Host compatible rhizobia induce the formation of legume root nodules, symbiotic organs within which intracellular bacteria are present in plant-derived membrane compartments termed symbiosomes. In Medicago truncatula nodules, the Sinorhizobium microsymbionts undergo an irreversible differentiation process leading to the development of elongated polyploid noncultivable nitrogen fixing bacteroids that convert atmospheric dinitrogen into ammonia. This terminal differentiation is directed by the host plant and involves hundreds of nodule specific cysteine-rich peptides (NCRs). Except for certain in vitro activities of cationic peptides, the functional roles of individual NCR peptides in planta are not known. In this study, we demonstrate that the inability of M. truncatula dnf7 mutants to fix nitrogen is due to inactivation of a single NCR peptide, NCR169. In the absence of NCR169, bacterial differentiation was impaired and was associated with early senescence of the symbiotic cells. Introduction of the NCR169 gene into the dnf7-2/NCR169 deletion mutant restored symbiotic nitrogen fixation. Replacement of any of the cysteine residues in the NCR169 peptide with serine rendered it incapable of complementation, demonstrating an absolute requirement for all cysteines in planta. NCR169 was induced in the cell layers in which bacteroid elongation was most pronounced, and high expression persisted throughout the nitrogen-fixing nodule zone. Our results provide evidence for an essential role of NCR169 in the differentiation and persistence of nitrogen fixing bacteroids in M. truncatula.Sinorhizobium | ineffective nodules | symbiotic host peptides | senescence | bacteroid differentiation

Legumes form endosymbiotic associations with nitrogenfixing bacteria and arbuscular mycorrhizal (AM) fungi which facilitate nutrient uptake. Both symbiotic interactions require a molecular signal exchange between the plant and the symbiont, and this involves a conserved symbiosis (Sym) signaling pathway. In order to identify plant genes required for intracellular accommodation of nitrogen-fixing bacteria and AM fungi, we characterized Medicago truncatula symbiotic mutants defective for rhizobial infection of nodule cells and colonization of root cells by AM hyphae. Here, we describe mutants impaired in the interacting protein of DMI3 (IPD3) gene, which has been identified earlier as an interacting partner of the calcium/ calmodulin-dependent protein, a member of the Sym pathway. The ipd3 mutants are impaired in both rhizobial and mycorrhizal colonization and we show that IPD3 is necessary for appropriate Nod-factor-induced gene expression. This indicates that IPD3 is a member of the common Sym pathway. We observed differences in the severity of ipd3 mutants that appear to be the result of the genetic background. This supports the hypothesis that IPD3 function is partially redundant and, thus, additional genetic components must exist that have analogous functions to IPD3. This explains why mutations in an essential component of the Sym pathway have defects at late stages of the symbiotic interactions.Legumes form nitrogen-fixing symbioses with soil bacteria collectively called rhizobia and, like most plant species, they also establish interactions with arbuscular mycorrhizal (AM) fungi. Both symbioses facilitate the uptake of mineral nutrients. Host plants and their symbionts form specialized structures during these interactions: rhizobia induce the development of root nodules on the host plant wherein the reduction of atmospheric nitrogen takes place, while AM fungal hyphae form a densely ramified structure in the inner cortical cells of the root where the transfer of nutrients occurs.The exchange of diffusible signaling molecules between symbionts and host plant takes place prior to physical contact. The flavonoid and isoflavonoid contents of root exudates activate rhizobia to produce nodulation (Nod) factors (NF), which induce the host plant to initiate nodulation ( Both rhizobia and mycorrhizal fungi exist as intracellular symbionts, which requires the coordinated development of both partners to allow invasion and appropriate differentiation (Jones et al. 2007;Oldroyd and Downie 2008;Parniske 2008). The best-characterized rhizobial infection strategy occurs via root hairs. Rhizobia become trapped in root hair curls and induce invaginations from these curled root hairs, forming tubular structures called infection threads (IT). The IT grows toward the newly divided cells of the developing nodule primordial, wherein the bacteria are released and colonize the host cells through endocytosis. The bacteria become enveloped with a plant-derived peribacteroid membrane, forming a cytoplasmic structure referred to as th...

SummaryThe chaplin and rodlin proteins together constitute the major components of the hydrophobic sheath that coats the aerial hyphae and spores in Streptomyces, and mutants lacking the chaplins are unable to erect aerial hyphae and differentiate on minimal media. We have gained insight into the developmental regulation of the chaplin (chp) and rodlin (rdl ) genes by exploiting a new model species, Streptomyces venezuelae, which sporulates in liquid culture. Using microarrays, the chaplin and rodlin genes were found to be highly induced during submerged sporulation in a bldNdependent manner. Using s BldN ChIP-chip, we show that this dependence arises because the chaplin and rodlin genes are direct biochemical targets of s