Abiotic stresses such as drought, salinity, extreme temperatures and oxidative stresses adversely affect plant growth and productivity (Fahad et al. 2017). Many of these stresses, either individually or in combinations, take a heavy toll on agricultural productivity in most parts of the semi-arid tropics (McCartney and Lefsrud 2018). In recent decades, the effects of such abiotic stresses have been further exacerbated by unprecedented changes in climate (Fedoroff et al. 2010). Conventional crop improvement approaches to render plants tolerant to abiotic stresses and resilient to climate change have been met with limited success, primarily due to the combination of the stressors and the multitude of plant traits involved in determining tolerance. A more recent and exciting approach has emerged from the use of endophytic fungi to alter plant responses and adaptation to abiotic stresses.
Endophytic fungi coexist with plants without causing any apparent disease symptoms. Several studies have demonstrated the role of endophytic fungi in enhancing plant fitness, within both, normal and stressful environments (Abdelaziz et al. 2017). The rationale for an endophyte-based adaptation rests on the fact that the endophytes adapt to environmental adversities faster than their host plants, and often are also able to collaterally share such adaptations with their respective host plants (Suryanarayanan et al. 2017). Careful exploitation of endophytic fungi could offer a strategic approach to alleviate stress effects in non-host plants, such as crop species (Rodriguez et al. 2008). Here, we briefly review studies that have explored the use of endophytes in modulating plant responses to abiotic stresses and discuss how these could potentially be used to mitigate abiotic stressors in crop plants.
Abiotic stresses such as drought, salinity, extreme temperatures and oxidative stresses adversely affect plant growth and productivity (Fahad et al. 2017). Many of these stresses, either individually or in combinations, take a heavy toll on agricultural productivity in most parts of the semi-arid tropics (McCartney and Lefsrud 2018). In recent decades, the effects of such abiotic stresses have been further exacerbated by unprecedented changes in climate (Fedoroff et al. 2010). Conventional crop improvement approaches to render plants tolerant to abiotic stresses and resilient to climate change have been met with limited success, primarily due to the combination of the stressors and the multitude of plant traits involved in determining tolerance. A more recent and exciting approach has emerged from the use of endophytic fungi to alter plant responses and adaptation to abiotic stresses.
Scores of studies have examined the role of endophytes in enabling plant adaptation to abiotic stresses. One of the most extensively studied fungi is Piriformospora, a root endophyte, isolated from woody shrubs of the Thar Desert, India (Varma et al. 1999). Recently, following a taxonomic revision, the fungus has been renamed Serendipita indica (Weiß et al. 2016). The fungus readily colonizes a wide array of plants and imparts tolerance to abiotic stresses such as drought, salinity, osmotic and heavy metals (Hosseini et al. 2017). Aside from conferring adaptation to abiotic stresses, S. indica colonization of soybean plants was shown to improve plant growth and also nutrient acquisition (Bajaj et al. 2018). Under salinity stress, maize plants colonised by S. indica produced higher biomass and maintained higher shoot potassium ion content compared to un-inoculated plants (Yun et al. 2018). Using rice as a model system, studies have shown that endophytic fungi from salt-adapted plants enhance growth and yield of salt sensitive rice varieties under salinity stress when compared to plants not colonized by such fungi (Redman et al. 2011; Yuan et al. 2016). Sangamesh et al. (2017) demonstrated the ability of endophytes isolated from plants adapted to deserts to not only successfully colonize non-host plants such as rice, but also to impart thermo-tolerance to them under laboratory conditions. In a meta-analysis conducted on 94 strains of endophytes and 42 host plants, Rho et al. (2018) reported that, overall, endophyte colonization led to effective mitigation of drought and salinity stress as well as nitrogen deficiency. The study also showed the ability of endophytes to readily colonize and establish plant–endophyte relationships. The existing evidences suggest that endophytes from stress-adapted plants could be transferred across plants of varied phylogenetic affiliations.
The immediate physiological and molecular basis of plant-endophyte interactions, including how endophytes from plants adapted to extreme habitats are able to confer resistance to non-host plants, are only beginning to be addressed. A cyclophilin A-like protein (PiCypA) obtained from Serendipita indica has been implicated in the ability of the fungus to impart salinity tolerance. Transgenic tobacco plants overexpressing PiCypA exhibited higher salt stress tolerance compared to wild type plants (Trivedi et al. 2013). A salt responsive gene, PiHOG1 from S. indica, was shown to impart osmotic and salt stress tolerance to rice plants when compared to mutants in-vitro. Treatment of S. indica knock down, KD-PiHOG1 resulted in decreased colonization and reduced tolerance to salt stress (Jogawat et al. 2016). Arabidopsis plants inoculated with S. indica exhibited ion homeostasis under salt stress. These plants had higher transcript levels of high affinity potassium transporters, HKT1 and inward rectifying K? channels, KAT1 and KAT2, compared to plants without the fungus (Abdelaziz et al. 2017). Under both normal and low phosphorous conditions, S. indica activated signaling, transport, metabolic and developmental programs in roots of Arabidopsis (Bakshi et al. 2017). The fungus was also shown to cause global reprogramming of host metabolic compounds and metabolic pathways in Chinese cabbage (Hua et al. 2017).
It is evident from existing studies that endophytes offer a promising option to mitigate abiotic stresses in crop plants. The single most important advantage of this approach is that it offers a non-genetic-invasive method to alter plant phenotype, when compared to conventional and molecular breeding approaches (Gopal and Gupta 2016); furthermore, it is rapid and cost-effective. Several initiatives have been launched to harness the power of endophytes, including BioEnsure, a product approved for use by the US Food and Drug Administration, and Rootonic, a mixture of S. indica biomass and magnesium sulphate (Jones 2013; Shrivastava and Varma 2014). The product Bioensure was able to stabilize yields of maize under drought, increase seed germination under freezing stress several-fold, and improve the water use efficiency of plants (Jones 2013). Rootonic treatment to seeds provided multifarious benefits to the plants, under both normal and stressful conditions (Shrivastava and Varma 2014). Rapid methods of deployment of select endophytes, either through seed priming or through foliar or floral dip methods, could offer a quick and safe agronomic strategy to mitigate abiotic stress in plants. This approach also resonates in its application to major crop plants that may have lost many of their native endophytes during the process of domestication. For these plants, the endophyte-enrichment approach could in fact be a process of returning to their roots (Oyserman et al. 2018).