Early nodulins in root nodule development

The symbiotic interaction between bacteria of the genus Rhizobium and leguminous plants leads to the formation of root nodules, which are specific nitrogen-fixing organs on the roots of plants. Bacteria enter the root by infection threads, and concomitantly cell divisons are induced in the root cortex, which lead to the formation of a meristern. From this meristern the different tissues of the root nodule originate. In the nodule bacteria are released in plant cells and then differentiate into the endosymbiotic bacteroids. These bacteroids are capable of nitrogen fixation.The formation of root nodules involves expression of both bacterial and plant genes. Rhizobium genes involved in nodule formation are the nodulation ( nod ) genes. Nodulespecific plant genes are termed nodulin genes. According to their timing of expression they can be divided into early and late nodulin genes. Early nodulin genes are expressed well before the onset of nitrogen fixation, at the time that the nodule tissue is formed and the roots become infected by bacteria, while expression of late nodulin genes starts shortly before the onset of nitrogen fixation, when the nodule structure has been formed. Therefore only early nodulins can be involved in the infection process and in nodule development. Early nodulin genes expressed during the pea ( Pisum sativum L.) - Rhizobium leguminosarum bv. viciae interaction are the subject of this thesis. Several cDNA clones representing pea early nodulin genes have been isolated and they have been used to study root nodule development and the communication between bacteria and host plant.In chapter 2 we review general aspects of plant development. Recent progresses in understanding the molecular mechanisms underlying animal development are listed, and the possible significance of such mechanisms for plant development is discussed. The features of the root nodule formation system that make it suitable to study particular questions on the molecular basis of p