Recent development and new insight of diversification and symbiosis specificity of legume rhizobia: mechanism and application

Abstract

SummaryCurrently, symbiotic rhizobia (sl., rhizobium) refer to the soil bacteria in aand b-Proteobacteria that can induce root and/or stem nodules on some legumes and a few of nonlegumes. In the nodules, rhizobia convert the inert dinitrogen gas (N 2 ) into ammonia (NH 3 ) and supply them as nitrogen nutrient to the host plant. In general, this symbiotic association presents specificity between rhizobial and leguminous species, and most of the rhizobia use lipochitooligosaccharides, so called Nod factor (NF), for cooperating with their host plant to initiate the formation of nodule primordium and to inhibit the plant immunity. Besides NF, effectors secreted by type III secretion system (T3SS), exopolysaccharides and many microbeassociated molecular patterns in the rhizobia also play important roles in nodulation and immunity response between rhizobia and legumes. However, the promiscuous hosts like Glycine max and Sophora flavescens can nodulate with various rhizobial species harbouring diverse symbiosis genes in different soils, meaning that the nodulation specificity/efficiency might be mainly determined by the host plants and regulated by the soil conditions in a certain cases. Based on previous studies on rhizobial application, we propose a '1+nÀN' model to promote the function of symbiotic nitrogen fixation (SNF) in agricultural practice, where '1' refers to appreciate rhizobium; '+n' means the addition of multiple trace elements and PGPR bacteria; and 'ÀN' implies the reduction of chemical nitrogen fertilizer. Finally, open questions in the SNF field are raised to future think deeply and researches. Current rhizobial taxonomic outline and use of genome sequence in rhizobial systematicsCurrently, all the symbiotic nitrogen-fixing bacteria associating with legumes are found in the Phylum Proteobacteria, mainly in the Classes Alphaproteobacteria (a-rhizobia) and Betaproteobacteria (b-rhizobia), but maybe also Gammaproteobacteria (c-rhizobia) (Shiraishi et al. 2010) , with about 180 species in 20 genera at the time of writing (Fig. 1). Among them, a-rhizobia are the most common group with a very wide distribution in biogeography and host plants, and beta-rhizobia are also well established with specific legumes though less widely distributed.It is common that symbiotic rhizobia are usually intermingled with nonsymbiotic bacteria at different taxonomic levels. For example, a group of nonsymbiotic bacteria isolated from maize root endosphere, Rhizobium

Nickel mine soil is a potential source for soybean plant growth promoting and heavy metal tolerant rhizobia

“…As a host plant, soybean is promiscuous and may be nodulated with both fast- and slow-growing rhizobia ( Chen et al, 2021 ). Based on the 16S rRNA gene analysis, three of our isolates were identified as Ensifer americanum, E. fredii or E. xinjiangense , i.e.…”

Biodiversity and Geographic Distribution of Rhizobia Nodulating With Vigna minima

“…Rhizobia are a kind of specific soil bacteria that could induce the formation of root and/or stem nodules and perform symbiotic biological fixation with legume hosts ( Long, 1996 ; Masson-Boivin and Sachs, 2018 ; Bisseling and Geurts, 2020 ; Lindström and Mousavi, 2020 ). Currently, rhizobia encompass more than 180 species in 21 genera of two classes ( Alphaproteobacteria and Betaproteobacteria ), and the genera Bradyrhizobium , Rhizobium , Mesorhizobium , and Ensifer harbored the most common and the majority of the rhizobial species; furthermore, nodulation was thought to arose from Bradyrhizobium ( Ormeño-Orrillo and Martínez-Romero, 2019 ; Chen et al, 2020 ).…”

A newly-evolved chimeric lysin motif receptor-like kinase in Medicago truncatula spp. tricycla R108 extends its Rhizobia symbiotic partnership

“…Different genera/species/strains may show either a narrow or promiscuous host range (Masson-Boivin et al, 2009). Similarly, some legumes are highly promiscuous whereas others show exquisite fidelity to a narrow range of rhizobia (Andrews & Andrews, 2017;Chen et al, 2021) Coevolution has ensured that Legume-Rhizobia couples are highly adapted to each other and has led to improvement of plant and bacterial fitness (Younginger & Friesen, 2019).…”

“…Various chemical substitutions on the basic structure lead to the primary mechanism of partner specificity (D'Haeze & Holsters, 2002;Oldroyd et al, 2011;Krönauer & Radutoiu, 2021). Although other signals may be important for some legume-Rhizobia interactions, such as cell-surface polysaccharides or protein effectors (Wang et al, 2018;Chen et al, 2021), the genomic linkage of the NF (nod) genes to nitrogen fixation (nif) genes (generally on a symbiotic megaplasmid or a genomic island), provides a means by which NF signalling can be used . CC-BY-NC-ND 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.…”

“…Different genera/species/strains may show either a narrow or promiscuous host range (Masson-Boivin et al ., 2009). Similarly, some legumes are highly promiscuous whereas others show exquisite fidelity to a narrow range of rhizobia (Andrews & Andrews, 2017; Chen et al ., 2021).…”