Plant Molecular Genetics, Institute of Agriculture and Center for Legume Research, The University of Tennessee, P.O. Box 1071, 37901-1071, Knoxville, TN, USA.
Theor Appl Genet. 1992 Aug;84(5-6):624-32. doi: 10.1007/BF00224161.
Rhizobium nodulation genes can produce active extracellular signals for legume nodulation. The R. meliloti host-range nodH gene has been postulated to mediate the transfer of a sulfate to a modified lipo-oligosaccharide, which in its sulfated form is a specific nodulation factor for alfalfa (Medicago sativa L.). We found that alfalfa was capable of effective nodulation with signal-defective and non-nodulating nodH mutants (Nnr) defining a novel gene-for-gene interaction that conditions nodulation. Bacteria-free nodules that formed spontaneously at about a 3-5% rate in unselected seed populations of alfalfa cv 'Vernal' in the total absence of Rhizobium (Nar) exhibited all the histological, regulatory and ontogenetic characteristics of alfalfa nodules. Inoculation of such populations with nodH mutants, but not with nodA or nodC mutants, produced a four- to five-fold increase in the percentage of nodulated plants. Some 10-25% of these nodulated plants formed normal pink nitrogen-fixing nodules instead of white empty nodules. About 70% of the S1 progeny of such Nnr(+) plants retained the parental phenotype; these plants were also able to form nodules in the absence of Rhizobium. If selected Nar(+) plants were self-pollinated almost the entire progeny exhibited the parental Nar(+) phenotype. Segregation analysis of S1 and S2 progeny from selected Nar(+) plants suggests that the Nar character is monogenic dominant and that the nodulation phenotype is controlled by a gene dose effect. The inoculation of different S1 Nar(+) progeny with nodH mutant bacteria gave only empty non-fixing nodules. Our results indicate that certain alfalfa genotypes can be selected for suppression of the non-nodulation phenotype of nodH mutants. The fact that the Nnr plant phenotype behaved as a dominant genetic trait and that it directly correlated with the ability of the selected plants to form nodules in the absence of Rhizobium suggests that the interaction of plant and bacterial alleles occurs early during signal transduction through the alteration of a signal reception component of the plant so that it responds to putative signal precursors.
根瘤菌的结瘤基因可以产生对豆科植物结瘤有活性的胞外信号。推测 R. meliloti 宿主范围 nodH 基因介导硫酸盐向修饰的脂寡糖的转移，而其硫酸化形式是紫花苜蓿（Medicago sativa L.）的特定结瘤因子。我们发现，苜蓿能够与信号缺陷和不结瘤的 nodH 突变体（Nnr）有效结瘤，这定义了一种新的基因对基因相互作用，从而调节结瘤。在没有根瘤菌（Nar）的情况下，在苜蓿 cv 'Vernal'未选择的种子群体中自发形成的约 3-5%的无细菌结瘤显示出苜蓿结瘤的所有组织学、调节和个体发生特征。用 nodH 突变体接种这些群体，但不用 nodA 或 nodC 突变体接种，结瘤植物的百分比增加了 4-5 倍。这些结瘤植物中约有 10-25%形成正常的粉红色固氮结瘤而不是白色空结瘤。这些 Nnr(+)植物的 S1 后代中约有 70%保留了亲本表型；这些植物在没有根瘤菌的情况下也能形成结瘤。如果选择 Nar(+)植物自交，几乎整个后代都表现出亲本 Nar(+)表型。从选择的 Nar(+)植物的 S1 和 S2 后代的分离分析表明，Nar 特征是单基因显性的，结瘤表型受基因剂量效应的控制。用 nodH 突变体细菌接种不同的 S1 Nar(+)后代只会产生空的非固氮结瘤。我们的结果表明，某些苜蓿基因型可以被选择来抑制 nodH 突变体的不结瘤表型。Nnr 植物表型表现为显性遗传特征的事实，以及它与所选植物在没有根瘤菌的情况下形成结瘤的能力直接相关，表明植物和细菌等位基因的相互作用发生在信号转导的早期，通过改变植物的信号接收成分，使其对假定的信号前体做出反应。
