Rhizosphere Bacterial Isolates Research Articles

Induction of defense-related proteins by mixtures of plant growth promoting endophytic bacteria against Banana bunchy top virus

Rhizosphere and endophytic bacterial isolates from the roots and corms of banana were tested for their biocontrol efficiency against Banana bunchy top virus (BBTV). Molecular characterization using RAPD and microsatellite markers revealed genomic variability in the endophytic Pseudomonas and Bacillus isolates. Bio-formulations of mixtures of the rhizobacterial isolate Pseudomonas fluorescens (Pf1) and endophytic Bacillus spp. (EPB22) were effective in reducing the incidence of BBTV under green-house (80%) and field conditions (52%). Reduction in virus titer (0.64) was noticed in the plants treated with compatible mixtures of rhizobacterial and endophytic bacterial isolates as evidenced by ELISA, in comparison to control plants (1.69). In addition to disease control, a significant increase in the yield (53.33%) was noticed in the bacterized plants when compared to the control plants. Pathogenesis-related (PR) proteins, chitinase and β-1,3-glucanase and defense-related proteins, peroxidase, polyphenol oxidase, phenylalanine ammonia-lyase and phenolic compounds were significantly activated in the bacterized plants, thus inducing resistance against bunchy top virus. Populations of endophytic bacteria also remained high and stable throughout the growing period. Thus, application of mixtures of rhizosphere and endophytic bacteria increases yield and has a potential role in inducing resistance against Banana bunchy top virus.

Evaluation of rhizosphere bacterial antagonists for their potential to bioprotect potato ( Solanum tuberosum) against bacterial wilt ( Ralstonia solanacearum)

Bacterial wilt ( Ralstonia solanacearum) is one of the production constraints of potato ( Solanum tuberosum). The intent of the study was to evaluate potential of bacterial antagonists to suppress bacterial wilt disease development and evaluate the role of the strains as plant growth-promoting rhizobacteria (PGPR) in potato. One hundred-twenty rhizosphere bacterial isolates were screened against virulent strain of Ralstonia solanacearum PPRC-Rs. After in vitro screening, six antagonistic strains (PFMRI, BS-DFS, PF9, PF20, BC, and BS-wly) with inhibition diameter >11 mm were selected and studied further in the greenhouse, in vivo. During in vivo study, the strains were evaluated for their effect in suppressing disease development in terms of area under disease progress curve (AUDPC) and increasing biomass (plant height and dry weight) of potato. Accordingly, PFMRI, BS-DFS, and PF9, significantly reduced AUDPC by 78.6, 66, and 64.3%, and wilt incidence by 82.7, 66.2, and 65.7%, respectively, compared to the control. During the sole application, the strains significantly ( P < 0.0001) increased plant height by 35.6, 45.9, and 45%, and dry matter by 111, 130.4, and 129%, respectively compared to non-bacterized control. In the presence of the pathogen strain PFMRI, BS-DFS, and PF9 increased plant height by 66, 50, and 48.2%, and dry matter by 153.8, 96.8, and 92.5%, respectively compared to the pathogen treated control. Hence, the study shows that PFMRI, BS-DFS, and PF9 strains have potential use in potato bioprotection, as PGPR or in an integrated bacterial wilt management; whose effectiveness under a variety of field conditions should be investigated.

