In planta regulation of phytopathogenic bacteria virulence genes: relevance of plant-derived signals

Abstract

Interactions between plants and pathogens are complex dynamic and multistep processes involving invasion of the host, avoidance or overcome of plant defence responses and multiplication of the pathogen, eventually culminating with expression of disease symptoms. In all these steps communication at different levels is important either between the pathogen and its host or between the members of the pathogen community. It is therefore not surprising that the genes involved in plant-pathogen interactions are under the control of complex regulatory circuits responding to numerous stimuli. The discovery by Stachel et al. (1985) that the Agrobacterium vir genes, normally expressed at a very low level, are specifically induced by cocultivation of the bacterium with plant cell suspensions and the further identification of plant-derived inducing signals, was the first demonstration of the importance of the direct signalling role of the plant in the initiation of the infection. Subsequently, the role of signals coming from the host in the regulation of genes involved in plant-bacteria interactions was investigated. In gram-negative plant pathogenic bacteria, several factors playing a role in their interactions with plants have been identified. These include the production of toxins or hormones interfering with the normal plant metabolism or development, the secretion of enzymes or signal proteins interacting with plants, polysaccharide production, and the expression of efficient iron acquisition systems. In all bacterial pathosystems studied, the production of these virulence factors is very finely tuned by complex regulatory circuits and several recent overviews on the role of these factors and their regulation in plant-pathogenic bacteria interactions are available (Daniels et al., 1988; Gross, 1991; Zambryski, 1992; Winans, 1992; Leigh and Coplin, 1992; Dow and Daniels, 1994; Barras et al., 1994; Fuqua et al., 1994; Expert et al., 1996; Schell, 1996). The goal of this review is to analyse the data relevant to the possible importance of plant signals in the regulation of the production of the different virulence factors so far characterised. The approaches which were investigated to directly identify genes which would be specifically induced in planta in a search for additional important genes in plant-bacteria interactions will then be discussed.