Dickstein R, Bisseling T, Reinhold V N, Ausubel F M
Department of Genetics, Harvard Medical School, Boston, Massachusetts.
Genes Dev. 1988 Jun;2(6):677-87. doi: 10.1101/gad.2.6.677.
To help dissect the molecular basis of the Rhizobium-legume symbiosis, we used in vitro translation and Northern blot analysis of nodule RNA to examine alfalfa-specific genes (nodulins) expressed in two types of developmentally defective root nodules elicited by Rhizobium meliloti. Fix- nodules were elicited by R. meliloti nif mutants; these nodules were invaded by rhizobia and contained differentiated bacteroids. 'Empty' nodules were elicited by R. meliloti exo and ndv mutants and by Agrobacterium tumefaciens strains carrying the R. meliloti nod genes; these nodules contained a nodule meristem but lacked infection threads, intracellular bacteria, and bacteroids. Fix- nodules contained a spectrum of nodulins similar to wild-type nodules. In contrast, only two nodulins, Nms-30 and a nodulin homologous to ENOD2 of soybean, were detected in empty nodules. Although R. meliloti ndv and exo mutants elicited nodules with the same defective phenotype, ndv and exo mutants (except for exoC mutants) had distinct biochemical phenotypes. R. meliloti ndvA and ndvB mutants were deficient in cyclic glucan production but not the acidic exopolysaccharide; the converse was true for exoA, exoB, and exoF mutants. exoC mutants were defective in both exopolysaccharide and cyclic glucan biosynthesis. Our results support the model that the R. meliloti nod genes produce a signal that results in nodule meristem induction. Both the exopolysaccharide and cyclic glucan, however, appear to act at the next step in the developmental process and are involved in the production of a signal (or structure) that allows infection thread formation and invasion of the nodule.
为了深入剖析根瘤菌与豆科植物共生的分子基础，我们利用体外翻译和根瘤RNA的Northern印迹分析，来检测在两种由苜蓿中华根瘤菌引发的发育缺陷型根瘤中表达的苜蓿特异性基因（根瘤蛋白）。固氮根瘤由苜蓿中华根瘤菌nif突变体引发；这些根瘤被根瘤菌侵染并含有分化的类菌体。“空”根瘤由苜蓿中华根瘤菌exo和ndv突变体以及携带苜蓿中华根瘤菌nod基因的根癌土壤杆菌菌株引发；这些根瘤含有根瘤分生组织，但缺乏感染丝、细胞内细菌和类菌体。固氮根瘤含有一系列与野生型根瘤相似的根瘤蛋白。相比之下，在空根瘤中仅检测到两种根瘤蛋白，即Nms - 30和一种与大豆ENOD2同源的根瘤蛋白。尽管苜蓿中华根瘤菌ndv和exo突变体引发的根瘤具有相同的缺陷表型，但ndv和exo突变体（除exoC突变体之外）具有不同的生化表型。苜蓿中华根瘤菌ndvA和ndvB突变体在环状葡聚糖产生方面存在缺陷，但酸性胞外多糖产生正常；exoA、exoB和exoF突变体则相反。exoC突变体在胞外多糖和环状葡聚糖生物合成方面均存在缺陷。我们的结果支持这样一种模型，即苜蓿中华根瘤菌nod基因产生一种信号，导致根瘤分生组织诱导。然而，胞外多糖和环状葡聚糖似乎在发育过程的下一步起作用，并参与产生一种允许感染丝形成和根瘤侵染的信号（或结构）。
