**Abstract**

Fungal endophytes that live inside plant tissues without causing any apparent symptoms in the host plant are important components of plant micro-ecosystems. Endophytic fungi confer profound impacts on their host plants by enhancing their growth, increasing their fitness, strengthening their tolerances to pests and diseases. Moreover, fungal endophytes symbiotic with host plant produce a plethora of bioactive secondary metabolites that are expressed as defensive weapons to protect the host plant against various abiotic stresses. Currently, main focus in endophytic fungi research is associated with the ability of these microorganisms to produce and accumulate biologically active metabolites as these are potent source of novel natural products useful in agriculture sector.

**Keywords:** fungal endophyte, symbiosis, secondary metabolites, stress, sustainable agriculture

## **1. Introduction**

Over reliance of synthetic pesticides in crop fields from late 1940 to mid-1960s resulted in a number of adverse environmental impacts such as secondary pest outbreak, insect resurgence, effects on non-target organisms, residual problem, environmental pollution, prompted an urgent need for alternative tactics to help make crop protection more sustainable. Biological control using micro-organism has gained much interest, being specific, low relative cost and low risk to ecosystem [1]. Among the various micro-organisms, endophytic fungi can make the chemical intensive crop production system more sustainable as it has ability to enhance plant growth, yield and increase plant fitness by providing biotic and abiotic stress tolerance [2, 3]. Endophytes ("endo" = within, "phyte" = plant) are the microorganisms that inhabit interior of plants especially leaves, stems, roots without causing any apparent harm to the host [4]. These are ubiquitous having rich biodiversity and found in every plant species as nearly 3,00,000 plant species exist on earth with each individual plant host having one or more than one endophytes [5]. Endophytic fungi are considered as plant mutualists as they receive nutrition and protection from host plant while the host plant may benefit from enhanced competitive abilities and increased resistance to herbivores, pathogens and various abiotic stresses [6]. It spends whole or part of their life cycle colonizing inters- and/or intracellularly within the healthy tissues of the host plant without causing visible signs

of infection [7, 8]. Moreover, fungal endophytes have gained significant interest in sustainable agriculture due to their great potential to contribute to secondary compounds with unique structure, including alkaloids, benzopyranones, chinones, flavonoids, phenolic acids, quinones, steroids, terpenoids, tetralones, xanthones, etc. [9–11] produced by the fungi or by the plant due to interaction with the fungi. Among the microorganisms, fungal endophytes are the largest group producing secondary metabolites. Fungal toxins produced by these biotic metabolites contribute to plants tolerance towards various biotic and abiotic stresses. Fungal endophytes are known to produce bioactive compounds toxic to insects, nematodes, produces extracellular enzymes (cellulases, proteinase, lipases, esterases) for degradation of dead soil biomass, solubilize insoluble phosphates and produce plant growth-promoting hormones (auxins, cytokinins, gibberellins). Endophyte infected plants manage plant growth under adverse conditions of drought, salinity, temperature and heavy metal stress through different mechanisms. This chapter outlines various approaches for the use of endophytic fungal inoculants to combat various stresses in agricultural fields, thus increasing global crop productivity.
