**2.2 Role of plant growth-promoting bacteria (PGPR) and fungi (PGPF) in sustainable agriculture**

In a comprehensive literature review on role of Plant Growth-Promoting microorganisms in Sustainable Agriculture and Environmental Remediation [42], highlighted the most influential accounts of soil microorganisms, particularly the rhizobacteria that have the potential to influence growth, yield and nutrient uptake by crops either directly or indirectly as well as maintain soil fertility and health.

PGPR can enhance the plant tolerance by promoting the plant growth, even in poor growth conditions and increase agricultural produce of different crops under stressful environment [43–45]. Apart from the above-mentioned facts, recent reports suggest that application of PGPRs also improves nutritional quality and antioxidant status of the crops [46, 47]. Harnessing the above-mentioned plantmicrobe interactions can also help in reclamation of degraded lands, reduction in usage of chemical fertilizers and agrochemicals [48]. A prominent agricultural symbiotic association exists between the rhizosphere bacteria and roots of the legumes by the formation of root nodules. Previous studies showed that plantfungal associations are much older than the rhizobia-legume interaction. In various plant-fungal interactions fungi help in phosphate acquisition and make it available to plants [49]. Some reports also indicate that the DMI-2 protein is required for the initiation of the plant-arbuscular mycorrhizae interaction, which helps in phosphate solubilization. Although, the underlying mechanism of the PGPR and PGPF interactions with the plants are quite different, some studies showed a similarity between them. In *Medicago truncatula*, a type of arbuscular mycorrhizal fungi (AMF), the interaction releases some small diffusible factors to activate the similar genes by the rhizobacterial nod factor [50]. This confirms the analogy of the PGPR and PGPF to some extent and needs further clarification from cutting-edge research on the topic. Supporting this view [51], stated that plants use receptor-like kinases to monitor environmental changes and transduce signals into plant cells. The study further added that *M. trancula* DOES NOT MAKE INFECTION 2 (DMI2) protein functions as a co-receptor of rhizobial signals to initiate nodule development and rhizobial infection during nitrogen-fixing symbiosis. However, the mechanisms regulating DMI2 protein level and folding associated with arbuscular mycorrhizae are still unknown.

Mutualistic association (co-inoculation) of PGPR and PGPF (arbuscular mycorrhizae fungi) increases the growth, nutrient uptake potential, and yield of the plants [52]. PGPF can directly enhance the nutrient uptake (P, Zn) and water use efficiency of the inhabiting plant by increasing the root surface with hyphal network. With increased water use efficiency, AMF also controls the N2O emission, which is a potent greenhouse gas emitted from agricultural fields [53]. AMF alone or in combination with certain PGPR enhances plant growth indirectly by inhibiting growth of root pathogens and optimizing soil structures [54]. Apart from this PGPF can also regulate soil health and fertility by improving the overall soil nutrient dynamics [55].

The negative effect of climate change is also mitigated by AMF through maintenance of proper soil aggregation and thereby providing another major advantage to agricultural crop production. More and more studies show that the mycorrhizae can play an essential role in plant growth by enhancing plant vigor in poorly performing soils, and through their ability to store large amounts of carbon, which in turn may improve some of the effects of climate change. In conclusion, we can say that

application of PGPR and PGPF in combination or alone can negate the hazardous effect of chemical fertilizers, improve soil health, reduce environmental stresses and promote sustainable agriculture [56–58]. The interaction of AMF and rhizobacteria thus can promote plant growth by improving soil structural properties as well as the enhanced availability of nutrients and reduce disease progression in a sustainable manner [43].
