RESUMEN
The interaction between the tropical legume Sesbania rostrata and the bacterium Azorhizobium caulinodans results in the formation of nodules on both stem and roots. Stem nodulation was used as a model system to isolate early markers by differential display. One of them, Srchi24 is a novel early nodulin whose transcript level increased already 4 h after inoculation. This enhancement depended on Nod factor-producing bacteria. Srchi24 transcript levels were induced also by exogenous cytokinins. In situ hybridization and immunolocalization experiments showed that Srchi24 transcripts and proteins were present in the outermost cortical cell layers of the developing nodules. Sequence analyses revealed that Srchi24 is similar to class III chitinases, but lacks an important catalytic glutamate residue. A fusion between a maltose-binding protein and Srchi24 had no detectable hydrolytic activity. A function in nodulation is proposed for the Srchi24 protein.
Asunto(s)
Quitinasas/química , Fabaceae/microbiología , Proteínas de la Membrana , Proteínas de Plantas/genética , Secuencia de Aminoácidos , Azorhizobium caulinodans/fisiología , Citocininas/farmacología , ADN Bacteriano , ADN Complementario , Regulación de la Expresión Génica de las Plantas , Ácido Glutámico/metabolismo , Hidrólisis , Hibridación in Situ , Lipopolisacáridos/metabolismo , Datos de Secuencia Molecular , Proteínas de Plantas/química , Proteínas de Plantas/metabolismo , Raíces de Plantas/metabolismo , Raíces de Plantas/microbiología , Tallos de la Planta/metabolismo , Tallos de la Planta/microbiología , Homología de Secuencia de Aminoácido , Simbiosis/genéticaRESUMEN
Lipo-chitooligosaccharide (LCO) Nod factors are produced and secreted by rhizobia and trigger nodule development in leguminous host plants. The products of the bacterial nodlJ genes are related to transporters of capsular polysaccharides and were proposed to be involved in LCO transport. We have studied nodlJ of Azorhizobium caulinodans ORS571 by analysis of cell-associated and secreted radioactively labelled Nod factors in wild-type ORS571, a nodJ mutant and a complemented strain. Secretion was strongly reduced in the nodJ mutant, and restored to wild-type levels after complementation. Constructs were made for expression of combinations of different nod genes in Escherichia coli DH5 alpha. The strain DH5 alpha (pUCNABCSU) synthesized LCOs, but they were only secreted when a plasmid containing both nodl and nodJ was supplied in trans. nodl or nodJ alone was not sufficient. In E. coli as well as in Azorhizobium, the nodlJ-encoded transporter showed a specificity for more hydrophilic LCOs.
