Plant gene expression in early stages of Rhizobium - legume symbiosis

<p>Upon infection of the root of a leguminous plant with bacteria of the genus <em>Rhizobium</em> or <em>Bradyrhizobium</em> root nodules are formed in which symbiotic dinitrogen fixation occurs. In the host plant a set of genes is only expressed during the formation of a root nodule. These plant genes are termed nodulin genes (Van Kammen, 1984). Based upon the stage of development of the root nodule in which nodulin genes become expressed a distinction is made between early and late nodulin genes (Nap and Bisseling, 1988). The genes encoding early nodulins are expressed when the roots become infected by the rhizobia and the nodule is formed. Late nodulin genes become first expressed at the onset of dinitrogen fixation, when the nodule structure is ready. This thesis deals with the identification and characterization of plant genes specifically expressed during the early stages of the symbiosis of <em>Rhizobium</em> and the host plant.<p>Thee work in this thesis can be divided into two parts. One part deals with the plant genes involved in early stages of root nodule formation (chapters II, III and IV), and the second part is related to the plant genes involved in the interaction between root hairs and <em>Rhizobium</em> (chapter V).<p>Both in root hairs and young nodule, plant genes involved in the interaction of the plant with the <em>(Brady)Rhizobium</em> bacteria were identified after <em>in vitro</em> translation of nodule and root hair RNA, respectively, followed by two-dimensional gel electrophoresis (chapters II and V). From the root hairs also the in vivo proteins were studied (chapter V, section 5.1). For the genes regulated by <em>Rhizobium</em> in the root hairs we studied the involvement of the bacterial nodulation <em>(nod)</em> genes and the genes required for exopolysaccharide synthesis.<p>Early nodulin gene expression in soybean nodules was studied by using <em>Bradyrhizobium</em> mutants that arrest nodule development at different stages. In these studies we also analyzed the expression of some late nodulin genes (chapter IV). Finally three early nodulin cDNA clones from soybean were isolated and characterized (chapter III).<p>In chapter VI the present knowledge about plant genes that are involved in preinfection, infection, nodule formation, and the involvement of bacterial genes and signals responsible for host responses in these stages are discussed. This is integrated with the results in the preceding chapters. It is speculated about the mechanism by which the bacterial signal elicits nodule formation