Cooper J. B., Long S. R.
Department of Biological Sciences, Stanford University, Stanford, California 94305.
Plant Cell. 1994 Feb;6(2):215-225. doi: 10.1105/tpc.6.2.215.
The development of nitrogen-fixing nodules is induced on the roots of legume host plants by Rhizobium bacteria. We employed a novel strategy to probe the underlying mechanism of nodule morphogenesis in alfalfa roots using pTZS, a broad host range plasmid carrying a constitutive trans-zeatin secretion (tzs) gene from Agrobacterium tumefaciens T37. This plasmid suppressed the Nod- phenotype of Rhizobium nodulation mutants such that mutants harboring pTZS stimulated the formation of nodulelike structures. Alfalfa roots formed more or fewer of these nodules according to both the nitrogen content of the environment and the position along the root at which the pTZS+ bacteria were applied, which parallels the physiological and developmental regulation of true Rhizobium nodule formation. This plasmid also conferred on Escherichia coli cells the ability to induce root cortical cell mitoses. Both the pattern of induced cell divisions and the spatially restricted expression of an alfalfa nodule-specific marker gene (MsENOD2) in pTZS-induced nodules support the conclusion that localized cytokinin production produces a phenocopy of nodule morphogenesis.
根瘤菌可诱导豆科宿主植物根部形成固氮根瘤。我们采用了一种新策略，利用pTZS来探究苜蓿根瘤形态发生的潜在机制，pTZS是一种广宿主范围质粒，携带来自根癌农杆菌T37的组成型反式玉米素分泌（tzs）基因。该质粒抑制了根瘤菌结瘤突变体的结瘤缺陷型，使得携带pTZS的突变体能够刺激类根瘤结构的形成。根据环境中的氮含量以及接种pTZS⁺细菌的根段位置，苜蓿根形成的此类根瘤数量或多或少，这与真正的根瘤菌根瘤形成的生理和发育调控情况相似。该质粒还赋予大肠杆菌细胞诱导根皮层细胞有丝分裂的能力。在pTZS诱导的根瘤中，诱导细胞分裂的模式以及苜蓿根瘤特异性标记基因（MsENOD2）在空间上的受限表达均支持这样的结论：局部细胞分裂素的产生导致了根瘤形态发生的拟表型。
