Split‐root systems applied to the study of the legume‐rhizobial symbiosis: What have we learned?

Split‐root system (SRS) approaches allow the differential treatment of separate and independent root systems, while sharing a common aerial part. As such, SRS is a useful tool for the discrimination of systemic (shoot origin) versus local (root/nodule origin) regulation mechanisms. This type of approach is particularly useful when studying the complex regulatory mechanisms governing the symbiosis established between legumes and Rhizobium bacteria. The current work provides an overview of the main insights gained from the application of SRS approaches to understand how nodule number (nodulation autoregulation) and nitrogen fixation are controlled both under non‐stressful conditions and in response to a variety of stresses. Nodule number appears to be mainly controlled at the systemic level through a signal which is produced by nodule/root tissue, translocated to the shoot, and transmitted back to the root system, involving shoot Leu‐rich repeat receptor‐like kinases. In contrast, both local and systemic mechanisms have been shown to operate for the regulation of nitrogenase activity in nodules. Under drought and heavy metal stress, the regulation is mostly local, whereas the application of exogenous nitrogen seems to exert a regulation of nitrogen fixation both at the local and systemic levels.     

Observation On The Germination Of The Seeds Of Crinum Macowani,Baker

Arid and semi-arid areas occupy an increasing fraction of the Earth's surface. Legume floras exist for most of these areas, but there is little information as to whether the plants nodulate and fix nitrogen (N) in their native habitats, although many have been used over millennia for food, forage and medicinal and other uses.

This review shows that, in those arid and semi-arid areas where data are available, the ability of legumes to nodulate is a significant attribute. It examines some host genera present and, where known, the bacteria that induce nodulation in them.

With some exceptions all legumes from well-studied arid areas have the potential to nodulate. Semi-arid areas vary between continents in terms of legume genera present, the probable extent of N fixation and in the endosymbionts (rhizobia) that induce nodulation in them.

With climate change and an increasing world population, there is an urgent need to develop the diverse range of nodulated legumes native to dry environments. With modern methods this goal is readily achievable.     

Genetic characterization of fast-growing rhizobia able to nodulate Prosopis alba in North Spain

Prosopis is a Mimosaceae legume tree indigenous to South America and not naturalized in Europe. In this work 18 rhizobial strains nodulating Prosopis alba roots were isolated from a soil in North Spain that belong to eight different randomly amplified polymorphic DNA groups phylogenetically related to Sinorhizobium medicae, Sinorhizobium meliloti and Rhizobium giardinii according to their intergenic spacer and 16S rRNA gene sequences. The nodC genes of isolates close to S. medicae and S. meliloti were identical to those of S. medicae USDA 1,037(T) and S. meliloti LMG 6,133(T) and accordingly all these strains were able to nodulate both alfalfa and Prosopis. These nodC genes were phylogenetically divergent from those of the isolates close to R. giardinii that were identical to that of R. giardinii H152(T) and therefore all these strains formed nodules in common beans and Prosopis. The nodC genes of the strains isolated in Spain were phylogenetically divergent from that carried by Mesorhizobium chacoense Pr-5(T) and Sinorhizobium arboris LMG 1,4919(T) nodulating Prosopis in America and Africa, respectively. Therefore, Prosopis is a promiscuous host which can establish symbiosis with strains carrying very divergent nodC genes and this promiscuity may be an important advantage for this legume tree to be used in reforestation.     

Promiscuity of Hosting Nitrogen Fixation in Rice: An Overview from the Legume Perspective

ABSTRACT:?

The subject area of this review provides extraordinary challenges and opportunities. The challenges relate to the fact that the integration of various fields such as microbiology, biochemistry, plant physiology, eukaryotic as well as bacterial genetics, and applied plant sciences are required to assess the disposition of rice, an alien host, for establishing such a unique phenomenon as biological nitrogen fixation. The opportunities signify that, if successful, the breakthrough will have a significant impact on the global economy and will help improve the environment. This review highlights the literature related to the area of legume-rhizobia interactions, particularly those aspects whose understanding is of particular interest in the perspective of rice. This review also discusses the progress achieved so far in this area of rice research and the possibility of built-in nitrogen fixation in rice in the future. However, it is to be borne in mind that such research does not ensure any success at this point. It provides a unique opportunity to broaden our knowledge and understanding about many aspects of plant growth regulation in general.     

Regulation of legume nodulation by acidic growth conditions

Legumes represent some of the most important crop species worldwide. They are able to form novel root organs known as nodules, within which biological nitrogen fixation is facilitated through a symbiotic interaction with soil-dwelling bacteria called rhizobia. This provides legumes with a distinct advantage over other plant species, as nitrogen is a key factor for growth and development. Nodule formation is tightly regulated by the plant and can be inhibited by a number of external factors, such as soil pH. This is of significant agricultural and economic importance as much of global legume crops are grown on low pH soils. Despite this, the precise mechanism by which low pH conditions inhibits nodule development remains poorly characterized.     

The effectiveness and competitiveness of some Indonesian Rhizobium strains on tropical legumes grown in four soil types of Java

Several Indonesian and some imported Rhizobium strains were assessed for their effectiveness in nodulating four legume species in four soil types of Java. Naturally occurring Rhizobia formed effective symbioses onVigna unguiculata, Macroptilium atropurpureum andDesmodium heterocarpon in all four soils and the applied strains, with some exceptions, did not infect a majority of nodules of these legumes.Centrosema pubescens was more specific in its Rhizobia requirements and applied strains formed effective symbioses in two clay soils, but not in two sandy loam soils.     

