Assessment of microbial biota associated with rhizosphere of wheat (Triticum aestivum) during flowering stage and their plant growth promoting traits.

摘要

Microbial biota associated with wheat rhizosphere during flowering-stage and their plant growth promoting traits was investigated. 16S rRNA gene sequencing was performed of the isolates, which were obtained on selective media. Isolates belonged to: alpha-proteobacteria, beta-proteobacteria, gamma-proteobacteria; Actinobacteria; Bacteroidetes and Firmicutes. Bacillus was the most dominant genus (34.7%) followed by Pseudomonas (14.4%). Among diazotrophs, Arthrobacter sp. (n=3), Cupriavidus sp. (n=3) and Stenotrophomonas sp. (n=4) occurred more frequently. Most of the isolates produced indole acetic acid. Ac. baumannii , Ps. lini, Ser. marcescens, C. respiraculi and Ag.tumfaciens solubilized phosphate. Two Acinetobacter strains, five Pseudomonas srains and four Bacillus strains produced siderophore. Strains of Ps. aeruginosa, Ps. lini and Ps. thivervalensis exhibited in vitro fungal growth inhibition. Arthrobacter globiformis Y2S3 exhibited indole acetic acid production, siderophore production and antifungal activity. Plant growth promoting traits of these rhizobacteria indicated beneficial relationship between rhizobacteria and wheat plants. Several of the strains could be further developed as an effective bio-inoculant. Introduction Rhizosphere soil is a “hot-spot” for microbial growth and major microbial activities. The growth of many microorganisms in the rhizospheric region depends on the root exudates released by the plants (Bais et al. 2006). Interactions between plant and microbes are intensely studied and especially those that benefit plant growth. Such free-living soil bacteria isolated from the rhizosphere of plants, which have been shown to be beneficial for plant growth are referred to as plant growth promoting rhizobacteria (PGPR) (Klopper et al. 1980). It has been well established that PGPR enhance plant growth by direct or indirect means. The direct means may includes : Fixation of atmospheric nitrogen (Zehr et al. 2003; Dixon 1984), production of siderophores (Machuca and Milagres 2003; Schwyn and Neilands 1986), solubilization of minerals like phosphorus (Tilak et al. 2005) and synthesis of phytohormones like indole acetic acid (IAA) (Chung and Tzeng 2004; Huddedar et al. 2002). Indirect mechanisms include inhibition of phytopathogens and thus promoting plant growth (Compant et al. 2005; Padalalu and Chopade 2006).Among all the plant growth promoting properties, N2 fixation is of prime importance for plant growth. N2 fixers, also called ‘diazotrophs’ play a critical role in the plant ecosystem by reducing dinitrogen (N2) to ammonia (NH3) (Dilworth 1974). N2 fixation is carried out by a diverse group of prokaryotes, Bacteria and Archaea (Zehr et al. 2003). These include symbiotic nitrogen fixing forms; Rhizobium, the obligate symbionts in leguminous plants and Frankia in non-leguminous trees, associative nitrogen fixers which adhere to the root surfaces (e.g. Achromobacter, Enterobacter etc.) and non-symbiotic (free living) forms such as Azospirillum, Azotobacter, Acetobacter diazotrophicus, cyanobacteria etc. Diazotrophs have been reported to exert a positive effect on plant growth when they, synthesize phytohormones and vitamins, improve nutrient uptake, enhance stress resistance and solubilize inorganic phosphate. Indirectly, diazotrophs are able to prevent the deleterious effects of pathogenic microorganisms, mostly through the synthesis of antibiotics and/or fungicidal compounds, through competition for nutrients by siderophore production or by the induction of systemic resistance to pathogens (Dobbelaere et al. 2003).Wheat is one of the major crops cultivated in India and all over the world. The different stages of life cycle of wheat consist of elongation (30days), flowering stage (45days), fruiting stage (60days) and ripened fruiting stage (75days) (Huddedar and Chopade 2000; Huddedar et al. 2002). It is found that rate of root exudates released by the roots of the wheat at flowering stage is higher as co