豆科植物已经进化出与根瘤菌的固氮共生相互作用,这种联系帮助他们应对土壤中有限的氮条件。寄主植物和根瘤菌之间的相容性相互作用导致根瘤的形成,其中根瘤菌内化和转变为它们的共生形式,被称为类细菌,发生。反向重复缺失的豆科根瘤中的根瘤菌,包括Medicagotruncatula,进行终末分化,导致拉长和内复制的类细菌。胞吞根瘤菌的这种转变是由truncatula中宿主产生的富含结节特异性半胱氨酸(NCR)肽的大基因家族介导的。最近发现很少有NCR对于类细菌的完全分化和持久性是必需的。这里,我们表明,M.truncatula共生突变体FN9285,在根瘤菌的完全过渡缺陷,缺乏NCR基因簇。更具体地说,我们表明,A17基因型中重复基因NCR086和NCR314的丢失,在紫花苜蓿littoralisR108中的单个拷贝中发现,是FN9285的无效共生表型的原因。NCR086和NCR314基因对编码相同的成熟肽,但它们的转录活性差别很大。然而,这两个基因都可以恢复FN9285中的有效共生,表明它们的互补能力不取决于其表达活性的强度。NCR086/NCR314肽的鉴定,对于完全的类细菌分化至关重要,扩展了肽的列表,来自一个有几百个成员的基因家族,这对于M.truncatula中有效的固氮共生至关重要。
Legumes have evolved a nitrogen-fixing symbiotic interaction with rhizobia, and this association helps them to cope with the limited nitrogen conditions in soil. The compatible interaction between the host plant and rhizobia leads to the formation of root nodules, wherein internalization and transition of rhizobia into their symbiotic form, termed bacteroids, occur. Rhizobia in the nodules of the Inverted Repeat-Lacking Clade legumes, including Medicago truncatula, undergo terminal differentiation, resulting in elongated and endoreduplicated bacteroids. This transition of endocytosed rhizobia is mediated by a large gene family of host-produced nodule-specific cysteine-rich (NCR) peptides in M. truncatula. Few NCRs have been recently found to be essential for complete differentiation and persistence of bacteroids. Here, we show that a M. truncatula symbiotic mutant FN9285, defective in the complete transition of rhizobia, is deficient in a cluster of NCR genes. More specifically, we show that the loss of the duplicated genes NCR086 and NCR314 in the A17 genotype, found in a single copy in Medicago littoralis R108, is responsible for the ineffective symbiotic phenotype of FN9285. The NCR086 and NCR314 gene pair encodes the same mature peptide but their transcriptional activity varies considerably. Nevertheless, both genes can restore the effective symbiosis in FN9285 indicating that their complementation ability does not depend on the strength of their expression activity. The identification of the NCR086/NCR314 peptide, essential for complete bacteroid differentiation, has extended the list of peptides, from a gene family of several hundred members, that are essential for effective nitrogen-fixing symbiosis in M. truncatula.