Accumulation of lipochitin oligosaccharides and NodD-activating compounds in an efficient plant--Rhizobium nodulation assay

Designing Novel Strategies for Improving Old Legumes: An Overview from Common Vetch

Common vetch (Vicia sativa L.) is a grain legume used in animal feeding, rich in protein content, fatty acid, and mineral composition that makes for a very adequate component to enrich feedstuff. In addition, relevant pharmacological properties have been reported in humans. The common vetch, similar to other legumes, can fix atmospheric nitrogen, a crucial feature for sustainable agricultural systems. These properties enhance the use of vetch as a cover crop and its sowing in intercropping systems. Moreover, several studies have recently pointed out the potential of vetch in the phytoremediation of contaminated soils. These characteristics make vetch a relevant crop, which different potential improvements target. Varieties with different yields, flowering times, shattering resistance, nutritional composition, rhizobacteria associations, drought tolerance, nitrogen fixation capacity, and other agronomic-relevant traits have been identified when different vetch accessions are compared. Recently, the analysis of genomic and transcriptomic data has allowed the development of different molecular markers to be used for assisted breeding purposes, promoting crop improvement. Here, we review the potential of using the variability of V. sativa genetic resources and new biotechnological and molecular tools for selecting varieties with improved traits to be used in sustainable agriculture systems.

Role of nodD gene product and flavonoid interactions in induction of nodulation genes in Mesorhizobium ciceri

Mesorhizobium ciceri is a host specific bacterium which nodulates the genus, Cicer. Host specificity is regulated at first step by induction of nodulation (nod) genes in the presence of NodD protein and inducers (flavonoids) of plant origin. The inducer specificity of M. ciceri nodD gene was studied in NodD-mutant strain HN-9 carrying heterologous nodD genes and nodAlacZ fusion. The induction profile of nod promoter in M. ciceri revealed that nodD gene product of M. ciceri is specifically activated by chickpea root exudates only. M. ciceri HN-9 (nodA-lacZ) containing heterologous nodD genes from Rhizobium leguminosarum bv. viciae, R. leguminosarum bv. trifolii and Sinorhizobium meliloti was induced in presence of a number of flavonoids. On the other hand, induction profile of nod promoter showed that heterologous nodD gene products were activated to different levels in NodD(-) mutant of M. ciceri in presence of root exudates from homologous as well as heterologous legume hosts. The transfer of FITA (Flavonoid independent transcription activation) nodD gene in NodD(-) mutant, M. ciceri HN-9, was able to break the inducer specificity barrier and nod promoter was induced to maximum level irrespective of the presence or absence of inducer. It is concluded from the results that host specificity in M. ciceri - chickpea (Cicer arietinum) symbiosis is regulated at first step by the host specific interaction of nodD gene product of M. ciceri and inducers present in the root exudates of chickpea.

The production of species-specific highly unsaturated fatty acyl-containing LCOs from Rhizobium leguminosarum bv. trifolii is stringently regulated by nodD and involves the nodRL genes

A proportion of the Nod factors of some Rhizobium leguminosarum bv. trifolii strains is characterized by the presence of highly unsaturated fatty acyl chains containing trans double bonds in conjugation with the carbonyl group of the glycan oligosaccharide backbone. These fatty acyl chains are C18:3, C20:3, C18:4, or C20:4 and have UV-absorption maxima at 303 and 330 nm. These Nod factors are presumed to be important for host-specific nodulation on clover species. However, in wild-type R. leguminosarum bv. trifolii ANU843, Nod factors with these characteristic acyl chains were not observed using standard growth conditions. They were observed only when nod genes were present in multiple copies or when transcription was artificially increased to higher levels by introduction of extra copies of the transcriptional regulator gene nodD. In a screen for the genetic requirements for production of the Nod factors with these characteristic structures, it was found that the region downstream of nodF and nodE is essential for the presence of highly unsaturated fatty acyl moieties. Mu-lacZ insertion in this region produced a mutant that did not produce detectable levels of the highly unsaturated fatty acyl-bearing Nod factors. The Mu-lacZ insertion was translationally fused to a putative new gene, designated nodR, in the nodE-nodL intergenic region; however, no predicted function for the putative NodR protein has been obtained from database homology searches. In a set of 12 wild-type strains of R. leguminosarum by. trifolii originating from various geographical regions that were analyzed for the presence of a nodR-like gene, it was found that seven strains carry a homologous NodR open reading frame. Taken together, our results suggest a tightly controlled regulation of nod genes, in which we propose that it is the balance of transcriptional levels of nodFE and the nodRL genes that is critical for determining the presence of highly unsaturated fatty acyl moieties in the Nod factors produced by R. leguminosarum bv. trifolii.

Invasion of Lotus japonicus root hairless 1 by Mesorhizobium loti involves the nodulation factor-dependent induction of root hairs

In many legumes, including Lotus japonicus and Medicago truncatula, susceptible root hairs are the primary sites for the initial signal perception and physical contact between the host plant and the compatible nitrogen-fixing bacteria that leads to the initiation of root invasion and nodule organogenesis. However, diverse mechanisms of nodulation have been described in a variety of legume species that do not rely on root hairs. To clarify the significance of root hairs during the L. japonicus-Mesorhizobium loti symbiosis, we have isolated and performed a detailed analysis of four independent L. japonicus root hair developmental mutants. We show that although important for the efficient colonization of roots, the presence of wild-type root hairs is not required for the initiation of nodule primordia (NP) organogenesis and the colonization of the nodule structures. In the genetic background of the L. japonicus root hairless 1 mutant, the nodulation factor-dependent formation of NP provides the structural basis for alternative modes of invasion by M. loti. Surprisingly, one mode of root colonization involves nodulation factor-dependent induction of NP-associated cortical root hairs and epidermal root hairs, which, in turn, support bacterial invasion. In addition, entry of M. loti through cracks at the cortical surface of the NP is described. These novel mechanisms of nodule colonization by M. loti explain the fully functional, albeit significantly delayed, nodulation phenotype of the L. japonicus ROOT HAIRLESS mutant.