It is known that potato tubers are highly susceptible to D. dadantii 3937 colonization. Mutants in genes well known for their contribution to pathogenicity, such as pectate lyase and hrp, retain the wild-type Doxorubicin virulence in this tissue, and only mutant strains with severe defects in virulence show differences when compared with the wild type (Lopez-Solanilla et al., 2001). It should be pointed out that the two plant tissues are very

different. In chicory, the assays were conducted on leaves whereas in potato the assay was conducted on a storage organ tissue. It could be expected that the plant defence response to Dickeya would be stronger in the leaves than in tubers, as the latter contains mainly starch. The Tat system may be important to export factors Y-27632 involved in counteracting these plant responses. Also, it has to be noted that roles of motility and chemotaxis have been demonstrated recently in D. dadantii pathogenesis (Antunez-Lamas et al., 2009); therefore, the decrease in the virulence of the D. dadantii tat mutant on chicory leaves might be related to the observed impairment in motility. One of the potential Tat-dependent proteins involved in D. dadantii 3937 virulence is

PehX (ABF00-14958, Table 1), described as a polygalacturonase located in the periplasm and culture supernatant (Kazemi-Pour et al., 2004). We compared the polygalacturonase activity of Mtat and wild-type strains on KB plates containing polygalacturonic acid (2%), and no significant differences in clearing zone diameters surrounding inoculation points were observed after a 24-h incubation at 28 °C (data not shown). Taking into account that D. dadantii has three additional

polygalacturonases (PehN, PehV and PehW) (Nasser et al., 1999; Hugouvieux-Cotte-Pattat et al., 2002), all predicted as Tat-independent proteins, we assume that the total polygalacturonase activity corresponding to four polygalacturonases in the wild type was similar, at least in plate assays, to that of the Mtat strain. An analysis of the virulence and growth of a ΔpehV–pehW–pehX triple mutant in D. dadantii showed a reduction of 30% of the rotting region Resveratrol on chicory leaves and a weak reduction (13%) of macerated tissues in potato tubers. Also, no growth defects were observed when the same triple mutant was grown with polygalacturonate as the sole carbon source (Nasser et al., 1999). These results are in agreement with our finding that the tat mutant, where PehX would be mislocated, showed diminished virulence in chicory leaves, but similar virulence in potato tubers and similar behaviour in polygalacturonate plates as regarding the wild-type strain. Also, it is known that the different pectolytic enzymes produced by D. dadantii are not equivalent, because virulence requires only some of them in specific plant hosts (Boccara et al., 1988).