| The symbiosis between organisms is a common life activity and ecological phenomenon on the earth,and is also the most basic and most important relationship between organisms.Plant and microbial symbiosis is one of the common symbiotic phenomena in nature.Among them,the mutualistic symbiosis between rhizobia and legumes is one of the most concerned models.However,Only 0.3%-0.5%of legume plants,the third largest family of angiosperms,have been studied the symbiotic relationship with rhizobia.The majority of legume hosts studied were in the superfamily Papilionoideae,including the model hosts Medicago truncatula and Lotus japonicus,which were studied deeply in symbiotic nitrogen fixation with rhizobia.However,little is known about the symbiosis between Sophora flavescens that phylogenetically locates in the basal phylogenetic tree of Papilionoideae and rhizobia.Sophora flavescens belongs to the genus Sophora,Fabaceae.As a traditional chinese medicinal plant,S.flavescens has the curative effects that clear heat,dryness,dampness and diuresis by taking its roots.The two scientific question we raised were:a,which rhizobia species form nodules with S.flavescens and what about of the phylogenetic status of their housekeeping genes and symbiotic genes;b,how physical and chemical factors of soil in different ecological zones affect the biogeographic distribution of rhizobia of S.flavescens.Through collecting nodules and isolating rhizobia from three different ecological zones(Shanxi province,Shaanxi province and Liaoning province),it was found that the rhizobium species of S.flavescens were distributed in 5 different genera and 17 different genospecies.Two populations of rhizobia with uncertain phylogenetic status were identified.Secondly,we found that soil physical and chemical factors had different levels of effects on the biogeographic distribution of rhizobia of S.flavescens,of which pH value was a key determinant.Further cross-nodulation assays showed that among the 54 representative strains in the class of ? and ?,most of the representative rhizobia from the a class could form effective and functional nodules with S.flavescens,while those from the ?were ineffective strains.These results indicate that S.flavescens is a highly promiscuous legume host,which establishes symbiotic relationship with rhizobia that have different genome background.This is called "symbiotic relationship with promiscuity".In order to explore the association between the promiscuous symbiosis and the infection mode of rhizobia,lacZ markered strains S.fredii CCBAU 45436 and R.yanglingense CCBAU 01603 were used to observe the infection process.It was found that the two rhizobium species entered the root cortex cells through the typical root hair infection thread,which also occurred in the hosts M.truncatula and L.japonicas,which both have specific symbiosis with rhizobia.Through the electron microscope slices of the root nodules,we found that there were some differences in the physical development of bacteroids in the nodules of S.flavescens formed by different rhizobia,such as the synthetic amount of PHB and the amount of the bacteroid embedded inside the perbacteroid membrane inside a host plant cell.Studies have shown that legume nod factor receptor NFP is one of the key factors to determine the symbiotic spectrum of host with rhizobia.In order to reveal the role of the NFP homologous gene(SfNFP)of S.flavescens in the symbiotic promiscuity with diverse rhizobia,and considering that the genome of S.flavescens was not sequenced,we used transcriptome strategy to obtain the candidate gene SfNFP(c51657_g1)sequence.Further,by constructing phylogenetic tree and amino acid sequences alignment,it showed that c51657_g1 and other identified NFP homologous were grouped together and Shared highly conserved amino acid sequences.To further verify the biological function of c51657_g1,we used the c51657_g1 gene to complement M.truncatula mutant nfp.The results showed that SfNFP(c51657_g1)fully rescued the phenotype of nfp mutant when inoculation with compatible S.meliloti 2011.Further,normally incompatible bacteria for M.truncatula triggered root cell differentiation and initiation of the developmental program leading to empty nodule organogenesis on the nfp background transformed SfNFP.This suggests that SfNFP can recognize different rhizobia and activate the downstream pathway of symbiotic signal.Finally,two strains that efficiently nodulate and fix nitrogen with S.flavescens in both vermiculite and soil were identified.In conclusion,so far S.flavescens is one of the most promiscuous hosts when interact with rhizobia.SfNFP is an important key to understand the molecular mechanism of symbiotic promiscuity.Overall,our study provided a glimpse into interaction between promiscuous legume hosts with symbionts,laying a foundation for further study of promiscuous symbiosis. |
PDF Full Text Request