Sinorhizobium meliloti bacteria produce a signal molecule that enhances root respiration in alfalfa (Medicago sativa L.) and also triggers a compensatory increase in whole-plant net carbon assimilation. Nuclear magnetic resonance, mass spectrometry, and ultraviolet-visible absorption identify the enhancer as lumichrome, a common breakdown product of riboflavin. Treating alfalfa roots with 3 nM lumichrome increased root respiration 21% (P < 0.05) within 48 h. A closely linked increase in net carbon assimilation by the shoot compensated for the enhanced root respiration. For example, applying 5 nM lumichrome to young alfalfa roots increased plant growth by 8% (P < 0.05) after 12 days. Soaking alfalfa seeds in 5 nM lumichrome before germination increased growth by 18% (P < 0.01) over the same period. In both cases, significant growth enhancement (P < 0.05) was evident only in the shoot. S. meliloti requires exogenous CO 2 for growth and may benefit directly from the enhanced root respiration that is triggered by lumichrome. Thus Sinorhizobium-alfalfa associations, which ultimately form symbiotic N 2-reducing root nodules, may be favored at an early developmental stage by lumichrome, a previously unrecognized mutualistic signal. The rapid degradation of riboflavin to lumichrome under many physiological conditions and the prevalence of riboflavin release by rhizosphere bacteria suggest that events demonstrated here in the S. meliloti-alfalfa association may be widely important across many plant-microbe interactions.
Of 102 rhizoplane and endophytic bacteria isolated from rice roots and stems in California, 37% significantly (P < or = 0.05) inhibited the growth in vitro of two pathogens, Achlya klebsiana and Pythium spinosum, causing seedling disease of rice. Four endophytic strains were highly effective against seedling disease in growth pouch assays, and these were identified as Pseudomonas fluorescens (S3), Pseudomonas tolaasii (S20), Pseudomonas veronii (S21), and Sphingomonas trueperi (S12) by sequencing of amplified 16S rRNA genes. Strains S12, S20, and S21 contained the nitrogen fixation gene, nifD, but only S12 was able to reduce acetylene in pure culture. The four strains significantly enhanced plant growth in the absence of pathogens, as evidenced by increases in plant height and dry weight of inoculated rice seedlings relative to noninoculated rice. Three bacterial strains (S3, S20, and S21) were evaluated in pot bioassays and reduced disease incidence by 50%-73%. Strain S3 was as effective at suppressing disease at the lowest inoculum density (106 CFU/mL) as at higher density (10(8) CFU/mL or undiluted suspension). This study indicates that selected endophytic bacterial strains have potential for control of seedling disease of rice and for plant growth promotion.
Structure of the Mesorhizobium huakuii and Rhizobium galegae Nod factors: a cluster of phylogenetically related legumes are nodulated by rhizobia producing Nod factors with a,b-unsaturated N-acyl substitutions SummaryRhizobia are symbiotic bacteria that synthesize lipochitooligosaccharide Nod factors (NFs), which act as signal molecules in the nodulation of speci®c legume hosts. Based on the structure of their N-acyl chain, NFs can be classi®ed into two categories: (i) those that are acylated with fatty acids from the general lipid metabolism; and (ii) those ( aU-NFs) that are acylated by speci®c a,b-unsaturated fatty acids (containing carbonyl-conjugated unsaturation(s)). Previous work has described how rhizobia that nodulate legumes of the Trifolieae and Vicieae tribes produce aU-NFs. Here, we have studied the structure of NFs from two rhizobial species that nodulate important genera of the Galegeae tribe, related to Trifolieae and Vicieae. Three strains of Mesorhizobium huakuii, symbionts of Astragalus sinicus, produced as major NFs, pentameric lipochitooligosaccharides O-sulphated and partially N-glycolylated at the reducing end and N-acylated, at the non-reducing end, by a C18:4 fatty acid. Two strains of Rhizobium galegae, symbionts of Galega sp., produced as major NFs, tetrameric O-carbamoylated NFs that could be O-acetylated on the glucosamine residue next to the non-reducing terminal glucosamine and were N-acylated by C18 and C20 a,b-unsaturated fatty acids. These results suggest that legumes nodulated by rhizobia synthesizing aUNFs constitute a phylogenetic cluster in the Galegoid phylum.
The nodulation genes of Mesorhizobium sp. (Astragalus sinicus) strain 7653R were cloned by functional complementation of Sinorhizobium meliloti nod mutants. The common nod genes, nodD, nodA, and nodBC, were identified by heterologous hybridization and sequence analysis. The nodA gene was found to be separated from nodBC by approximately 22 kb and was divergently transcribed. The 2.0-kb nodDBC region was amplified by PCR from 24 rhizobial strains nodulating A. sinicus, which represented different chromosomal genotypes and geographic origins. No polymorphism was found in the size of PCR products, suggesting that the separation of nodA from nodBC is a common feature of A. sinicus rhizobia. Sequence analysis of the PCR-amplified nodA gene indicated that seven strains representing different 16S and 23S ribosomal DNA genotypes had identical nodA sequences. These data indicate that, whereas microsymbionts of A. sinicus exhibit chromosomal diversity, their nodulation genes are conserved, supporting the hypothesis of horizontal transfer of nod genes among diverse recipient bacteria.Rhizobia are soil bacteria that can form nodules, in which they fix nitrogen, on leguminous plants in a host-specific manner. Nodulation (nod) genes have been identified that control the specific infection and nodulation of the plant hosts. The initial infection event is regulated by a NodD protein or proteins which activate the transcription of other nod genes in the presence of host-produced flavonoids (12, 25, 37). The nod-ABC genes are called common nod genes because they are present in all rhizobia. Other nod genes, such as nodFE, nodH, nodSU, and nodZ (12,25,37), are present in various combinations in rhizobial species and are called host-specific nod genes.Expression of common and host-specific nod genes results in the production of lipochitooligosaccharides (Nod factors) that act as morphogenic signal molecules on specific legume hosts (12, 37). All Nod factors have a -1,4-linked N-acetyl glucosamine oligosaccharide backbone ranging in length from 3 to 5 residues and substituted for by an N-acyl chain at the nonreducing end and other chemical groups on the glucosamine residues. The common nodABC gene products are involved in the synthesis of the N-acylated oligosaccharide core, while the host-specific nod gene products are involved in the decoration of this backbone with substitutions that confer plant specificity. The nodABC genes encode an acyltransferase, a chitin oligosaccharide deacetylase, and a chitin oligosaccharide synthase, respectively (3, 33). The common nod genes are also involved in determining host range specificity to some extent. For example, different NodA proteins recognize and transfer different fatty acid chains to the chitooligosaccharide chain, the length of which is determined by NodC (11, 27, 32). The common nodABC genes are essential for nodule formation. Mutation in any of them abolishes the ability to produce Nod factors and results in a nonnodulating (Nod Ϫ ) phenotype (12). Astragalus sinicus L. (Chinese ...
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