Studies on legume root hair development : correlations with the infection process by Rhizobium bacteria

Rhizobia-legume interaction leading to the formation of specific organs, namely root nodules, starts at the epidermis of the root. Bacteria interfere with the develomental programme of the epidermal cells by inducing a number of responses, as new root hair growth, root hair deformation and curling, and formation of special structures, the infection threads, via which the bacteria enter the plant. It has been postulated that infection threads grow in a similar manner as root hairs, but to the opposite direction. We have initiated a programme aiming originally at studies on the development of root epidermis/root hairs. That resulted in the isolation of a number of cDNA clones that represent genes involved in different aspects of epidermis/root hair development. These clones were used as molecular tools to unravel whether infection thread growth employs the same programmes as growing root hairs and germinating pollen and play a role in polar growth. To further understand the mechanisms of infection thread formation we performed studies on epitope-tagged early nodulins involved in the infection process.In chapter 3, 4, and 5 we report the characterization of three root hair specific cDNA clones and the expression pattern of the corresponding genes. In chapter 2 we describe the exploratory studies on the potentialities of using epitope tagging of early nodulins to examine the role of early nodulins in the infection process and the formation of infection threads.The various steps involved in the infection of legume roots by rhizobia and the formation of nitrogen fixing root nodules are described in the review presented in chapter 1, while in the concluding chapter 6 is discussed to what extent infection thread formation is compatible to growth of root hairs and pollen tubes.