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Induction of pathogen resistance in fruit trees by transiently altering the flavonoid metabolism with specific enzyme inhibitors

Contract nr: QLK5-CT-1999-01583
Project type: RS (Research and Technological Development Project)
Starting date: 01-04-2000
Duration: 36 months
Total cost: 1,945,369 EUR
EC Contribution: 1,498,885 EUR
Scientific Officer: Richard HARDWICK
Research topic: 5.1.1. Sustainable agriculture/Plant Systems

Apple and pear trees treated with the plant growth regulator prohexadione-ca are significantly less affected by fire blight (a bacterial disease) and by scab (caused by a fungus). Initial results indicate that prohexadione-ca , which is inactive as a bactericide or fungicide, transiently alters the spectrum of endogenous flavonoids and related substances, thereby inducing natural resistance. The project aims at an in-depth elucidation of the underlying physiological, biochemical and molecular biological mechanisms. As a result, optimised methods for fire blight control, which are urgently needed, particulary in pear-growing areas, will become available. The disputed use of antibiotics to combat this disease will no longer be required. Likewise, the understanding of the new type of resistance mechanism will lead to new efficient, safe and environment friendly products for pathogen control in several further crop plants.

Fire blight, caused by the bacterium Erwinia amylovora and scab, caused by the fungus Venturia inaequalis are important diseases of pear and apple trees. Today, the economical importance of fire blight is still increasing due to the lack of satisfactory and reliable treatments and to its continuing expansion in Europe and other countries. Major losses in commercial orchards are expected, particularly in regions where climatic conditions facilitate the spread of the bacterium. In the case of scab, a chemical control is possible and commonly used in orchards. However, up to 15 fungicidal treatments per year may be needed, which imposes potential risks to growers, consumers and the environment. Resistant cultivars, even if available, do not completely fit commercial requirements.

Previous studies have shown that the successful restriction of the scab fungus Venturia inaequalis in tissues of resistant apple cultivars is accompanied by an induced biosynthesis and accumulation of phenylpropanoids and flavonoids. The induction of such compounds occurs also in restriction zones of tissues infected by Erwinia amylovora. In general, the resistance of many host plants against a wide range of fungi and bacteria is connected with the biosynthesis of phenolic compounds, in particular with flavonoids. Therefore, it can be expected that a defined steering of the flavonoid pathway could improve the defence reactions of several plant species.

Recent orchard trials revealed that apple trees pretreated with the plant growth regulator prohexadione-Ca were significantly less affected by fire blight and scab than control trees. Prohexadione-Ca acts as structural mimic of 2-oxoglutarate and, thus, may competitively inhibit 2-oxoglutarate-dependent dioxygenases. It has been hypothesised that this may induce an accumulation of distinct metabolites, which would, directly or indirectly, improve the resistance of plants against pathogens. First chemical analyses of apple tissues revealed the formation of unusual flavonoid compounds after prohexadione-Ca application, indicating an involvement of flavonoid biosynthesis in the defence reaction. Several 2-oxoglutarate-dependent dioxygenases are involved in the pathway leading to flavonoids. In particular, the inhibition of flavanone 3-hydroxylase (FHT) catalysing hydroxylation of flavanones to dihydroflavonols would interfere with the synthesis of the constitutively formed flavonols, catechins, proanthocyanidins and anthocyanins. As a result, an alternative pathway would be induced leading to flavan 4-ols and derived compounds, which are most probably responsible for the enhanced resistance. These effects are tissue specific, strictly localised in distinct cell compartments and, due to the rapid disintegration of prohexadione-Ca, of transient nature. Thus, no negative effects on fruit quality are to be expected.

The following objectives are dealt with in this project:
1.Chemical, biochemical and molecular elucidation of the basic flavonoid pathway in apple and pear shoot tissues, its modulation after pathogen infection and after prohexadione-Ca treatment. This involves the isolation and identification of flavonoids and phenylpropanoids, the characterisation of the involved enzymes, the isolation of the respective genes as well as studies on gene expression.
2.Identification of the substance or combination of substances, which represent the active principle. Evaluation of the potential biological activity of bacterial endotoxin(s) versus Erwinia amylovora in vitro and in planta.
3.Determination of the dynamics of fire blight and scab infections in pear and apple as related to dioxygenase inhibitor(s) application.
4.Identification of additional compounds influencing flavonoid enzyme activity and evaluation of their effects on plant resistance.
5.Optimisation of practical application (formulation, dosage, timing) of dioxygenase inhibitors and active resistance principles.
6.Defining further crop plants and their respective pathogens as potential additional targets for the use of dioxygenase inhibitors.

The elucidation of the flavonoid metabolism at the chemical, biochemical and molecular levels will improve our understanding of plant defence mechanisms and their use in crop protection and will most likely lead to the development of novel inductors of resistance. The investigations will finally provide cost-effective, safe and environment friendly methods of pathogen control in apple, pear and probably in a number of other important crop plants.

TU Wien
Tel.: +43 15880117320
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TU Munich-Freising Weihenstephan
Tel.: +49 81617 13753
Fax: +49 81617 15385
TU Munich-Freising Weihenstephan
Tel.: +49 8161 713416
Fax: +49 81617 13886
  Guglielmo COSTA
University of Bologna
Tel.: +39 051 2091492
Fax: +39 051 2091500
  Carlo BAZZI
University of Bologna
Tel.: +39 051 2091446
Fax: +39 051 2091446
BASF-Aktiengesellschaft ( ZOH-B1)
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Fax: +49 6216027176