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

The interaction between two Hrp mutants of Erwinia amylovora, the parental virulent strain, and a host plant (Malus sp.) was studied with apple seedli ngs and apple flowers, using inoculation procedures which reduce the possib ility of injury. We showed that one hrp regulatory mutant protected apple t issue against the virulent strain to a greater degree than a secretion muta nt. 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 res ponses, phenylalanine-ammonia lyase and guaiacol peroxidase, increased in l eaves treated with the regulatory mutant, but not in the leaves treated wit h the secretion mutant. The role of regulatory genes of the hrp cluster in this interaction is discussed. (C) 1999 Federation of European Microbiologi cal Societies. Published by Elsevier Science B.V. All rights reserved.