GETTING TO THE ROOT - THE ROLE OF THE AGROBACTERIUM-RHIZOGENES ROL GENES IN THE FORMATION OF HAIRY ROOTS

Agrobacterium tumefaciens and A. rhizogenes are the causative agents o f the crown gall and hairy root diseases, respectively. The pathogenic ity of both species is caused by an inter-kingdom transfer of DNA from the bacteria to wounded plant cells. This 'transfer-DNA' (T-DNA) cont ains oncogenes whose expression transforms the plant recipient cell in to a rapidly dividing tumour cell. In the case of A. tumefaciens, thre e of these oncogenes have been shown to encode enzymes catalyzing the biosynthesis of the plant growth hormones auxin and cytokinin. Therefo re, the unorganized cell division in the crown gall tumour can be larg ely explained by an unrealated overproduction of these plant growth re gulators. In contrast, the hairy root disease is characterized by a ma ssive growth of adventitious roots at the site of infection. Because o f the similarities of the infection processes, and because A. rhizogen es and A. tumefaciens are very closely related, it has been suggested that the most important A. rhizogenes oncogenes, the so called rol gen es, are also encoding proteins involved in the regulation of plant hor mone metabolism. However, recent data indicate that this is not the ca se. Thus the rol genes have functions that most Likely are different f rom producing mere alterations of plant hormone concentrations. This r eview summarizes recent results concerning the expression and function of the rol genes, and presents a model for the role of these genes, e specially rolB and rolC, in the A. rhizogenes infection process.