Rhizobia interact with host legumes to induce the formation of nitrogen-fixing nodules,which is very important in agriculture and ecology.The development of nitrogen-fixing nodules is stringently regulated by host plants and rhizobial symbionts.In our previous work,a new Sinorhizobium meliloti LysR regulator gene(lsrB)was identified to be essential for alfalfa nodulation.However,how this gene is involved in alfalfa nodulation was not yet understood.Here,we found that this gene was associated with prevention of premature nodule senescence and abortive bacteroid formation.Heterogeneous deficient alfalfa root nodules were induced by the in-frame deletion mutant of lsrB(lsrB1-2),which was similar to the plasmid-insertion mutant,lsrB1.Irregular senescence zones earlier appeared in these nodules where bacteroid differentiation was blocked at different stages from microscopy observations.Interestingly,oxidative bursts were observed in these nodules by DAB staining.The decreased expression of lipopolysaccharide core genes(lpsCDE)was correspondingly determined in these nodules.S.meliloti lipopolysaccharide is required for suppression of oxidative bursts or host cell defense.These findings demonstrate that the S.meliloti lsrB gene is involved in alfalfa root nodule development and bacteroid differentiation by suppressing oxidative bursts or defense responses in host cells.
Emerging evidence has demonstrated that ROP GTPases play important roles in symbiosis,but the molecular mechanisms on their regulation in symbiosis are largely unknown.In this study,we showed that MtROP8 is involved in the symbiotic interaction between Medicago truncatula and Sinorhizobium meliloti.Expression analyses showed that MtROP8 was down-regulated in the early infected roots,but significantly up-regulated in nodules compared to the roots.Phenotypic analysis of RNA interference(RNAi)-mediated silencing of MtROP8 revealed that knock-down of MtROP8 expression resulted in various developmental defects of root hairs,including branched hairs,short bulbous root hairs,and even root hairs with apparent swollen bases,which were caused by the modification of the distribution and the levels of reactive oxygen species(ROS).Moreover,infection events were increased in transgenic roots harboring the MtROP8 RNAi construct in response to S.meliloti inoculation,concomitant with enhanced nodulation.These results indicate that MtROP8participates in root hair development and the establishmentof the symbiotic interaction by regulating ROS production and distribution.
Qi WangMingjuan LeiAimin ChenRuigang WangGuojing LiYanzhang Wang
Small peptides function as key signals in processes,such as plant cell differentiation,organ development and defenses to biotic stresses.A large number of small peptide precursor genes have been predicted from the analysis of the soybean(Glycine max) whole genome DNA sequence.However,most of these genes have unknown characteristics and functions.In this report,we systemically searched for the gene families of small peptide precursors that are up-regulated in soybean nitrogen-fixing root nodules.We found 212 genes(encoding peptides shorter than 150 amino acids) that were up-regulated,and among them,79 genes belong to 38 multiple-gene families,but the other 133 genes are unique.Twenty-eight of 38 families are conserved in Arabidopsis,but the other 10 only exist in legumes.We also identified 16 out of the 38 members of the wound-induced polypeptide(WIP) gene family to be upregulated in nitrogen-fixing nodules.We further analyzed homologs of WIP genes in Medicago,Lotus,Arabidopsis and Oryza species and found that a few homologous genes from Medicago truncatula and Lotus japonicus were also upregulated in their nodules and some WIP genes were induced by specific fungal pathogens on soybean and rice.Structure prediction indicated that all WIP prepropeptides contain a conserved DUF3774 domain(including two hydrophobic regions) and most of them have an N-terminal signal sequence.Fluorescence microscopy analysis of two WIP prepropeptides fused to GFP revealed that these proteins are located on the plasma membrane of tobacco leaf cells.Interestingly,34 soybean WIP genes are clustered onto three soybean chromosomes,different from known peptide gene families(such as CLE).Among them,11 highly identical genes are aligned on the 6th chromosome,12 on the 12th,and 11 on the 13th chromosomes.Most of WIP genes from the 12th chromosome share the highest identities with their homologs on the 13th chromosome,suggesting that ancestral WIP genes could have originated from the 13th chromosome,then spread onto the 12th chromosome by chrom
