Multifaceted Interactions Between Endophytes and Plant: Developments and Prospects

Abstract

Microbial endophytes are present in all known plant species. The ability to enter and thrive in the plant tissues makes endophytes unique, showing multidimensional interactions within the host plant. Several vital activities of the host plant are known to be influenced by the presence of endophytes. They can promote plant growth, elicit defense response against pathogen attack, and can act as remediators of abiotic stresses. To date, most of the research has been done assuming that the interaction of endophytes with the host plant is similar to the plant growth-promoting (PGP) microbes present in the rhizosphere. However, a new appreciation of the difference of the rhizosphere environment from that of internal plant tissues is gaining attention. It would be interesting to explore the impact of endosymbionts on the host's gene expression, metabolism, and other physiological aspects essential in conferring resistance against biotic and abiotic stresses. A more intriguing and inexplicable issue with many endophytes that has to be critically evaluated is their ability to produce host metabolites, which can be harnessed on a large scale for potential use in diverse areas. In this review, we discuss the concept of endophytism, looking into the latest insights related to the multifarious interactions beneficial for the host plant and exploring the importance of these associations in agriculture and the environment and in other vital aspects such as human health.

Keywords: abiotic stress; biotic stress; endophytes; genomics; metabolomics; secondary metabolites.

FIGURE 1

Pictorial representation showing multifaceted interaction of endophytes with host plants. (1) Fungal endophytes change chemical and physical characteristics of the leaf such as high-cellulose content and lamina density, which provide toughness resulting in reduced herbivory rates, specifically by leaf–cutting ants. (2) Endophytes prime the host plant’s defensive responses against phytopathogens. Early detection of the phytopathogen by cell surface receptor kinases (RK) and subsequent cytoplasmic kinases (CK) mediate intracellular responses and trigger ethylene/jasmonic acid transduction pathway. (3) Abiotic and biotic stress (signals positively induce expression of the stress-responsive genes, preinvasion defense, and enhanced callose deposition. However, ABA affects negatively signals that trigger systemic acquired resistance. The endophyte significantly modulate stress through the downregulation of ABA. Gibberellins synthesized by plants or endophytes hamper the inhibitory effects of DELLA proteins over the plant-growing signals. (4) Reactive oxygen species (ROS), generated by the plant, are neutralized by the production of enzymes such as superoxide dismutases (SOD), catalases (CatA), peroxidases (POD), alkyl hydroperoxide reductases (AhpC), and glutathione-S-transferases (GSTs ) in endophytes. (5) Protein secretion systems (SSs), which deliver effector proteins (EF) into the plant are either absent or present in low abundance in mutualistic endophytic bacteria. Endophytes also encode specific genes for utilizing aspartate/maltose and dipeptides metabolism. (6) Fungal endophytes modulate the plant’s immune system by the production of chitin deacetylases, which deacetylate chitosan oligomers and, hence, prevent themselves from being recognized by chitin-specific receptors (PR-3) of the plants that recognize chitin oligomers. Perception of flagellin (FLS 2) from endophytes also differs from phytopathogens. (7) Endophytic microbes alleviate metal phytotoxicity via extracellular precipitation, intracellular accumulation, sequestration, or biotransformation of toxic metal ions to less toxic or non-toxic forms. Where, RK, receptor kinase; CK, cytoplasmic kinase; ET, ethylene; JA, jasmonic acid; SK, sensor kinase; GA, gibberellic acid; DELLA, DELLA protein; ABA, abscisic acid; SA, salicylic acid; HSP, heat shock protein; ROS, reactive oxygen species; SOD, superoxide dismutases; CatA, catalases; POD, peroxidases; AhpC, alkyl hydroperoxide reductases; GSTs, glutathione-s-transferases; EF, effector protein; PR-3, chitin-specific receptors; FLS 2, flagellin; TTSS, type III secretion system; SS, secretion system; MT, metal transporters; IC, ion channels; CW, bacterial cell wall.)