Resistance of the plane tree (Platanus spp-Ceratocystis fimbriata f. sp platani) to canker stain: defense reactions and futur prospects for improvement

摘要

The London plane tree, Platanus X acerifolia (A) Willd., is widespread in southern Mediterranean countries. This tree is highly regarded for its ornamental properties and its ability to survive in the extreme conditions of the urban environment. However, the tree is the target of many pathogens threatening its survival. The most serious problem involves a fungal disease, canker stain, caused by the Ascomyceteae Cemtocystis fimbriata f. sp platani (Ell and Halst) Walter which has already killed 25,000 trees in the south of France since its appearance in 1945. The symptoms of the disease are those of a wilt disease with coloration and creaking of the stem bark. While the symptomatology and etiology of the disease and preventive control methods havebeen well described, little was known with respect to plane tree defense responses. The objectives of this study were therefore to investigate specific plane tree defense reactions by using young plants (3-month-old) inoculated with a conidial suspensionof Cemtocystis fimbriata f. sp platani (Cfp). This paper summarizes the salient results and suggests some approaches to stimulate plane tree defense responses and improve resistance against the pathogenic fungus. The plane tree reacted to infection by Cfp with histological and biochemical responses that included the formation of tyloses, the production of gels, the accumulation of constitutive (hydroxycinnamic acids, flavans and flavonols) and de novo phenolic compounds (phytoalexins: the coumarins scopoletin, umbelliferone and xanthoarnol). These phenolic compounds displayed a high level of fongitoxidty and accumulated in infected tissues as well as in neighbouring areas free of infection, presumably as a strategy to prevent the ingress of Cfp. However, these responses were not sufficient to keep the plant from being infected because wilt symptoms were observed above the inoculation site but the plant sunned since leafy branches were developing under the inoculation site. So, in order to improve thelevel of disease resistance, treatments with fungal extracts or jasmonic acid were carried out to further elicit plane tree defense reactions. These treatments led to reduced disease symptoms which were correlated with the activation of the lipoxygenasepathway (involved in signal transduction) and the phenylpropanoid pathway (invoved in phytoalexins synthesis). In addition, other researches involved interspecific hybridization as an alternate method to improve resistance. To that end, resistant parents were selected in the P. occidentalis species, then crossed with those of the P. orientalis species susceptible to produce clones of P. acerifolia. This approach yielded some clones with a good level of resistance against the canker stain agent. Beforeany of the two approaches (induced resistance or interspecific hybridization) may be deemed applicable, they must now be evahiated in terms of durability according to their interaction with different factors of disease development such as the age of theplants, the conditions of urban environment, inoculum pressure. Cfp fitness and the presence of other parasitic: fungi and insects. Moreover, molecular markers for plane tree resistance to Cfp need to be developed for a better characterization of resistance since its evaluation is currently based on symptomatology which includes multiple variability factors. In the meantime, only protective measures can reduce the incidence of canker stain in affected areas and prevent its spread to unaffected countries