Phytohormones as Fundamental Regulators of Plant–Microbe Associations Under Stress Conditions

Abstract

Stress conditions, be they abiotic or biotic, have detrimental impacts on agricultural yields. They also slow down bioremediation and lead to changes in ecosystems. These effects are primarily caused by rapid climate change due to various different factors and activities. To adapt to climate change conditions, plants have developed complex physiological and molecular mechanisms to prevent disaster. Phytohormones produced by root-associated microbes are essential for plant growth and also contribute to stimulation of plant tolerance of various stresses. Hormones act either by activating secondary messengers or via phosphorylation cascades involved in gene regulation. The roles of microbes under various types of environmental stress can be appreciated with a particular focus on production of phytohormones and their associations with host plants. Moreover, they also contribute to tolerance of biotic stresses such as pathogenic organisms via activation of induced systemic resistance and systemic acquired resistance mechanisms in plants. The combination of plants, plant growth–promoting microbes and phytohormones represents a tripartite consortium to provide a suitable environment for the spread of beneficial microbes, which, in turn, enhance plant growth. However, the association of such microbes with plants for management of stresses in agricultural systems still needs to be explored in greater depth.