Sustainable Agroecosystem: An Outcome from Healthy Plant–Microbe Association

Plant growing under field conditions is a complex interaction between plant and rhi- zosperic microbes (Lundberg et al. 2012). The well-defined microbial community is

Various stress factors affecting plant growth and PGPRs combating environmental stress

FIGURE 1.5 Various stress factors affecting plant growth and PGPRs combating environmental stress.

TABLE 1.2

Seed Yield (kg ha-1) of Red Gram Influenced by Native Rhizobium Inoculation (Sethi 2019)

Treatments

CHRS-7*

RAN-1

RAB-1

MEAN-N

N„*

902

682

759

781

n5

1529

1001

968

1187

Njoo

1793

1331

1463

1529

N,50

1749

1309

1441

1500

MEAN-S

1493

1081

1158

LSD(P=0.05)

S=52

N=60

SXN=103

* CHRS-7(native strain), RAN-1 and RAB-1 are isolated Rhizobium strains

# Level of N application (N0- No nitrogen, N50- 50%, N100- 100%, and N|W- 150% of N requirement)

always associated with the crop and these communities are phytomicrobiomes; the phytomicrobiomes with a plant is the holobiont (Bulgarelli et al. 2015; Smith et al. 2017). This microbial community has been associated with terrestrial plants with challenges to access nutrients, novel and often-stressful conditions, and pathogens. There are elements (including bacteria and fungi) of the phytomicrobiome associated with all major plant structures (flowers, fruits, stems, leaves, roots). The most comprehensible example is the nitrogen-fixing rhizobia associated with legumes (Gray and Smith 2005). Many members of the phytomicrobiome cannot be cultured, and it has only been since the advent of metagenomics (Hirsch and Mauchline 2012) and related methods that we are able to assess how membership is changed by conditions, plant genotype (Delaplace et al. 2015; Poli et al. 2016; Wintermans et al. 2016), and plant development. Current microbes differ in their efficiency, and are used for the production of crops and enhancement of soil fertility for sustainable agriculture. The stress-tolerant microbe’s inoculation increases yield of pulse up to 20-30% over an uninoculated crop. The enhancement red gram yields up to 35% over nonnative rhizobium inoculation in an acidic condition (Table 1.2) This is due to the easy acclimatization of the inoculated native strain in comparison to non-native strains (Sethi 2019).

The yield and quality of green gram seed, black gram seed, okra fruit, finger millet grain, berseem grain (Pattanayak 2012, 2016) rice grain (Pattanayak et al. 2007, 2006a, b) increase due to the application of microbes in the plant rhizosphere, with integrated application nutrients and soil amendments for maintaining soil health.

1.6 POTENTIAL OF PLANT-MICROBE INTERACTION FOR

Potential of Plant–Microbe Interaction for Developing a Low-Input Sustainable Agriculture

Inorganic fertilizers are not cost-effective and, on the other hand, its injudicious use worsens soil health. The application of organic fertilizers or inoculation of plant growth-promoting rhizobacteria enhances root activity by producing different exopolysachharides, which create a suitable rhizosphere environment for microbe and host root proliferation.

As with diazotrophs, the production of growth hormones assists with amelioration of the root surface area, which may have indirect effects on the P-solubilizing ability of the bacteria (Sharma et al. 2016). It appears that the evaluation of ability to solubilize P in vitro does not participate as efficiently as under field conditions. Thus, it may be concluded that the biofertilizers with dual action might have been mediated by directly solubilizing the inorganic source of P, mineralizing organic P, and stimulating root growth or formation of mycorrhiza.

The significant quantities of N can be supplied by rhizobacteria in crop plants (Dobbelaere et al. 2003). However, inoculation of Azospirillum sp. in cereal (wheat, sorghum, and maize) contributed 5 kg N lur'year-1. This quantity is extremely small in comparison to 150-200 kg N ha_lyear' (as is common practice in modern agriculture). Contribution of free-living rhizobacteria for crop plants in Australia is even less than 10 kg N ha_lyear' (Unkovich and Baldock 2008). Peoples et al. (2002) also pointed out that the range of N2 fixation varied from 0-15 kg N ha-1 year-1. Further, value of N, fixation ranged between 1-10 kg N lur'year-1 as suggested by Bottomley and Myrold (2007). Therefore, the ability of plant-growth-promoting bacteria to fix atmospheric N, is considered an important criterion when classifying them as a biofertilizer which can be supplemented for enhancing the bioavailability of nutrients to crops. Thus, it can be used as an input for sustainable agriculture.

Conclusion

Microbes of the phytomicrobiome form a holobiont in combination with the plant, and this plant-microbe association provides a wide range of resources and benefits to the plant, as well as the microbial community. The plant-microbe association is of great importance to agriculture, owing to the rich diversity of root exudates and plant cell debris that attracts diverse and unique patterns of microbial colonization. Microbes of the rhizomicrobiome play key roles in enhancing plant growth by nutrient acquisition and assimilation, improved soil structure, secreting and modulating extracellular molecules such as hormones, secondary metabolites, antibiotics, and various signal compounds. The plant-microbe intercommunication can improve the dynamics and efficiency of many nutrients, as well as sustained growth and yield of crop plants, due to interactive influences of the soil, plant, and environmental factors, as described in this chapter. Any one or more of these factors may adversely affect the availability of nutrients. Along with these, PGPR enhances plant growth and productivity by enhancing various signaling mechanisms (direct and indirect). Studies have been undertaken of the healthy plant-microbe association, whereby crops escape from various abiotic as well as biotic stresses, thereby providing optimum conditions for their growth throughout their life cycle. The role of plant- microbe interaction in phytoremediation, bioremediation, and rhizoremediation has also been thoroughly discussed in this chapter. Ultimately, the process leads to a sustainable agroecosystem. How the interaction between plant and microbes could help make steps towards a sustainable agroecosystem has been thoroughly discussed in this chapter. An understanding of plant-microbe interaction and intercommunication is of the utmost importance when making efforts towards a sustainable ecosystem - and even towards a very predictable agroecosystem. Plants in companionship with rhizobial microbes can lead to a significantly more productive and sustainable agroecosystem, which in turn will support the life of all living organisms.

 
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