The Nodule Infection And Competition Of Rhizobium Etli CFN42 Through The Effect Of NifA Gene

A mutualism was formed due to a long-term evolution between rhizobium and their host.The bacteria fixed N2 in atmosphere to ammonia and then offered to their host,in return,the legume contributed carbon source to their symbion.How could this balance remained on this versatile environment?when one of the system changed(especially the symbacteria),what the other one will do?As for this question there were two different theories,one was already been accepted called "host sanction",mean that when the bacteria lose their symbiotic ability and still infected with host,the host plant will bring a punishment on these non-cooperated rhizobia.Another theory was raised rencently:"partner choice",the supporters regarded the relationship between rhizobia and host as a friendly one.The legume host and rhizobia will select the most suitable one among others,and then establish the symbiotic system.Our recent research showed that the absence of a nitrogen fixation regulated gene nifA in Rhizobium etli CFN42 would lead to some obvious differences in bacteria,the lack of NifA regulator would first lose the control of nif-connected genes and result in deficiency of nitrogenase(the nitrogenase of ?nifA was below 0.1?mol C2H4·(min-1·g-1)pure nodule),and the balance of autoregulation of nodulation would be broken,and we shown with nodulation kinetics that mutant strain would form nodules 30%more than wild type.This kind of mutation would increase competitive edge while mixed infected with wild type strain with the ratio 1:1 and also increase fitness of rhizobia while co-exist in one nodule with wild type in vivo through sevral different ratio(the AnifA was about 102-105 times than WT).In our result of reseach the host could not successfully identified the non-nitrogen fixation strain(?nifA)in nodule level,behaved at the ?nifA would not suffered a decrease from host.But,as for whole plant,the percent of wildtype would add with the growth of legume.According to the charactristics of nifA mutant,we designed an experiment to detect the ability of two strains while coinfected with other hosts' rhizobia,we selected two other thizobia that could form nodules with phaseolus vulgaris.During the in-vitro test strain Rhizobium fabae(R.fabae)was more competitive in nutrition,and Azorhizobium caulinodans(AZC)was weaker.In nodulation occupancy.R.fabae behaved similarly while competed with R.etli,and in this experiment ?nifA suffered a decrease co-infected with R.fabae in one single nodule while WT could make a balance with R.fabae.Competition belong R.etli and AZC 1 made a equal result:nifA mutant behaved powerful competed with wild type but not while co-cultured with nonhomologous competitors.During the results of this eaperiment,we only discovered the nodulation occupancy increase of WT in mixed cultulted with ?nifA but not in single nodule.We considered that the regulated protein NifA may include in other metabolism,so the configurable gene of nitrogase will be studied in others experiment in our research group.The test found that rhizobia with stronger competition in obtaining nutrition also have a competitive edge in nodulation occupancy,this result could regard as a guide in the selection of rhizobium during legume cultivation.