**1. Introduction**

In the era of sustainable crop production, the interaction between plants and soil microbes play an important role in the transformation, solubilization, mobilization, etc. of nutrients from a limited nutrient pool and make it available for the uptake of plants in order to realize their full genetic potential. Microorganisms perform numerous metabolic functions which are essential for their own maintenance and can directly or indirectly be beneficial to the biosphere through environmental detoxification, soil health improvement, nutrient recycling, waste water treatment, etc. [1].

For more than three centuries, endophytic microorganisms which colonize and reside in plant roots have been known to be in existence. Though their value in increasing crop yields and buffering environmental conditions have become appreciated in recent decades. When describing the formation of galls on roots in 1967, Malpighi reported the symbiotic association between microorganisms and plants. His report was not seen as scientific rather borne out of curiosity. After two centuries, Hellriegel and Wilfath, showed that these galls are nodules composed of both bacterial (Rhizobiaceae) and plant cells which fix N2 from the atmosphere, providing leguminous plants with an essential nutrient known as ammonia (NH3) [2]. In 1882, fungi which are presently known as arbuscular mycorrhizal fungi

(AMF) were found to symbiotically improve plants' productivity by colonizing their roots [3]. In the 1920s and 30s, *Trichoderma,* the commonly known soil-inhabiting fungi were found to biologically control pathogenic fungi, thus having the potential in protecting agricultural crops [4]. In addition to protecting plants against pathogenic fungi, some strains of *Trichoderma* have been found to induce multiple benefits to plants when they colonize their roots [5]. Another group of fungi known as *Piriformospora indica* that beneficially colonizes and inhabits plant roots was discovered in the 1990s [6]. Once these microorganisms colonize and inhabit plant roots, they induce physiological changes and modify the expression of genes in the plant they reside in, thus improving plants' productivity and resilience.