Screening of rhizosphere and soil bacteria for transformability

Natural transformation is assumed to be the most likely mechanism by which DNA from transgenic plants could be horizontally transferred to bacteria. In order to determine the occurrence of naturally transformable bacteria amongst bulk and rhizosphere soil bacteria, different transformation strategies were employed using either plasmid DNA (IncQ plasmids pSM1890 and pSM1885, conferring GFP, Sm(r), Gm(r) and GFP, Sm(r), Tc(r), respectively) or genomic DNA from rhizosphere isolates, which were chromosomally tagged with mini-Tn5 (GFP, Tc(r)), as transforming DNA. Transformation assays were done in microtiter plates (262 isolates and pSM1890 or pSM1885), on filters (i) with rhizosphere bacterial community mixed with pSM1890 or pSM1885, (ii) with 24 rhizosphere or soil bacterial isolates mixed with genomic DNA of the corresponding mini-Tn5-tagged strains, and in the rhizosphere of tobacco plants inoculated with rifampicin-resistant bacterial isolates and genomic DNA of the corresponding mini-Tn5-tagged strains added. One transformant colony was obtained when Brevundimonas vesicularis was transformed with genomic DNA of the corresponding mini-Tn5-tagged strain. Attempts to reproduce this result were unsuccessful. With this single exception, transformants were neither detected in the collection of isolates nor in the rhizosphere bacterial community. Acinetobacter baylyi BD413 used as a positive control showed drastically reduced transformation frequencies with plasmid pSM1890 as transforming DNA when mixed with the rhizosphere pellet. All transformants were characterized by BOX-PCR fingerprints, and three different BOX patterns were revealed. Sequencing the 16S rRNA gene showed that all transformants could be assigned to Acinetobacter sp. Since transformants were only observed in the positive control, the introduced BD413 either underwent genomic rearrangements, or competence of the Acinetobacter population present in the rhizosphere was stimulated by the introduction of BD413. The various transformation assays performed indicate that the proportion of rhizosphere or bulk soil bacteria which are naturally transformable is negligibly low.

Differential cross-utilization of heterologous siderophores by nodule bacteria of Cajanus cajan and its possible role in growth under iron-limited conditions

Siderophores are ferric specific ligands that are involved in receptor specific iron transport into bacteria. Partially purified siderophores from 37 different rhizospheric bacterial isolates, and 17 Cajanus cajan root nodule isolates were tested for their utilization by the 17 nodule isolates as test organisms. Significant variation in the siderophore cross-utilization pattern was observed. Catecholate siderophores were produced predominantly by nodule isolates whereas rhizospheric isolates produced catecholates, as well as hydroxamates. The quantity of hydroxamate produced was higher than that of catecholates. Considerable variation was observed with respect to siderophore cross-utilization amongst nodule isolates obtained from a particular location. Isolates collected from different nodules of same plant also showed considerable variation with respect to siderophore cross-utilization. Isolates, which showed, higher siderophores cross-utilizing ability, when tested for homologous (from other nodule bacteria) siderophores also gave good cross-utilization of heterologous (from non-nodule rhizospheric bacteria) siderophores. Isolate GHU(iii), the highest cross-utilizer of homologous and heterologous siderophores showed a significant increase in growth in the presence of exogenously added siderophores. Similar results were observed during the co-inoculation studies with rhizospheric isolate under iron-limited condition. Both isolates GAU(1)1 and AP(6)4, low cross-utilizers of siderophores, failed to show growth increase in response to co-inoculation and addition of siderophores. The iron regulated outer membrane protein (IROMP) profiles of strains showing maximum or least cross-utilization did not differ. The significance of these findings in context to survivability under natural conditions and the possible reasons accounting for high siderophore cross-utilizing status of isolate GHU(iii) is discussed.

Selective plant growth suppression by shoot application of soil bacteria

Selected rhizosphere bacterial isolates, previously determined as plant growth deleterious, were tested for their ability to suppress plant growth after foliar spray applications, for selectivity with regard to plant species, and in pilot field experiments for their potential as weed biocontrol agents. Inundative foliar applications of aqueous bacterial suspension were performed on a range of weed and crop species. Plant symptoms after spraying ranged from rapid necrosis and wilting to an overall growth suppression or stunting. Significant and selective reductions in biomass of up to 90% fresh weight, as well as large reductions in plant survival and plant height were recorded in greenhouse pot experiments. However, monocotyledonous plants were affected weakly or not at all by two isolates extensively tested. Effects of these were dose- and plant age-dependent, and were for some plants enhanced by high relative humidity. For one isolate, A153, effects were also expressed in cell-free culture filtrates pointing to involvement of specific metabolites. In pilot field experiments, strong growth suppression was observed on broad-leaved plants, while barley crop plants were unaffected.