**6. Conclusion**

*Sustainable Crop Production*

NaCl solution for 14 days.

geothermal soils [72].

**5.4 Heavy metal stress**

**5.3 Temperature stress (low and high)**

High salinity due to extreme climatic conditions and misuse of agricultural land over the past few decades has led to high salinity, which is a limiting factor to global agricultural productivity [97]. Soil salinity is the accumulation of water soluble salts in soil that affects its physical and chemical properties thereby reducing soil's agricultural output [98]. Reactive oxygen species (SOD, CAT, APX) are formed in plants on onset of salt and osmotic stress. Endophytic *Piriformospora indica* induces salt stress tolerance by elevation of antioxidant enzymes [99]. These are involved in the removal of reactive oxygen species either directly or indirectly via regeneration of ascorbate and glutathione in the cell. Experimental studies by Rodriguez et al. [100] reported that constant exposure of non-symbiotic plants dunegrass (*Leymus mollis*) to 500 mmol/l NaCl solution, became severely wilted and desiccated within 7 days and were dead after 14 days. In contrast, symbiotic plants infected with *F. culmorum* showed wilting symptoms only after they were exposed to 500 mmol/l

High temperature is a major obstacle in crop production that results in major cellular damage such as protein degradation and aggregation [101]. Whereas, low temperature can cause impaired metabolism due to inhibition of enzyme reactions, interactions among macromolecules, changes in protein structure, and modulating cell membrane properties [102]. Endophytic, *Curvularia* spp. is proven to confer thermal tolerance ability plants like tomato, watermelon, and wheat [103]. Herbal plant wooly rosette grass (*Dichanthelium lanuginosum*) that lives in the areas where soil temperatures can reach up to 57°C, the presence of endophytic fungi *Curvularia* sp. protects the plant from temperature stress better than endophyte free plants [104]. Experimental demonstration by Redman et al. [103] showed that grass *D. lanuginosum* survival in soil temperatures ranging between 38 and 65°C is directly linked to its association with the fungus *C. protuberata* and its mycovirus, *Curvularia* thermal tolerance virus (CThTV). Moreover, cold stress tolerance was conferred in germinated seeds of rice under laboratory conditions by *C. protuberata* isolated from *D. lanuginosum* thriving in

Heavy metal contamination due to increased industrialization has recently received attention because heavy metals cannot be itself degraded [105]. Toxicity by heavy metals can cause the loss of about 25–80% of various cultivated crops. Heavy metals being very toxic to roots of cultivated crop plants can cause poor development of the root system [106]. Endophytic fungi possess metal sequestration or chelation systems that increases tolerances of their host plants to heavy metals via enhancements of antioxidative system thereby changing heavy metal distribution in plant cells and detoxification of heavy metal, thus assisting their hosts to survive in contaminated soil [107, 108]. For instance, dark septate root endophytes (DSEs), *Phialocephala fortinii* can produce the black biopolymer melanin, which can be synthesized from phenolics and binds heavy metals [109] that keep heavy metal ions away from living, plant cells [110]. Siderophores being metal-chelating compounds [111, 112] released from roots into the rhizosphere can be helpful in inhibiting absorption of heavy metals into plant cells as siderophores can form complexes with heavy metals which are not easily absorbed by plant

**5.2 Salinity stress**

**218**

Fungal endophytes can be a significant component of sustainable agriculture, being safe, cost-effective, have ability to produce various compounds like phytohormones, defensive compounds, solubilize phosphates, extracellular enzymes, siderophore production, inhibiting plant pathogens, and promoting plant growth. Over the last decade, sharp rise in study of fungal endophytes is seen as they hold huge potential in agricultural sector. However, most of the research on endophytes is still at an experimental level in lab or greenhouse. For permitting the practical use of these endophytes in agriculture it is extremely necessary to encourage field experiments to determine the effectiveness of the endophytes under real world conditions. Simultaneously, it is also necessary to build awareness of this new research field among farmers to improve interactions and collaboration with scientists working in different fields, thereby encouraging the adoption of endophytes in agriculture and maximizing their benefits. If endophytes become feasible in agricultural sector, their practical aspects will also have to be researched so that farmers can learn how to integrate these endophyte species within pre-existing eco-friendly agricultural methods so as to ensure continuity in the approach to sustainability. Moreover, scientific research has to be also focused on use of genetically modified endophytes made by combining endophytes having two or more different ecological roles, such as the suppression of diseases and insect pests to simultaneously improve plant yields and its defensive properties. Thus, optimization of microbial functions to enhance crop production and protection is also required.
