Crosstalk amongst phytohormones from planta and PGPR under biotic and abiotic stresses

Phytohormones are endogenously produced organic substances indispensable for regulating plant growth and yield and also play major role in inducing tolerance to plants against various biotic and abiotic stresses. The convergence points among hormone signal transduction cascades are considered as cross-talk which are crucial for plant development as well as for plant responses to biotic and abiotic stresses. Hormones interact by activating either a second messenger or through a phosphorylation cascade. These transduction cascades lead to the regulation of gene expression that directly affects the biosynthesis or action of different hormones and developmental processes in coordination with multiple stimuli. Hormone synthesis, signal transduction, perception and cross-talk create a complex network. Interaction of plant growth promoting rhizobacteria (PGPR) which form intimate association with the roots of higher plants also modulate the level of endogenous phytohormones and demonstrate a new paradigm for hormonal interaction. The ratio of hormones changes with ontogeny of plant and the specific ratio of growth promoting and growth inhibiting hormones determine the response of plants. Furthermore, the sensitivity of plant tissue to each hormone changes with the exposure to stresses. This review is a compilation of the interactions between phytohormones and plant development. The cross talk between different hormones under abiotic and biotic stresses will be enumerated. Hormone and stress-responsive cis elements and the trans-regulation capabilities of miRNAs for the coordination of multiple hormonal responses will be discussed. Finally the role of PGPR will be evaluated under various environmental stresses with particular emphasis on phytohormone production and its interaction with host plant physiology. PGPR provides cross protective properties through improvement in defense mechanism controlling pathogen resistance through induced systemic resistance (ISR) and alleviating abiotic stress through influencing the phytohormones metabolism. PGPR isolates from stressed soil/stressed host plants impart tolerance to plants against abiotic and biotic stresses by modulating the production of phytohormones and alteration in their sensitivity to respond. Bacteria communicate with each other through quorum sensing molecules which also regulate gene expression and phytohormone production. The intricate relationship between other microbes/fungi and their residual effects on plant rhizosphere phytohormones need further investigation for better understanding of bacterial coordination with plants.