Secretion and regulation Hrp mutants of Erwinia amylovora trigger different responses in apple

Abstract

The interaction between two Hrp mutants of Erwinia amylovora, the parental virulent strain, and a host plant (Malus sp.) was studied with apple seedlings and apple flowers, using inoculation procedures which reduce the possibility of injury. We showed that one hrp regulatory mutant protected apple tissue against the virulent strain to a greater degree than a secretion mutant. The dynamics of bacterial growth in this interaction indicated that the protection was associated with the inhibition of the multiplication of the virulent strain. The activity of two enzymes involved in plant defence responses, phenylalanineammonia lyase and guaiacol peroxidase, increased in leaves treated with the regulatory mutant, but not in the leaves treated with the secretion mutant. The role of regulatory genes of the hrp cluster in this interaction is discussed. z

Protection of apple against fire blight induced by an hrpL mutant of Erwinia amylovora

“…It has been shown previously that hrp mutants of E. amylovora altered in their regulatory functions were the most effective in protecting against fire blight (Tharaud et al 1997, Faize et al 1999. In this paper, we investigated the possible basis of the protective ability of a regulatory hrpL mutant.…”

“…The protective ability of the engineered hrpL mutant M73 was tested against the virulent strains CFBP1430 and FB2224 on young apple seedlings and apple flowers as described by Tharaud et al (1997) and Faize et al (1999), respectively. This ability was compared to that of the hrcV mutant PMV6023, previously shown to have a low protective ability (Tharaud et al 1997).…”

“…To test the ability of the engineered hrpL mutant M73 to elicit plant defense, POX activities were assessed in leaves of apple seedlings previously infiltrated with bacterial suspension as described by Faize et al (1999), using water, the hrcV mutant PMV6023 and the virulent CFBP1430 as controls. Leaf tissues were ground in 50 mM acetate buffer pH 5.8 containing 0.01 % Triton X-100, 1 mM β-mercaptoethanol, 1 mM PMSF and 8 % (m/v) of polyvinylpolypyrrolidone.…”

“…The hrp mutants of E. amylovora have been shown to protect apple seedlings and apple flowers from developing fire blight symptoms caused by the virulent parental strain (Tharaud et al 1997, Faize et al 1999. When inoculated either in advance of − or simultaneously with − the virulent strain, hrpS and hrpL mutants were significantly more protective than hrp secretory mutants (hrcV) (Tharaud et al 1997).…”

“…In a first approach to elucidate the mechanisms leading to protection we showed (Faize et al 1999) that the protective ability of a hrpS mutant was correlated with the induction in the host plant of defense reactions including increased activities of phenylalanine-ammonia lyase (PAL; EC 4.3.1.5) and of peroxidase (POX; EC 1.11.1.7). Because of the partial control exerted on the hrp cluster by the hrpS gene these results suggested that in this mutant some pathogenicity factors were released at a level sufficient to induce plant defense response but not high enough to cause disease.…”