Plant growth-promoting rhizobacteria (PGPR) are free-living bacteria which actively colonize plant roots, exerting beneficial effects on plant development. The PGPR may (i) promote the plant growth either by using their own metabolism (solubilizing phosphates, producing hormones or fixing nitrogen) or directly affecting the plant metabolism (increasing the uptake of water and minerals), enhancing root development, increasing the enzymatic activity of the plant or "helping" other beneficial microorganisms to enhance their action on the plants; (ii) or may promote the plant growth by suppressing plant pathogens. These abilities are of great agriculture importance in terms of improving soil fertility and crop yield, thus reducing the negative impact of chemical fertilizers on the environment. The progress in the last decade in using PGPR in a variety of plants (maize, rice, wheat, soybean and bean) along with their mechanism of action are summarized and discussed here.

Nodulation gene inducer flavonoids increase the overall production of autoinducers and the expression of N-acyl homoserine lactone synthesis genes in rhizobia Running title: AHL production in rhizobia is induced by flavonoids.

Rhizobium (now Sinorhizobium) fredii is a symbiotic nitrogen-fixing bacterium that can nodulate soybean in a cultivar-specific manner. This process is governed by a set of negatively acting nodulation genes termed nolXWBTUV. These genes prevent R. fredii strain USDA257 from infecting soybean cultivars such as McCall, but they do not block nodulation of cultivar Peking. R. fredii strain USDA191 contains DNA sequences that hybridize to nolXWBTUV, yet it forms normal nitrogen-fixing nodules on both McCall and Peking soybean. These sequences were isolated and their structure and function examined in comparison to nolXWBTUV of strain USDA257. Restriction maps of the two loci are identical, as is a 2-4 kb DNA sequence that corresponds to nolX and its promoter region. Expression of nolX by strain USDAl91 is f lavonoid-dependent in culture and readily detectable in nodules. The gene is not inducible in a mutant of strain USDA191 that lacks the regulatory nodDl gene, and its expression is greatly attenuated in a nodD2 mutant. nolX is also present and flavonoid-inducible in HH103, a second R. fredii strain that nodulates McCall soybean normally. Inactivation of nolX in strain HH103, USDA191 or USDA257 leads to retardation of initial nodulation rates on soybean cultivars such as Peking and to acquisition of the capacity to form nitrogen-fixing nodules on two species of Erythrina. nolX is thus of symbiotic significance in all three strains, even though it regulates soybean cultivar specificity only in strain USDA257.1

We mutagenized Sinorhizobium fredii HH103-1 with Tn5-B20 and screened about 2,000 colonies for increased β-galactosidase activity in the presence of the flavonoid naringenin. One mutant, designated SVQ287, produces lipochitooligosaccharide Nod factors (LCOs) that differ from those of the parental strain. The nonreducing Nacetylglucosamine residues of all of the LCOs of mutant SVQ287 lack fucose and 2-O-methylfucose substituents. In addition, SVQ287 synthesizes an LCO with an unusually long, C20:1 fatty acyl side chain. The transposon insertion of mutant SVQ287 lies within a 1.1-kb HindIII fragment. This and an adjacent 2.4-kb HindIII fragment were sequenced. The sequence contains the 3′ end of noeK, nodZ, and noeL (the gene interrupted by Tn5-B20), and the 5′ end of nolK, all in the same orientation. Although each of these genes has a similarly oriented counterpart on the symbiosis plasmid of the broad-host-range Rhizobium sp. strain NGR234, there are significant differences in the noeK/nodZ intergenic region. Based on amino acid sequence homology, noeL encodes GDP-D-mannose dehydratase, an enzyme involved in the synthesis of GDP-Lfucose, and nolK encodes a NAD-dependent nucleotide sugar epimerase/dehydrogenase. We show that expression of the noeL gene is under the control of NodD1 in S. fredii and is most probably mediated by the nod box that precedes nodZ. Transposon insertion into noeL has two impacts on symbiosis with Williams soybean: nodulation rate is reduced slightly and competitiveness for nodulation is decreased significantly. Mutant SVQ287 retains its ability to form nitrogen-fixing nodules on other legumes, but final nodule number is attenuated on Cajanus cajan.Sinorhizobium fredii was initially described as a fastgrowing bacterium that nodulates soybean (Glycine max (L.) Merr.) in a cultivar-specific manner and that can enter into symbiosis with several other legume species (Keyser et al. 1982). Although the first, and many subsequently, isolated S. fredii strains were from China (Keyser et al. 1982;Dowdle and Bohlool 1985;Chen et al. 1988), similar microorganisms have been isolated from diverse geographical regions, including Panama (Hernandez and Focht 1984), Malaysia (Young et al. 1988), the United States (Shen and Davis 1992), and Vietnam (Rodriguez-Navarro et al. 1996). One of the most interesting characteristics of S. fredii is its exceptionally broad host range. Strain USDA257, for example, produces nitrogenfixing nodules on more than 60 legume species (Krishnan and Pueppke 1994b; S. G. Pueppke, unpublished). Rhizobium sp. strain NGR234, a versatile symbiont that is closely related to and perhaps a strain of S. fredii (Jarvis et al. 1992), is even more promiscuous.Although the unusual symbiotic properties of S. fredii make it an attractive model for experimental manipulation, genetic analysis of broad-host-range sinorhizobia is less advanced than that of narrower-host-range symbionts. Bacterial genes that function in nodulation are termed nodulation or nod genes, and their expression is induc...

Strain SVQ121 is a mutant derivative of Sinorhizobium fredii HH103 carrying a transposon Tn5-lacZ insertion into the nolO-coding region. Sequence analysis of the wild-type gene revealed that it is homologous to that of Rhizobium sp. NGR234, which is involved in the 3 (or 4)-O-carbamoylation of the nonreducing terminus of Nod factors. Downstream of nolO, as in Rhizobium sp. NGR234, the noeI gene responsible for methylation of the fucose moiety of Nod factors was found. SVQ121 Nod factors showed lower levels of methylation into the fucosyl residue than those of HH103-suggesting a polar effect of the transposon insertion into nolO over the noel gene. A noeI HH103 mutant was constructed. This mutant, SVQ503, produced Nod factors devoid of methyl groups, confirming that the S. fredii noeI gene is functional. Neither the nolO nor the noeI mutation affected the ability of HH103 to nodulate several host plants, but both mutations reduced competitiveness to nodulate soybean. The Nod factors produced by strain HH103, like those of other S. fredii isolates, lack carbamoyl residues. By using specific polymerase chain reaction primers, we sequenced the nolO gene of S. fredii strains USDA192, USDA193, USDA257, and 042B(s). All the analyzed strains showed the same -1 frameshift mutation that is present in the HH103 nolO-coding region. From these results, it is concluded that, regardless of their geographical origin, S. fredii strains carry the nolO-coding region but that it is truncated by the same base-pair deletion.