Is Endophytic Colonization of Host Plants a Method of Alleviating Drought Stress? Conceptualizing the Hidden World of Endophytes

Abstract

In the wake of changing climatic conditions, plants are frequently exposed to a wide range of biotic and abiotic stresses at various stages of their development, all of which negatively affect their growth, development, and productivity. Drought is one of the most devastating abiotic stresses for most cultivated crops, particularly in arid and semiarid environments. Conventional breeding and biotechnological approaches are used to generate drought-tolerant crop plants. However, these techniques are costly and time-consuming. Plant-colonizing microbes, notably, endophytic fungi, have received increasing attention in recent years since they can boost plant growth and yield and can strengthen plant responses to abiotic stress. In this review, we describe these microorganisms and their relationship with host plants, summarize the current knowledge on how they "reprogram" the plants to promote their growth, productivity, and drought tolerance, and explain why they are promising agents in modern agriculture.

Keywords: antioxidants; bio-priming; drought stress; endophytes; epigenetic effects; hormones; metabolites; osmolytes; water relations.

Figure 1

The response of plants to drought stress. The plant’s internal structure under drought stress is schematically described, showing the intracellular signal transduction pathways, along with the molecular regulation mechanism of plants. Drought-regulating substances, reactive oxygen species (ROS), and scavenging processes in plants are shown. An analysis of physiological and biochemical responses relevant to osmotic regulation metabolism, drought-induced protein metabolism, and reactive oxygen metabolism is shown (modified from Lamaoui et al. [102]; Zenda et al. [103]; Yang et al. [78]).

Figure 2

Endophyte-mediated drought tolerance in plants. As part of a plant–endophyte association, the latter employs a variety of strategies to mitigate the abiotic stresses from the host’s natural habitat. A lack of nutrients in the habitat can be countered by mechanisms such as the production of growth-promoting hormones, other compounds, and nitrogen fixation by plant endophytes. As an additional benefit, some endophytes can go a step further and provide host plants with crucial protection against harsh environmental conditions by activating antioxidant enzymes and the production of stress-responsive molecules (modified from Vurukonda et al. [158]; Ullah et al. [159]; Verma et al. [160]).