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| Competition between growth, secondary metabolism, and resistance of apple |
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S. Rühmann, C. Leser, M. Bannert, D. Treutter
Institute of Fruit Science, Center of Life and Food Sciences Weihenstephan
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Defensive activities of apple trees are costly since they often depend on enhanced biosynthesis and accumulation of secondary compounds. This is particularly known for the defence of Venturia inaequalis, a fungus causing the scab of apple by infecting very young leaves and fruits. The management of this disease is rather sophisticated and often frustrating since the plant's susceptibility as well as the virulence of the fungus depend on the genotypes and environmental conditions including cultivation practices. The latter also substantially modify the growth vigour of the trees. The most common instrument for increasing shoot growth and fruit cropping is fertilization with nitrogen in supporting the periodic renewal of fruit bearing shoots which is a prerequisite for quality production.
In this paper we show that N-induced vigorous shoot growth increases susceptibility of apple trees to Venturia inaequalis. This is due to a weakened defence which can obviously be deduced from the fact that the number of infected leaves of the high-N variants increased after natural infection. Furthermore, these susceptible leaves showed large lesions with excessive sporulation whereas infected leaves from the low-N variants exhibited successful defence with small chlorotic lesions and without sporulation. This might be explained by a well functioning biosynthesis of phenylpropanoids in the young leaves of the resistant trees. It could be found that there is a negative correlation between shoot growth of apple trees and the concentration of phenolic compounds in their young leaves. Both growth related metabolic activities and biosynthesis of phenylpropanoids require carbohydrates and energy and compete for them.
Studies on in-vitro shoot-cultures revealed that the availability of sugars for the phenylpropanoid pathway is a strong regulatory factor.The ratio of medium-supplied sucrose and nitrogen not only influenced the total level of secondary products in the in-vitro grown plantlets. Moreover, the relative deficiency of sugars is responsible for a metabolic block at the level of the enzymes chalcone synthase/dihydrochalcone synthase and glucosyltransferases which may explain the accumulation of p-coumaroyl-esters of phloridzin and the aglycone phloretin.
It is speculated that a strong sink in the apex of vigorously growing shoots may lead to a, at least, transiently reduced allocation of sugars to the very young leaves thus inhibiting the phenylpropanoid pathway just at that moment when the pathogen is attacking these organs.
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