The strains of Pss were isolated from apricot, cherry, peach and wheat in Fars and Kohgiluyeh and Boyer-Ahmad provinces. All P. syringae pv. syringae strains used in this study were negative for oxidase, potato rot, and arginine dihydrolase, but, positive for levan production and the hypersensitive response on tobacco. All strains caused symptoms showing disorders around wounds on fruitlets. After 24h from fruit inoculations with strains of Pss deep black brown necroses were observed, while strain of P. fluorescens did not induce any symptoms. To find out whether production of syringomycin and syringupptin in different isolates is affected by plant material, inhibitory effect of these toxins on G. candidum and B. megaterium were studied. When no plant extracts were added to the SRM medium, the most inhibitory effect on the G. candidum was observed in isolate C1 (Fig. 1). The highest production of syringomycin was achieved by this strain in the presence of Takdane cultivar leaf extract. Most of the isolates produced more syringomycin in the presence of Takdane leaf extract. Syringomycin inhibition was not significantly different between three strains A1, W1 and P1 on the media containing leaf extract of Ghaheri cultivar (Fig. 1). The inhibitory difference of all isolates in the presence of leaf extract of all three cherry cultivars except P2 was significant in comparison to absence of leaf extract. leaf extract of Surati variety had a lower effect on the increasing of syringomycin and syringopeptin production (Fig. 1 and 2). After Takdane cultivar, Ghaheri and Surati cultivars increased the production of toxin. As a result, Takdane, Qahiri and Surati cultivars were more susceptible to bacterial canker of stone fruits respectively. Significant increase in the production of syringomycin and syringopeptin in the presence of leaf extract in comparison to the conditions without the presence of leaf extract indicates the high effect of plant signal molecules on the expression of pathogenicity genes of Pss. The production of syringomycin and syringopptin is significantly related to the amount of plant signal molecules, especially phenolic and sugar compounds (Wang et al., 2006). Among the isolates tested, inhibition zones were observed for all strains of P. syringae pv. syringae (Fig. 1 and 2). The largest inhibition zones were obtained with strains A1 and C1 in presence of plant extracts. The zones of inhibition were generally larger on medium supplemented with extracts of cherry cultivar Takdaneh, but the sizes of the inhibition zones varied by strain. The strains A1 and C1 produced substantially more syringomycin and syringopeptin on SRM and PDA agar media in presence of all plant extracts. In SRM medium without any extracts, the strain A1 formed 7-mm zones of inhibition of G. candidum, compared to 14-mm zones on medium supplemented with Takdaneh extracts and 11mm zones on medium with Ghaheri extracts (Fig. 1). Similar results were observed on PDA agar developed specifically for the production of syringopeptin under defined culture conditions (Fig. 2). Syringomycin and syringopeptin production by the strains was relatively sensitive to plant extracts. The production of syringomycin and syringopeptin by all isolates was significantly higher in the presence of plant extracts on SRM and PDA agar medium. On SRM agar medium, strains A1 and C1 produced zones of antifungal activity whose diameters were almost two times those produced in the absence of the plant extracts. Also on PDA agar medium, strains W1, A1 and C1 produced zones of antibacterial activity whose diameters were almost two times those produced in the absence of the plant extracts. There is growing evidence that virulence genes in bacteria respond to environmental stimuli (Mo et al., 1995), and Pss is no exception. Because virulence determinants are not constitutively expressed in most bacteria, activation of virulence genes upon perception of a specific chemical or physical stimulus imparts order and balance to pathogenesis that will optimize the bacterium’s chances for long term survival. The induction of toxigenesis in P. syringae pv. syringae by specific plant signal molecules reflects an ability of the bacterium to adapt to a dynamic plant environment. It was reported that syringomycin production is activated by specific plant signal molecules in diverse strains of P. syringae pv. syringae (Quigley & Gross, 1994). It recently was established that phytotoxin production by Pss is modulated by the perception of specific plant metabolites (Mo & Gross, 1991). Certain phenolic glucosides, such as arbutin, serve as signals that induce production of syringomycin, a cyclic lipodepsinonapeptide toxin that causes necrotic symptoms in host plants. In addition, the syrB gene, which is conserved in toxigenic strains of Pss and is predicted to encode a synthetase (Quigley & Gross, 1994), is activated by phenolic signal compounds. Cherry tissues appear to contain plant signal molecules that are perceived by Pss based on evidence that the syrB-ZucZ fusion is transcriptionally activated in an environment containing plant constituents (Mo & Gross, 1991). Inoculations of immature cherry fruits demonstrated that there is a rapid and strong expression of syrB in situ. When SRM medium was not amended with the cherry leaf extracts, strains failed to increase syringomycin production. This demonstrated that the cherry leaf extract contained a constituent or signal that was sensed by the bacterium, eventually leading to activation of genes responsible for toxin induction. Studies by Krzesinska et al., (1993) indicated that susceptibility of cherry genotypes to bacterial canker is correlated with signal activity. Extracts from the stems of 12 cherry genotypes were tested for syrB-inducing activity, and the genotypes most susceptible to bacterial canker contained higher signal activities than resistant genotypes. Consequently, both the quantity and quality of plant metabolites with signal activity may have an acute effect on disease development. Because high amounts of flavonoid glycoside signals occur in cherry leaves, one can speculate that their sudden release would be quickly sensed by the bacterium and lead to activation of genes, such as syrB, necessary for phytotoxin production. In addition, it appears that a broad spectrum of Pss strains would be capable of recognizing the flavonoid glycoside signals from cherry. This is based on evidence that most strains of Pss attack a wide range of plant hosts and that they recognize phenolic signal molecules that are found in the leaves, bark, and flowers of many plant species (Mo & Gross, 1991; Quigley & Gross, 1994). Host resistance as observed, for example, in cherry cultivars (Krzesinska et al., 1993) may reflect qualitative and quantitative differences in signal molecules or the balance of plant substances that antagonize induction by plant signals.

Fig. 1: Effects of plant extracts on syringomycin production by different strains of Pseudomonas syringae pv. syringae. The amounts of syringomycin produced were measured as the diameters of inhibition zones of the fungus G. candidumon SRM agar medium

Fig. 2: Effects of plant extracts on syringopeptin production by different strains of Pseudomonas syringae pv. syringae. The amounts of syringopeptin produced were measured as the diameters of inhibition zones of the bacterium B. megaterium on PDA agar medium

In conclusion, it was found that the cultivars of Takdaneh, Ghaheri and Surati respectively induce more toxin production by Pss strains. These cherry cultivars in southwest Iran have the highest cultivation area. In susceptible cultivars, the pathogenicity factors of Pss is produced at a higher level compared to resistant cultivars.