Novel root nodule bacteria belonging to the genus Caulobacter

Aims: Aim of this study is to determine the genetic variation of rhizobia associated with horse gram [Macrotyloma uniflorum (Lam.) Verdc.] plants grown in different regions of Andhra Pradesh, India. Methods and Results: Four representative isolates having most representative characters from the previous characterization were selected for 16S rRNA sequence. The sequences were submitted to the NCBI GenBank and Ribosomal Database Project (RDP). The isolates HGR‐4, 6 and 13 showed more than 99% homology between them and they were grouped with Rhizobium reference strains where as the isolate HGR‐25 showed 87·1, 87·4 and 87·2% homology with the isolates HGR‐4, 6 and 13, respectively, and were grouped with reference strains for Caulobacter. The nodulation ability of these isolates on horse gram was confirmed by inoculation tests. Conclusions: The isolate HGR‐25 was identified as Caulobacter isolated from the plants growing in soil samples collected from Khareemnagar district, Andhra Pradesh, India. Inoculation tests revealed that Caulobacter formed nodules on horse gram. It was also confirmed by RDP. Significance and Impact of the Study: This is the first report that a legume was nodulated by a member of the genus Caulobacter, which belongs to the family Caulobacteriaceae in the order Caulobacterales of Alphaproteobacteria.     

Growth and root responses of woody species to rocky substrate: implications for gully rehabilitation

Gullies formed in the Velhas River basin in Brazil have been filled with urban construction waste for physical stabilisation purposes. Aimed at rehabilitating gullies, we selected woody species from the Brazilian Cerrado that can grow on rocky substrates under greenhouse conditions. An assessment was made regarding plant growth in both rocky and natural soil substrates by analysing the height, diameter, fresh and dry weights of shoots and roots, plant water content, root occupation and architecture. Principal component analysis and Chi-squared tests segregated rock-tolerant species based on the specific influence on root dry and fresh weights. Fast-growing species reduced the emergence of their lateral roots under rocks, compromising their growth in height and biomass production. In contrast, slow-growing woody species were particularly suitable for gully rehabilitation because these species exhibited a genetic pattern of low lateral root emergence that prevented damage to their roots. Most slow-growing species demonstrated a similar growth pattern in both substrates, and some of them, such as Copaifera langsdorffii, achieved better growth in height and biomass production on rocks than on soil, a finding attributed to the root plastic response involving primary root elongation and lateral root emergence. Therefore, slow-growing species are recommended for gully rehabilitation procedures.     

The effects of salinity and sodicity upon nodulation and nitrogen fixation in chickpea (Cicer arietinum)

Production of grain legumes is severely reduced in salt-affected soils because their ability to form and maintain nitrogen-fixing nodules is impaired by both salinity and sodicity (alkalinity). Genotypes of chickpea, Cicer arietinum, with high nodulation capacity under stress were identified by field screening in a sodic soil in India and subsequently evaluated quantitatively for nitrogen fixation in a glasshouse study in a saline but neutral soil in the UK. In the field, pH 8.9 was the critical upper limit for most genotypes studied but genotypes with high nodulation outperformed all others at pH 9.0-9.2. The threshold limit of soil salinity for shoot growth was at ECe 3 dS m(-1), except for the high-nodulation selection for which it was ECe 6. Nodulation was reduced in all genotypes at salinities above 3 dS m(-1) but to a lesser extent in the high-nodulation selection, which proved inherently superior under both non-saline and stress conditions. Nitrogen fixation was also much more tolerant of salinity in this selection than in the other genotypes studied. The results show that chickpea genotypes tolerant of salt-affected soil have better nodulation and support higher rates of symbiotic nitrogen fixation than sensitive genotypes.     

氮添加对七彩花生与根瘤菌共生关系及生物量分配的影响

为了探明不同氮条件下根瘤菌对花生生物量分配的影响,在盆栽条件下以七彩花生为材料,设置接种和不接种慢生根瘤菌处理,探讨添加0、8、16、32、64和128 mmol·L^-1氮条件下七彩花生生物量分配和共生结瘤特征。结果表明: 1)不接菌条件下,与无氮添加(0 mmol·L^-1)相比,低水平氮(8～32 mmol·L^-1)添加对植物生物量影响较小,氮添加达64 mmol·L^-1时植株总生物量、叶生物量、叶面积和总净光合速率开始显著增加,分别增加82.1%、116.6%、116.1%和122.1%。2)接菌条件下,与无氮添加(0 mmol·L^-1)相比,植株总生物量、叶生物量、叶面积和总净光合速率显著增加发生在16 mmol·L^-1氮添加下,分别增加65.3%、97.5%、91.7%和112.8%;植株结瘤生物量和豆血红蛋白总量随氮浓度的增加呈现先增加后减少趋势,两者均在16 mmol·L^-1氮添加下达最大值,分别为49.00和0.12 mg·株^-1,当氮添加水平达到64 mmol·L^-1时显著降低,当氮添加水平为128 mmol·L^-1时根系不结瘤。3)与不接种根瘤菌相比,接种根瘤菌在氮添加8～64 mmol·L^-1下显著增加了植株叶生物量、地上生物量、叶面积和总净光合速率,总体增幅分别为43.3%、37.6%、34.5%和53.8%;而在0和128 mmol·L^-1氮添加下对以上指标均无显著影响;整体上,接种根瘤菌显著提高了植株叶-根和叶-总生物量的异速生长常数,降低了根-茎和根-总生物量的异速生长常数。综上,七彩花生在环境氮吸收与共生固氮权衡中积极调整对各器官的营养分配策略,以实现资源投资利益最大化。在本试验条件下,16 mmol·L^-1为七彩花生-慢生根瘤菌共生结瘤的最佳氮添加水平。