紫云英根瘤菌的exo与nod基因在宿主结瘤过程中的协同作用

紫云英根瘤菌菌株107经转座子Tn5诱变的胞外多糖合成缺陷型变种(Exo~-),在共生性状方面的改变有四种类型。我们选用了仅在宿主根部形成瘤状突起(Calli)的A类(NA-01,NA-02)、形成无效瘤(Nod~ ,Fix~-)的B类(NA-12)及不结瘤的(Nod~-)D类(NA-14)中的四个突变株分别与消除了共生质粒的Exo~ Nod~-变种(热处理变种及ANU-1116)混合接种紫云英幼苗,观察到与D类变种配合的接种组仍不能结瘤,而与A,B两类变种配合的接种组均诱导宿主产生形态正常的无效根瘤,并且该无效瘤全部被Nod~-变种侵占。说明一个表型仍为Exo~ ,但失去共生质粒的Nod~-变种,可在一个含有该质粒的Exo~-变种的帮助下进入根瘤。这提示共生质粒上的结瘤基因(nod)仅与侵染过程的早期有关,瘤的发育尚需根瘤菌的其他基因,其中包括exo基因的参与。 此外,共生质粒缺失的三个异种根瘤菌突变株分别与紫云英根瘤菌Exo~-变种混合接种时,也都在紫云英根部诱导出无效瘤,并且从瘤中能分离到这三个异种菌。表明在最初的识别作用发生后,植物对共生菌的专一性要求有所降低。

Two Classes of Genes are Involved in the Nodulation of Astragalus sinicui

Several mutants of Rhizobium astragalus strain 107 with transposon Tn5 insertion fail to produce normal amount of extracellular polysaccharides, and induce on the host Astragalus sinicus L. only ineffective nodules void of the nitrogen-fixation activity. To investigate the possibility that R. astragalus Exo~ Nod~- mutants, which have lost their Sym-plasmid and are thoroughly unable to in.fect the host, might "help" the Exo- mutants to invade the host, we co-inoculated the plants with both types of the mutants and found unexpectedly that the former type did not improved their symbiotic behavior with the help of Exo~ Nod~- mutants while the latter occupied the nodules exclusively. The result indicate that the mutants which have lost their Sym-plasmid and their symbiotic activity may enter the plant cells after the nodule initiation was induced by the mutant defective in the EPS synthesis but containing the Sym-plasmid. Thus we conciude that certain genes located on the Symplasmid are involved only in the very early stages of the rhizobium infection process whereas the exo genes are necessary for the development of nodules. In other experiments in which the pSym-cured Exo~ Nod~- mutants of other three species of Rhizobium were applied to co-inoculate A. sinicus respectively with R. astragalus Nod~ Exo~- mutants, only ineffective nodules were formed on the plant roots and the other species of bacteria were re-isolated from these nodules, indicating that the specificity requirement of the plants for the symbiotic bacteria had declined after the early recognition took place.