Phytoalexin formation in apple roots responding to apple replant disease (ARD) soil

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Apple is a valuable fruit tree, however, its propagation is affected by apple replant disease (ARD), which leads to stunted growth of shoots and roots. The causes are poorly understood but changes in the microbiome of the rhizosphere appear to play an important role. Recently, biomass decline and root damage have been observed by our cooperation partner in susceptible rootstock M26 plants in ARD soils, when compared to roots in heat-treated and gamma-irradiated soils. For the first time, occurrence of biphenyl and dibenzofuran phytoalexins in ARD-affected roots was then detected. In addition, we have observed exudation of the phytoalexins from elicitor-treated in vitro-grown roots to the culture medium, suggesting release of the compounds from roots in ARD soils to the rhizosphere. We will study various in vitro grown rootstock genotypes after elicitor treatment for qualitative and quantitative differences in their phytoalexin production. Elicitor preparations from abundant species in ARD soils will also be tested. The major phytoalexins will be synthesized and examined for their effects on various rhizosphere (micro)organisms. These effects are expected to cause changes in the biome composition. The activity of phytoalexins toward roots is also examined, revealing potential allelopathy and autotoxication. Furthermore, the temporal and spatial regulation of phytoalexin biosynthesis in ARD-affected roots is studied. Our results will contribute to understanding the complex changes in soils, caused by apple growth, and in apple itself when planted into ARD soils.