress resistance. In our analysis, we relate to specific roles uncovered for the Aux pathway TRANSPORT INHIBITOR RESISTANT 1 (TIR)/AUXIN SIGNALING F-BOX (AFB) and Gretchen Hagen 3 (GH3) proteins, the CK pathway isopentenyl transferase (IPT), CK oxidase (CKX), and Arabidopsis histidine kinase (AHK) proteins, and the GA pathway DELLA and GA20 oxidase proteins, in plant immunity and disease resistance. Additionally, we discuss microbial phytohormones that can enhance plant growth and resistance against pathogens, and the effect of phytohormones on modulation of the plant microbiome. Finally, we provide a concise overview of the possible approaches for modulating hormonal cross talk to improve plant health and resistance.