Plant growth promoting rhizobacteria reduce aphid population and enhance the productivity of bread wheat

Plant growth promoting rhizobacteria increase plant growth and give protection against insect pests and pathogens. Due to the negative impact of chemical pesticides on environment, alternatives to these chemicals are needed. In this scenario, the biological methods of pest control offer an eco-friendly and an attractive option. In this study, the effect of two plant growth promoting rhizobacterial strains (Bacillus sp. strain 6 and Pseudomonas sp. strain 6K) on aphid population and wheat productivity was evaluated in an aphid susceptible (Pasban-90) and resistant (Inqlab-91) wheat cultivar. The seeds were inoculated with each PGPR strain, separately or the combination of both. The lowest aphid population (2.1tiller-1), and highest plant height (85.8cm), number of spikelets per spike (18), grains per spike (44), productive tillers (320m-2), straw yield (8.6Mgha-1), and grain yield (4.8Mgha-1) were achieved when seeds were inoculated with Bacillus sp. strain 6+Pseudomonas sp. strain 6K. The grain yield of both varieties was enhanced by 35.5-38.9% with seed inoculation with both bacterial strains. Thus, the combine use of both PGPR strains viz. Bacillus sp. strain 6+Pseudomonas sp. strain 6K offers an attractive option to reduce aphid population tied with better wheat productivity.

Rhizobia promote the growth of rice shoots by targeting cell signaling, division and expansion

The growth-promotion of rice seedling following inoculation with Sinorhizobium meliloti 1021 was a cumulative outcome of elevated expression of genes that function in accelerating cell division and enhancing cell expansion. Various endophytic rhizobacteria promote the growth of cereal crops. To achieve a better understanding of the cellular and molecular bases of beneficial cereal-rhizobia interactions, we performed computer-assisted microscopy and transcriptomic analyses of rice seedling shoots (Oryza sativa) during early stages of endophytic colonization by the plant growth-promoting Sinorhizobium meliloti 1021. Phenotypic analyses revealed that plants inoculated with live rhizobia had increased shoot height and dry weight compared to control plants inoculated with heat-killed cells of the same microbe. At 6 days after inoculation (DAI) with live cells, the fourth-leaf sheaths showed significant cytological differences including their enlargement of parenchyma cells and reduction in shape complexity. Transcriptomic analysis of shoots identified 2,414 differentially-expressed genes (DEGs) at 1, 2, 5 and 8 DAI: 195, 1390, 1025 and 533, respectively. Among these, 46 DEGs encoding cell-cycle functions were up-regulated at least 3 days before the rhizobia ascended from the roots to the shoots, suggesting that rhizobia are engaged in long-distance signaling events during early stages of this plant-microbe interaction. DEGs involved in phytohormone production, photosynthetic efficiency, carbohydrate metabolism, cell division and wall expansion were significantly elevated at 5 and 8 DAI, consistent with the observed phenotypic changes in rice cell morphology and shoot growth-promotion. Correlation analysis identified 104 height-related DEGs and 120 dry-weight-related DEGs that represent known quantitative-trait loci for seedling vigor and increased plant height. These findings provide multiple evidences of plant-microbe interplay that give insight into the growth-promotion processes associated with this rhizobia-rice beneficial association.

Potential of Rice Stubble as a Reservoir of Bradyrhizobial Inoculum in Rice-Legume Crop Rotation

