The disease cycle of Xf, the bacterial pathogen that causes PD, involves intriguing interactions with plant and insect hosts. The bacterium persists and multiplies in both types of hosts. In plants, colonization is limited to the xylem. Several species of xylem-feeding insects, predominantly leafhoppers (Almeida and Purcell, 2003; Hewitt et al., 1942; Purcell, 1979) but also spittlebugs (Severin, 1950), can transmit Xf while feeding on host plants. Juvenile insects can transmit the pathogen until they molt (Purcell and Finlay, 1979) and adult insects transmit Xf throughout their lifespans (Severin, 1949). After entering the plant, the pathogen multiplies, forming microcolonies at the inoculation site (Hopkins, 1985; Tyson et al., 1985). The bacteria then efficiently and systemically colonize the plant by moving within and between xylem vessels (Newman et al., 2003). Movement of the pathogen between vessels is correlated with the expression of pathogen genes encoding degradative enzymes that are predicted to facilitate movement by degrading the pit membranes between vessels (Scarpari et al., 2003). Although the pathogen spreads through the plant and is detected in low numbers in many vessels, symptoms do not appear unless vessels contain high populations of bacteria (S.M. Fry and Miholland, 1990; Newman et al., 2003). Xf is not observed outside of the plant’s vascular system. Recent genome sequence data and comparative analyses with sequences of other bacterial pathogens show that Xf colonizes insect and plant hosts and induces disease in the plant host using genes that are expressed from a relatively small (2.5 Mb) genome (Simpson et al., 2000).
The complete genome sequence of Xf (including sequences for two strains and draft sequences for another two) and the microarrays based on the sequence are facilitating the identification of genes that could lead to effective management strategies. In fact, many genes have been implicated or eliminated from consideration based on comparison of the Xf genome sequences with other pathogenic bacteria (Van Sluys et al., 2002). For example, genes encoding the type III secretion system, which are common to and essential for virulence in many mammalian and plant pathogenic bacteria, are not found in Xf.
Several genes similar to those encoding putative virulence factors were identified in Xf. The presence of sequences related to virulence genes found in other organisms can inform the process of creating hypotheses; it does not prove that the genes are involved in pathogenicity. To determine gene function, in this case in the induction of disease, requires systematic analysis of mutagenesis and complementation, gene expression, physiologic and biochemical activity, and pathogen–host plant interactions. Several hypotheses that describe putative roles of particular genes in disease have been built from comparisons of genome sequences. Targeting genes for functional analysis will allow critical questions related to disease to be addressed. Some of those questions are presented below.