Legumes form a symbiotic association with rhizobia and fix atmospheric nitrogen in specialized root organs known as nodules. It is well known that salt stress inhibits root nodule symbiosis by decreasing rhizobial growth, rhizobial infection, nodule number, and nitrogenase activity in diverse legumes. Despite this knowledge, the genetic and molecular mechanisms governing salt stress\'s inhibition of nodulation and nitrogen fixation are still elusive. In this Viewpoint, we summarize the most recent knowledge of the genetic mechanisms that shape this symbiosis according to the salt levels in the soil. We emphasize the relevance of modulating the activity of the transcription factor Nodule Inception to properly shape the symbiosis with rhizobia accordingly. We also highlight the knowledge gaps that are critical for gaining a deeper understanding of the molecular mechanisms underlying the adaptation of the root nodule symbiosis to salt-stress conditions. We consider that filling these gaps can help to improve legume nodulation and harness its ecological benefits even under salt-stress conditions.

UNASSIGNED: Rhizobium etli CE3-DsRed pMP604 drives infection 12-24 h earlier than R. etli CE3-DsRed and it is an excellent tool in live-cell imaging studies of IT developement in P. vulgaris roots. The study of the cellular dynamics of nodulation has frequently been limited by the difficulty of performing live-cell imaging in nodule primordia and legume roots, which are constituted by multiple cell layers, such is the case of Phaseolus vulgaris. Seeking conditions to reduce the time it takes for rhizobia to infect P. vulgaris root, we decided to explore the nodulation properties of Rhizobium etli CE3 pMP604, a strain that constitutively produces Nod factors through a flavonoids-independent transcriptional activation which is often used to purify Nod factors. Even though the strain infects 12-24 h earlier than the parental R. etli CE3 strain, infection thread (IT) formation, nodule organogenesis processes and N2-fixation activity are similar for both strains. Additionally, we have confirmed that R. etli CE3-DsRed pMP604 is an excellent tool to trace IT development in P. vulgaris roots.

The symbiosis of soybean with Bradyrhizobium diazoefficiens USDA110, which always competes with other rhizobia in the field, is of great agronomic and environmental importance. Herein, a dual-luciferase reporter assay was utilized to monitor the dynamics of two dominant bradyrhizobia infecting roots of soybean. More explicitly, luciferase-tagged B. diazoefficiens USDA110 (USDA110-FLuc) and Bradyrhizobium elkanii USDA 94 (USDA94-RLuc) were designed, co-inoculated into soybean seeds, and observed for their colonization in root nodules by bioluminescence imaging. The results showed that USDA110-FLuc initiated infection earlier than USDA94-RLuc, but its occupancy in the nodules decreased as the plant grew. A nodulation test showed that nodD1 mutant USDA110 strains, including CRISPR engineered mutants, were less competitive than wild type. I constructed siRNAs to knockdown nodD1 at different target sites and transformed them into the bacteria. Surprisingly, although siRNAs - with 3\' end target sites - were able to repress up to 65% of nodD1 expression, the profiling of total RNAs with a bioanalyzer revealed that 23S/16S-rRNA ratios of siRNA-transformed and wild type USDA110 strains were similar, but lower than that of nodD1 mutant. In short, the current work - while reporting the competitiveness of B. diazoefficiens USDA110 in early occupancy of soybean nodules and the gene nodD1 as a key determinant of this infection - gives an insight on siRNA silencing in microbes, and demonstrates a highly efficient imaging approach that could entail many new avenues for many biological research fields.

Increased biomass and yield of plants grown under elevated [CO2 ] often corresponds to decreased grain N concentration ([N]), diminishing nutritional quality of crops. Legumes through their symbiotic N2 fixation may be better able to maintain biomass [N] and grain [N] under elevated [CO2 ], provided N2 fixation is stimulated by elevated [CO2 ] in line with growth and yield. In Mediterranean-type agroecosystems, N2 fixation may be impaired by drought, and it is unclear whether elevated [CO2 ] stimulation of N2 fixation can overcome this impact in dry years. To address this question, we grew lentil under two [CO2 ] (ambient ~400 ppm and elevated ~550 ppm) levels in a free-air CO2 enrichment facility over two growing seasons sharply contrasting in rainfall. Elevated [CO2 ] stimulated N2 fixation through greater nodule number (+27%), mass (+18%), and specific fixation activity (+17%), and this stimulation was greater in the high than in the low rainfall/dry season. Elevated [CO2 ] depressed grain [N] (-4%) in the dry season. In contrast, grain [N] increased (+3%) in the high rainfall season under elevated [CO2 ], as a consequence of greater post-flowering N2 fixation. Our results suggest that the benefit for N2 fixation from elevated [CO2 ] is high as long as there is enough soil water to continue N2 fixation during grain filling.

