**5. Autotrophy and nitrogen fixation capability**

*X. autotrophicus* can use H2 from thiosulfate as source of energy for CO2 fixation, when grown heterotrophically in the presence of gas mixture, *Xanthobacter* species fix CO2 mainly via the ribulose-biphosphate pathway but phosphoenolpyruvate carboxylase activity also has been reported. Have shown that the fixation of CO2 plays an important role in the degradation of aliphatic epoxides and ketones by novel carboxylases [5, 8, 10, 24]. *X. autotrophicus* fixes N2 under heterotrophic growth conditions with sucrose as a carbon source; however, N2 fixation was showed for several strains of *X. autotrophicus* with 15N2 incorporation into cell protein [12, 18]. The biochemical studies on the enzyme and its relationship to oxygen have been restricted to *X. autotrophicus*. The nitrogenase in these two strains is similar to that in other aerobic diazotrophs [2, 6, 36, 37]. There is strong variation among the strains in respect to the optimal O2 concentration for growth under N2-fixing conditions, for *X. autotrophicus*. The optimal partial pressures of O2 for acetylene reduction are 5 and 2.5 kPa to 0.36 kPa. However, the alternative vanadium nitrogenase system could not yet be shown through substantial ethane synthesis or improving its growth when vanadium is added to molybdenum deprived medium [1, 14, 38].

### **6. Natural habitats**

The known habitats of *Xanthobacter* are depending on its physiological properties, underline its catabolic versatility [39]. The sources for isolated strains include oil-contaminated soil and sludge from Japan [5], marine sediments, water and sediment samples from fresh- water lakes, soil of flooded rice fields, rhizosphere of wetland street ditches and wet meadow soil and garden soil from Europe, South Africa, North America, and Asia, sewage samples [3, 13, 40] and tree leaves [20, 41]. *Xanthobacter* is ubiquitous in microaerophilic environments with decaying organic material or matter [19] containing sufficient concentrations of H2 and CO2 and other metabolic compounds products of anaerobic microbial activity, such as organic acids and alcohols. *Xanthobacter* species are important in the microaerophilic interface between the anaerobic and aerobic habitats. Therefore, it is very likely that *Xanthobacter*, and possibly also other N2-fixing Knallgas bacteria, can be found in habitats other not yet known [42, 43]. According to literature, no thermophilic, psychrophilic, or halophilic strains have been isolated [44]. Furthermore, it is not clear whether *Xanthobacter* contributes significantly as an associative bacterium to the nitrogen cycle in agriculture issues, even though in greenhouse experiments [17, 45, 46], *X. autotrophicus* strains isolated from several environmental samples have been shown to stimulate and growth yields of rice, tomato and lettuce at reduced dose of nitrogen or phosphate fertilizer [1, 17, 38, 47].

Xanthobacter autotrophicus *an Endophytic Beneficial Bacterium for Wheat and Other Plants… DOI: http://dx.doi.org/10.5772/intechopen.102066*

## **6.1 Ecological interactions with other domestic plants**

In Japan was reported a survey of N2-fixing bacteria from roots of rice, with strains called group 2 were *X. autotrophicus* and other isolated *Xanthobacter*-like of group 5, which could be a new genus [22, 31, 48, 49]. Some of these isolations were identified as a *X. flavus* on the basis of morphological and physiological properties. Up to 25% of the nitrogen fixed by soil bacteria was incorporated into rice plants and other reported. In one soil like soils of Kasakh, Armenia *Xanthobacter* was up 40–70% were N2-fixing population they may contribute to N balance in the soil of paddy rice. Also was demonstrated that strains close to *X. autotrophicus* could be found in the sediment of patty rice fields in Arkansas, United States, with more than 105 cells per g dry weight of roots in the rhizosphere of rice clearly as an endophyte [50]. A positive interaction among Xanthobacter and some domestic crops due to enhance biomass of plant as well as nitrogen content compared to those crops without Xanthobacter at limited dose of nitrogen fertilizer [17, 40]. Therefore, *Xanthobacter* can be classified as an associative diazotroph [19, 38, 44, 51, 52]. The possible role of *Xanthobacter* as a contributor of fixed N2, a growth factor stimulant on bean [45] lettuce, tomato, rice, rootbeet, wheat, plants, and an associative N2 fixer through either the phyllosphere or even stems nodules if in the future *Azorhizobium* is incorporated into the genus *Xanthobacter* needs to be investigate [20, 41, 53]. These studies should examine: (1) the role of the slime produced by *Xanthobacter* in its adherence to the rhizosphere and phyllosphere an involvement of slime in adherence processes was shown for various anaerobic bacteria; (2) the possible role of the polyglutamine polymer produced under high-nitrogen conditions directly after nitrogen fertilization [7, 38, 54] and (3) the role of plant growth stimulant formation by root and stem-associated *Xanthobacter* cells [13, 28, 55–57]. It has been reported than cultures of *X. autotrophicus* are producing indoleacetic acid when grown in medium with tryptophan [3]. Until now there are no reports about *Xanthobacter* isolated associated with any plant disease [18].

