The genus Bacillus is one of the predominant bacterial genera found in soil, and several species of this genus have been reported from diverse ecological niches. Endowed with tremendous genetic and metabolic diversity, Bacillus spp. serve multiple ecological functions in soil ecosystem from nutrient cycling to conferring stress tolerance to plants. Members of the genus Bacillus are known to have multiple beneficial traits which help the plants directly or indirectly through acquisition of nutrients, overall improvement in growth by production of phytohormones, protection from pathogens and other abiotic stressors. This functionally versatile genus is one of the most commercially exploited bacteria in the agro-biotechnology industry. Still its potential has not been realized sufficiently and requires an emphasis towards translating the relevant technologies from laboratory to land for the benefit of mankind.

In this review, a comprehensive discussion exclusively on bacterial xylanases; their gene organization; different factors and conditions affecting enzyme yield and activity; and their commercial application have been deliberated in the light of recent research findings and extensive information mining. Improved understanding of biological properties and genetics of bacterial xylanase will enable exploitation of these enzymes for many more ingenious biotechnological and industrial applications.

Soil microorganisms play an important role in enhancing soil fertility and plant health. Arbuscular mycorrhizal fungi and plant growth promoting rhizobacteria form a key component of the soil microbial population. Arbuscular mycorrhizal fungi form symbiotic association with most of the cultivated crop plants and they help plants in phosphorus nutrition and protecting them against biotic and abiotic stresses. Many species of Bacillus occurring in soil are also known to promote plant growth through phosphate solubilization, phytohormone production and protection against biotic and abiotic stresses. Synergistic interaction between AMF and Bacillus spp. in promoting plant growth compared to single inoculation with either of them has been reported. This is because of enhanced nutrient uptake, protection against plant pathogens and alleviation of abiotic stresses (water, salinity and heavy metal) through dual inoculation compared to inoculation with either AMF or Bacillus alone.

Endophytic bacilli of ethano-botanical plant Ocimum tenuiflorum were screened for salt stress-alleviating traits in tomato. Four promising O. tenuiflorum endophytes (Bacillus safensis BTL5, Bacillus haynesii GTR8, Bacillus paralicheniformis GTR11, and Bacillus altitudinis GTS16) were used in this study. Confocal scanning laser microscopic studies revealed the inter-genera colonization of O. tenuiflorum endophytes in tomato plants, giving insights for widening the applicability of potential endophytes to other crops. Furthermore, in a pot trial under 150 mM NaCl concentration, the inoculated endophytes contributed in reducing salt toxicity and improving recovery from salt-induced oxidative stress by different mechanisms. Reduction in reactive oxygen species (ROS) (sub-cellular H2O2 and superoxide) accumulation was observed besides lowering programmed cell death and increasing chlorophyll content. Endophyte inoculation supplemented the plant antioxidant enzyme system via the modulation of enzymatic antioxidants, viz., peroxidase, ascorbate peroxidase, superoxide dismutase, and catalase, apart from increasing proline and total phenolics. Antioxidants like proline have dual roles of antioxidants and osmoregulation, which might also have contributed to improved water relation under elevated salinity. Root architecture, viz., root length, projection area, surface area, average diameter, tips, forks, crossings, and the number of links, was improved upon inoculation, indicating healthy root growth and enhanced nutrient flow and water homeostasis. Regulation of Na+/K+ balance and water homeostasis in the plants were also evident from the modulation in the expression of abiotic stress-responsive genes, viz., LKT1, NHX1, SOS1, LePIP2, SlERF16, and SlWRKY39. Shoot tissues staining with light-excitable Na+ indicator Sodium GreenTM Tetra (tetramethylammonium) salt showed low sodium transport and accumulation in endophyte-inoculated plants. All four endophytes exhibited different mechanisms for stress alleviation and indicated complementary effects on plant growth. Furthermore, this could be harnessed in the form of a consortium for salt stress alleviation. The present study established inter-genera colonization of O. tenuiflorum endophytes in tomato and revealed its potential in maintaining Na+/K+ balance, reducing ROS, and improving root architecture under elevated salinity.

The intensive use of chemical fertilizers, monoculture and irrigation with surface saline water has resulted in the deterioration of soil health by enhancing the level of salinity in Eastern IndoGangetic Plains of India. Therefore, diversity of halotolerant bacteria adapted to that environment and possessed the ability to produce plant growth hormones was explored, that could be used for salt stress amelioration. 16S rRNA gene sequencing and fatty acid methyl ester (FAME) were used for diversity analysis of salt tolerant bacilli. Among the 95 isolates, 55 strains showed plant growth promotion traits, production of industrially important enzymes (amylase, protease and cellulase) and tolerance to more than 4% NaCl. Using partial 16S rRNA sequences and FAME comparisons 21 different species of Bacillus and Bacillus derived genera were identified viz. Bacillus megaterium, B. subtilis, B. licheniformis, B. firmus, B. horikoshii, B. pumilus, Bacillus sp., B. safensis, B. thuringiensis, B. simplex, B. agri, B. flexus, B. oceanisediminis, B. cereus, B. arsenicus, Paenibacillus dendritiformis, Lysinibacillus sp., L. sphaericus, B. marisflavi, Terribacillus sp., and B. mycoides. These isolates possess ability to tolerate high salt, endospore former, can sustain under harsh environment and also have the potential for plant growth promotion, which could be useful in formulation of new inoculants, to enhance the availability of nutrients for crop growth under saline conditions.