In this study, we investigated the natural nodulation of legume forage crops were widely grown in the natural pastures in Sanliurfa, Turkey. This legume forage crops are Vicia sativa L. subsp. sativa L., Vicia narbonensis L. var. narbonensis L., Vicia palaestina Boiss., Vicia hybrida L., Vicia lutea L. var. lutea Boiss. ET Ball., Pisum sativum L. subsp. sativum L. var. sativum L, Cicer echinospermum P.H. Davis, Trifolium tomentosum L., Trifolium retusum L., Trifolium campestre SCHREB., Medicago truncatula GAERTN. var. truncatula Schultz Bip., Trigonella mesopotamica Hub.-Mor., Lens culinaris Medik., Onobrychis crista-galli, Lathyrus cassius Boiss., Melilotus officinalis (L.) DESR., Coronilla scorpioides (L.) W.D.J. Koch. Nodulation, nodule colors and shapes were examined at the blooming period of forage legumes. In this study, the colour of the interior of nodules are pink-red colour and may be related to high rates of nitrogen fixation in legume crops.

Several fast-growing and multipurpose tree species have been widely used in West Africa to both reverse the tendency of land degradation and restore soil productivity. Although beneficial effects have been reported on soil stabilization, there still remains a lack of information about their impact on soil microorganisms. Our investigation has been carried out in exotic and native tree plantations of 28 years and aimed to survey and compare the abundance and genetic diversity of natural legume-nodulating rhizobia (LNR). The study of LNR is supported by the phylogenetic analysis which clustered the isolates into three genera: Bradyrhizobium, Mesorhizobium, and Sinorhizobium. The results showed close positive correlations between the sizes of LNR populations estimated both in the dry and rainy seasons and the presence of legume tree hosts. There were significant increases in Rhizobium spp. population densities in response to planting with Acacia spp., and high genetic diversities and richness of genotypes were fittest in these tree plantations. This suggests that enrichment of soil Rhizobium spp. populations is host specific. The results indicated also that species of genera Mesorhizobium and Sinorhizobium were lacking in plantations of non-host species. By contrast, there was a widespread distribution of Bradyrhizobium spp. strains across the tree plantations, with no evident specialization in regard to plantation type. Finally, the study provides information about the LNR communities associated with a range of old tree plantations and some aspects of their relationships to soil factors, which may facilitate the management of man-made forest systems that target ecosystem rehabilitation and preservation of soil biota.

The pea leaf weevil, Sitona lineatus L., is periodically a significant pest of pea, Pisum sativum L., in the Palouse region of northern Idaho and eastern Washington, USA. Previous on-station research demonstrated significantly greater adult pea leaf weevil colonization, immature survival, adult emergence and plant damage in conventional-tillage compared to no-tillage plots of pea. In experiments conducted during the 2006 and 2007 growing seasons, aerial and ground adult pea leaf weevil colonization of large-scale commercial pea fields under different tillage regimes in northern Idaho and eastern Washington was examined for the first time. Initial pea leaf weevil feeding damage, immature weevil densities and subsequent adult emergence from the fields were also assessed. During both years, significantly more adult pea leaf weevils were captured in conventional-tillage than in no-tillage fields during the crop establishment period in May. No-tillage soils remained wet longer in the spring and could not be planted by growers until later than conventional-tillage fields. Pea planted under conventional-tillage emerged earlier and had significantly greater feeding damage by the pea leaf weevil than no-tillage pea. Significantly, greater immature pea leaf weevil densities and subsequent adult emergence were observed in conventional-tillage than in no-tillage pea fields. Delayed development of root nodules in the cooler, moister conditions of no-tillage pea fields likely resulted in escape from attack and injury during the critical growth stages that ultimately influence yield. Results indicate that large-scale commercial no-tillage pea fields are less suitable for colonization and survival of the pea leaf weevil and suffer less weevil damage than fields under conventional tillage.

Legumes acquired the ability to engage in a symbiotic interaction with soil-borne bacteria and establish a nitrogen-fixing symbiosis in a novel root organ, the nodule. Most legume crops and the model legumes Medicago truncatula and Lotus japonicus are infected intracellularly in root hairs via infection threads that lead the bacteria towards a nodule primordium in the root cortex. This infection process, however, does not reflect the great diversity of infection strategies that are used by leguminous plants. An alternative, intercellular invasion occurs in the semiaquatic legume Sesbania rostrata. Bacteria colonize epidermal fissures at lateral root bases and trigger cortical cell death for infection pocket formation and subsequent intercellular and intracellular infection thread progression towards the primordium. This infection mode evolved as an adaptation to waterlogged conditions that inhibit intracellular invasion. In this review, we discuss the molecular basis for this adaptation and how insights into this process contribute to general knowledge of the rhizobial infection process.

We used a split-root system to determine the timing for induction of the autoregulation of nodulation (AUT) in Lotus japonicus (Regel) Larsen after inoculation with Mesorhizobium loti. The signal took at least five days for full induction of AUT and inhibition of infection thread formation. Strain ML108 (able to nodulate but unable to fix nitrogen) induced full AUT, but ML101 (unable to nodulate or to fix nitrogen) did not induce autoregulation. These results indicate that Nod factor-producing strains induce AUT, but that the nitrogen fixed by rhizobia and supplied to the plant as ammonia does not elicit the AUT in L. japonicus.