Bradyrhizobium encompasses a variety of bacteria that can live in symbiotic and endophytic associations with leguminous and nonleguminous plants, such as rice. Therefore, it can be expected that rice endophytic bradyrhizobia can be applied in the rice-legume crop rotation system. Some endophytic bradyrhizobial strains were isolated from rice (Oryza sativa L.) tissues. The rice biomass could be enhanced when supplementing bradyrhizobial strain inoculation with KNO₃, NH₄NO₃, or urea, especially in Bradyrhizobium sp. strain SUTN9-2. In contrast, the strains which suppressed rice growth were photosynthetic bradyrhizobia and were found to produce nitric oxide (NO) in the rice root. The expression of genes involved in NO production was conducted using a quantitative reverse transcription-PCR (qRT-PCR) technique. The nirK gene expression level in Bradyrhizobium sp. strain SUT-PR48 with nitrate was higher than that of the norB gene. In contrast, the inoculation of SUTN9-2 resulted in a lower expression of the nirK gene than that of the norB gene. These results suggest that SUT-PR48 may accumulate NO more than SUTN9-2 does. Furthermore, the nifH expression of SUTN9-2 was induced in treatment without nitrogen supplementation in an endophytic association with rice. The indole-3-acetic acid (IAA) and 1-amino-cyclopropane-1-carboxylic acid (ACC) deaminase produced in planta by SUTN9-2 were also detected. Enumeration of rice endophytic bradyrhizobia from rice tissues revealed that SUTN9-2 persisted in rice tissues until rice-harvesting season. The mung bean (Vigna radiata) can be nodulated after rice stubbles were decomposed. Therefore, it is possible that rice stubbles can be used as an inoculum in the rice-legume crop rotation system under both low- and high-organic-matter soil conditions.IMPORTANCE This study shows that some rice endophytic bradyrhizobia could produce IAA and ACC deaminase and have a nitrogen fixation ability during symbiosis inside rice tissues. These characteristics may play an important role in rice growth promotion by endophytic bradyrhizobia. However, the NO-producing strains should be of concern due to a possible deleterious effect of NO on rice growth. In addition, this study reports the application of endophytic bradyrhizobia in rice stubbles, and the rice stubbles were used directly as an inoculum for a leguminous plant (mung bean). The degradation of rice stubbles leads to an increased number of SUTN9-2 in the soil and may result in increased mung bean nodulation. Therefore, the persistence of endophytic bradyrhizobia in rice tissues can be developed to use rice stubbles as an inoculum for mung bean in a rice-legume crop rotation system.

Proteomic analysis of rice seedlings infected by Sinorhizobium meliloti 1021

Rhizobial endophytes infect and colonize not only leguminous plants, but several non-leguminous species as well. Using green fluorescent protein tagging technique, it has been shown that Rhizobia infect different varieties of rice species and migrate from plant roots to aerial tissues such as leaf sheaths and leaves. The interaction between them was found to promote the growth of rice. The growth promotion is the cumulative result of enhanced photosynthesis and stress resistance. In addition, indole-3-acetic acid also contributes to the promotion. Gel-based comparative proteomic approaches were applied to analyze the protein profiles of three different tissues (root, leaf sheath and leaf) of Sinorhizobium meliloti 1021 inoculated rice in order to get an understanding about the molecular mechanism. Upon the inoculation of rhizobia, proteins involved in nine different functional categories were either up-regulated or down-regulated. Photosynthesis related proteins were up-regulated only in leaf sheath and leaf, while the up-regulated proteins in root were exclusively defense related. The results implied that there might have been an increase in the import and transport of proteins involved in light and dark reactions to the chloroplast as well as more efficient distribution of nutrients, hence enhanced photosynthesis. Although the initiation of defensive reactions mainly occurred in roots, some different defense mechanisms were also evoked in the aerial tissues.

Long-term effects of crop management on Rhizobium leguminosarum biovar viciae populations

Little is known about factors that affect the indigenous populations of rhizobia in soils. We compared the abundance, diversity and genetic structure of Rhizobium leguminosarum biovar viciae populations in soils under different crop managements, i.e., wheat and maize monocultures, crop rotation, and permanent grassland. Rhizobial populations were sampled from nodules of pea- or vetch plants grown in soils collected at three geographically distant sites in France, each site comprising a plot under long-term maize monoculture. Molecular characterization of isolates was performed by PCR-restriction fragment length polymorphism of 16S-23S rDNA intergenic spacer as a neutral marker of the genomic background, and PCR-restriction fragment length 0polymorphism of a nodulation gene region, nodD, as a marker of the symbiotic function. The diversity, estimated by richness in types and Simpson's index, was consistently and remarkably lower in soils under maize monoculture than under the other soil managements at the three sites, except for the permanent grassland. The highest level of diversity was found under wheat monoculture. Nucleotide sequences of the main rDNA intergenic spacer types were determined and sequence analysis showed that the prevalent genotypes in the three maize fields were closely related. These results suggest that long-term maize monoculturing decreased the diversity of R. leguminosarum biovar viciae populations and favored a specific subgroup of genotypes, but the size of these populations was generally preserved. We also observed a shift in the distribution of the symbiotic genotypes within the populations under maize monoculture, but the diversity of the symbiotic genotypes was less affected than that of IGS types. The possible effect of such changes on biological nitrogen fixation remains unknown and this requires further investigation.