Wall L G, Favelukes G
Cátedra de Química Biológica I, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Argentina.
J Bacteriol. 1991 Jun;173(11):3492-9. doi: 10.1128/jb.173.11.3492-3499.1991.
Adsorption of Rhizobium meliloti to alfalfa roots before their infection and nodule formation shows the specificity of the symbiotic association (G. Caetano-Anollés and G. Favelukes, Appl. Environ. Microbiol. 52:377-382, 1986). The time course of specific adsorption of R. meliloti (10(3) to 10(4) cells per ml) to roots shows an initial lag period of 3 h, suggesting that either or both symbionts must become conditioned for the adsorption process. Preincubation of R. meliloti L5-30 for 3 h with dialyzed alfalfa root exudate (RE) markedly increased early adsorption of rhizobia to alfalfa roots. The activity in RE was linked to a nondialyzable, thermolabile, trypsin-sensitive factor(s), very different from the root-exuded flavonoid compounds also involved in early Rhizobium-legume interactions. The lack of activity in the RE from plants grown in 5 mM NO3- suggested its negative regulation by the nitrogen nutritional status of the plant. Preincubation of R. meliloti with heterologous clover RE did not stimulate adsorption of rhizobial cells to roots. A short pretreatment of RE with homologous (but not heterologous) strains eliminated the stimulatory activity from solution. The stimulation of adsorption of R. meliloti to alfalfa roots was strongly dependent on the growth phase of the rhizobia, being greater at the late exponential stage. Nevertheless, the capacity of R. meliloti L5-30 to eliminate from solution the stimulatory activity in RE appeared to be constitutive in the rhizobia. The low concentration of rhizobial cells used in these experiments was critical to detect the stimulation of adsorption. The early interaction of spontaneously released alfalfa root macromolecular factor(s) and free-living R. meliloti, which shows the specificity and regulatory properties characteristic of infection and nodulation, would be an initial recognition event in the rhizosphere which triggers the process of symbiotic association.
苜蓿中华根瘤菌在感染和形成根瘤之前吸附到苜蓿根上，这显示了共生关系的特异性（G. 卡埃塔诺 - 阿诺莱斯和G. 法韦卢克斯，《应用与环境微生物学》52:377 - 382，1986年）。苜蓿中华根瘤菌（每毫升10³至10⁴个细胞）吸附到根上的时间进程显示出最初3小时的延迟期，这表明共生体中的一方或双方必须为吸附过程做好准备。用透析后的苜蓿根分泌物（RE）对苜蓿中华根瘤菌L5 - 30进行3小时预培养，显著增加了根瘤菌对苜蓿根的早期吸附。RE中的活性与一种不可透析、对热不稳定、对胰蛋白酶敏感的因子有关，这与也参与根瘤菌 - 豆科植物早期相互作用的根分泌的类黄酮化合物非常不同。在5 mM NO₃⁻中生长的植物的RE缺乏活性，这表明其受植物氮营养状况的负调控。用异源三叶草RE对苜蓿中华根瘤菌进行预培养不会刺激根瘤菌细胞吸附到根上。用同源（但不是异源）菌株对RE进行短时间预处理会消除溶液中的刺激活性。苜蓿中华根瘤菌对苜蓿根吸附的刺激强烈依赖于根瘤菌的生长阶段，在指数生长后期更大。然而，苜蓿中华根瘤菌L5 - 30从溶液中消除RE中刺激活性的能力在根瘤菌中似乎是组成型的。这些实验中使用的低浓度根瘤菌细胞对于检测吸附刺激至关重要。自发释放的苜蓿根大分子因子与自由生活的苜蓿中华根瘤菌的早期相互作用，显示出感染和结瘤的特异性和调节特性，这将是根际中的一个初始识别事件，触发共生关系的过程。
