Role Of Key Inorganic Phosphate Transporters In Non-terminally Differentiated Bacteroids

Rhizobia have developed mutually beneficial symbiotic associations with legume plants for biological nitrogen fixation.Rhizobia infect roots or stems nodules through the plant-made infection thread.Then the bacteria enlarge and differentiate into nitrogen-fixing bacteroids in the nodules.Nitrogen fixation in legume nodules involves a complex exchange of nutrients between the plant and bacteroids.In its simplest terms,this symbiosis is often viewed as an exchange of the reduced carbon of plant photosynthesis for the reduced nitrogen by rhizobia nitrogen fixation.However,it is clear that optimum nodule function also involves a balanced flow of other nutrients.One nutrient that has been shown to be important for this symbiosis is phosphorus.The transport of inorganic phosphate or other sources of phosphorus is essential for growth of all living organisms.Phosphate homeostasis is tightly modulated in all organisms,including bacteria,which harbor both high-and low-affinity transporters acting under conditions of fluctuating phosphate levels.It was thought that nitrogen-fixing rhizobia,named bacteroids,inhabiting root nodules of legumes are not phosphate limited.Here,we show that the high-affinity phosphate transporter PstSCAB,rather than the low-affinity phosphate transporter OrfA-Pit,is essential for effective nitrogen fixation of Sinorhizobium fredii CCBAU45436(SF45436)in soybean nodules.And orfA~pit expression is negatively regulated by PhoB,while expression of the stSCAB genes requires PhoB.Symbiotic and growth defects of the pst mutant can be effectively restored by knocking out PhoB,the transcriptional repressor of pit.These results uncover that symbiotic S,fredii bacteroids can be Pi limited in G.max nodules,and is quite different from finding that rhizobia are not phosphate limited in alfalfa root nodules.In further research,it was demonstrated that the pst homologs of representative rhizobia were actively transcribed in bacteroids without terminal differentiation in nodules of diverse legumes(soybean,pigeonpea,cowpea,common bean,and Sophora flavescens)but exhibited a basal expression level in terminally differentiated bacteroids(alfalfa,pea,and peanut).Rhizobium leguminosarum bv.viciae Rlv3841 undergoes characteristic nonterminal and terminal differentiations in nodules of S.flavescens and pea,respectively.The pst mutant of Rlv3841 showed impaired adaptation to the nodule environment of S.flavescens but was indistinguishable from the wild type strain in pea nodules.Taken together,root nodule rhizobia can be either phosphate limited or non-limited regarding the rhizobial differentiation fate,which is a host-dependent feature.These findings have important implications for further study of the differentiation and metabolic mechanisms of the two types bacteroids and the differential adaptation mechanisms of different legumes-rhizobia symbiotic nitrogen-fixing systems to the low phosphorus stress conditions.Understanding the diverse mechanisms by which bacteroids acquire phosphorus should provide many useful information for engineering rhizobia of improved symbiotic performance.