Plant Physiol, March 2000, Vol. 122, pp. 757-766

Salicylic Acid Mediated by the Oxidative Burst Is a Key Molecule in Local and Systemic Responses of Cotton Challenged by an Avirulent Race of Xanthomonas campestris pv malvacearum

Institut de Recherches pour le Développement, GeneTrop, UR Résistance des Plantes, BP 5045, 34032 Montpellier, France (C.M., E.B., J.-F.D., A.J., J.-P.G., K.A., M.N.); Université des Sciences et Techniques du Languedoc, Laboratoire de Génie Biologique et Sciences des Aliments (J.-C.B.); and Laboratoire des Substances Naturelles Végétales (Y.B.), Place Eugène Bataillon, 34095 Montpellier, France; and Commissariat à l'Energie Atomique, Département d'Ecophysiologie Végétale et de Microbiologie, Cadarache, 13108 Saint-Paul-Lez Durance, France (J.-L.M.)

We analyzed the production of reactive oxygen species, the accumulation of salicylic acid (SA), and peroxidase activity during the incompatible interaction between cotyledons of the cotton (Gossypium hirsutum) cv Reba B50/Xanthomonas campestris pv malvacearum (Xcm) race 18. SA was detected in petioles of cotyledons 6 h after infection and 24 h post inoculation in cotyledons and untreated leaves. The first peak of SA occurred 3 h after generation of superoxide (O₂^·), and was inhibited by infiltration of catalase. Peroxidase activity and accumulation of SA increased in petioles of cotyledons and leaves following H₂O₂ infiltration of cotyledons from 0.85 to 1 mM. Infiltration of 2 mM SA increased peroxidase activity in treated cotyledons and in the first leaves, but most of the infiltrated SA was rapidly conjugated within the cotyledons. When increasing concentrations of SA were infiltrated 2.5 h post inoculation at the beginning of the oxidative burst, the activity of the apoplastic cationic O₂^·-generating peroxidase decreased in a dose-dependent manner. We have shown that during the cotton hypersensitive response to Xcm, H₂O₂ is required for local and systemic accumulation of SA, which may locally control the generation of O₂^·. Detaching cotyledons at intervals after inoculation demonstrated that the signal leading to systemic accumulation of SA was emitted around 3 h post inoculation, and was associated with the oxidative burst. SA produced 6 h post infection at HR sites was not the primary mobile signal diffusing systemically from infected cotyledons.