N-induced vigorous shoot growth (Tab.1) increases susceptibility of apple trees to Venturia inaequalis (Fig. 3), (Fig.4) and weakens the defense reaction against Phytophthora cactorum (Fig. 7), supporting hypotheses I, V and VI.
A negative correlation between shoot growth of apple trees and the concentration of phenolic compounds in leaves (Fig. 5) (Fig. 7) and bark was found.
The availability of sugars is a strong regulatory factor of the phenylpropanoid pathway (Lux-Endrich et al. 2000). The ratio of medium-supplied sucrose and nitrogen influences the total level of secondary products of in-vitro shoot tip cultures (Fig. 6) and is responsible for a metabolic block at the level of enzymes (Fig. 8).
Based on these findings, effective defense of apple may only occur when growth is reduced thus allowing partitioning of resources in a way that supports secondary metabolism.
Results: Phase III
• Among the genes differentially expressed upon V. inaequalis inoculation flavonoid genes, PR-genes and other defense related genes as well as some genes of the primary metabolism were found.
• PAL gene expression and thus flavonoid biosynthesis was stimulated in Golden Delicious under low N-supply simultaneously with the development of disease symptoms (Fig.9) .
• On enzyme level, under high N-nutrition a downregulation of the PAL activity was observed, resulting in a generally decreased flavonoid accumulation (Phase II, Strissel et al. 2005).
• Genes playing a role in signal transduction were stimulated in Golden Delicious under low N-supply simultaneously with the development of disease symptoms (Fig.10) .
• As a result of the infection, PR-genes accumulated independently from the N-supply in Golden Delicious (Fig.11) .
• Transcripts for a highly redundant metallothionein gene accumulated independently from the scab affection in both cultivars, especially under high N-supply (Fig.12) .
• In quantitative RT-PCR analyses fungal cDNA in the leaves was not detected in high quantity until 25 days post inoculation. In Golden Delicious, leaves under low N-supply showed less scab incidence than leaves from highly fertilized trees (Fig.13) . The resistant cultivar Rewena did not show any symptoms.
• Increased N-supply led to reduced concentrations of phenylpropanoids and flavonoids. The contents of phloridzin and of the hydroxycinnamic acids decreased by time. Inoculated Golden Delicious leaves had higher concentrations of phenolics compared to the non-inoculated control leaves.
Conclusion:
Differences in the reaction of the apple cultivars upon V. inaequalis inoculation and different N-supply were observed on the level of metabolites, of enzyme activity, and of transcripts. |