### **6.2 Biofertilizer application of endophytic plant growth promoting bacteria in modern sustainable agriculture**

Biofertilizer is key action of organic farming and a main element for the economy in general modern agricultural production on a world scale [55, 56, 58, 59]. The biofertilizers play an important role in improving the fertility of the soil [60, 61]. In addition, their application in soil improves the structure of the soil minimizes the sole application of chemical fertilizer. Grain yield and harvest index also increase with use of biofertilizers. Inoculation with *Azotobacter* + *Rhizobium* + mycorrhizae gave the highest increase in straw and grain yield of wheat plants with rock phosphate as a P fertilizer. *Azolla* is inexpensive, economical, friendly, which provide benefit in terms of carbon and nitrogen enrichment of soil [62]. Some commercially available biofertilizers are also used for the crop. Raj [63] recorded that microorganism: *B. subtilis, Thiobacillus thioxidans*, and *Saccharomyces*) can be used as bio-fertilizers for solubilization of fixed micronutrients like Zn (zinc). As well for biological control, a modern approach of disease management a key role in sustainable agriculture [64–66]. Biofertilizers can be defined as carriers that contain living endophytic plant growth promoting bacteria (EPGPB) and/or microorganisms (EPGPM); when they are applied to seeds, plant surfaces, to soil, or to hydroponic agricultural system, they colonize the root system or interior of the plant, and to stimulate plant growth by increasing the demanding or availability of macro or micro minerals: nitrogen (N), phosphorus (P), potassium (K), cupper (Cu), iron (Fe), etc., to the host plant [67, 68]. According to Mishra et al. [69], biofertilizer could be mixture of

active or latent microbial cell for several important mechanisms to improve plant growth and yield as the well-known: nitrogen fixing, phosphate solubilizing, or cellulolytic microorganisms for applications to soil, seed, roots, or composting involving any microbial process with the aims for enhancing plant growth, augment the availability of nutrients that can then be easily absorbed by the plants, as well as for biological control of plant pathogenic agents. Malusá and Vassilev [70] proposed that a biofertilizer is the formulated product containing one or more microorganisms that enhance the mineral availability for health growth and yield profitable performance of the plants by either replacing soil nutrients and/or by making nutrients more available to plants and/or by increasing plant availability to basic minerals [66, 71].

Biofertilizer products are usually based on the EPGPB or PGPM can be classified into three main types of microorganisms: arbuscular mycorrhizal fungi or AMF [72], plant growth promoting rhizobacteria or PGPR [73], and nitrogen fixing rhizobia and free nitrogen fixing bacteria for non-leguminous plant [74, 75] which are applied and approved as beneficial for domestic crops growth based in mineral nutritional, underline reported that PGPR are recommend worldwide as biofertilizers, contributing to maintain profitable yield without soil deterioration and preventing environmental pollution. Hence, with the potential contribution of the PGPR, to sustainable agriculture and forestry when pandemic condition of COVID 19 caused economic world depress [76, 77]. Sufficient densities of PGPR and/or EPGPB like *X. autotrophicus* in biofertilizer provide a beneficial role in creating a proper rhizosphere for plant growth and converting nutritionally important elements through biological process, for example increasing the availability of N, P, K, as well as inhibiting pathogens growth [67, 71].

The increasing availability of N, P, and K is enhancing soil fertility, to improve antagonistic capacity of PGPR or EPGPB to biocontrol of plant pathogens agents [58] as well as the survival time in all types of soil [78]. Previous studies show that a biofertilizer prepared by mixing all types of PGPR with composts or carriers could enhance growth- promoting effects and biocontrol of plants [79]. *Bacillus* spp [80] and *Pseudomonas* spp [81] are two PGPR that have been reported to effective biocontrol agents. Among these bacteria species, *Bacillus subtilis, B. amyloliquefacients*, and *B. cereus* are the most effective species for controlling plant diseases in domestic crops by several mechanisms [82]. Due endospores of the genus and species of *Bacillus* are tolerant to adverse environmental conditions allows PGPR, to survive and even to grow in a wide range of soils, thus facilitating the effective formulation of biofertilizer [83]. Based in this biochemicals qualities as well as the biorestauration of hyper fertilized or deteriorated soil [43, 74, 84, 85]. However, *X. autotrophicus* has many biological mechanisms to avoid environmental stress without any specific resistance structure a quality of this genus and specie [3, 21, 35] that has been useful to treat environments contaminated by chemical agents [86].
