Does induced phytotoxicity in apple replant disease affect Malus domestica roots?

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Apple (Malus domestica) is one of the most important cultivated fruits worldwide. In tree nurseries, plants are replanted frequently (2-3 years), which leads to diminished crop productivity, also known as apple replant disease (ARD). ARD can be defined as “a harmfully disturbed physiological and morphological reaction of apple plants to soils that faced alterations in their (micro-) biome due to the previous apple cultures”. Formerly, biologically broad-spectrum soil fumigants were used to control ARD. Nevertheless, application intricacy, high cost and hazards to environment and human health are problematic. Therefore, breeding and/or selecting less sensitive genotypes would present a more sustainable solution to ARD. However, the development of ARD trait associated markers relies on a better understanding of molecular reactions in planta to unravel disease etiology. Recently, phytoalexin biosynthesis genes were found to be strongly up-regulated after seven days of culture on ARD soil compared to disinfected ARD soil. It was revealed that the phytoalexins accumulated in the root system in very high concentrations leading to potential phytotoxicity. ABC transporters which are involved in the translocation and exudation of phytoalexins did not show any regulation leading to the assumption that phytoalexins may not be exuded into the soil under ARD conditions and therefore accumulate in the roots in very high concentrations. Additionally, the vacuolar transport may be hindered as well leading to the lack of detoxification of the accumulated substances. A potential reason for the proposed lack of exudation of phytoalexins or of sequestration into vacuoles via ABC transporters may be the generation of toxic cyanide levels in ARD affected plants leading to less ATP availability for ABC transporters to transport phytoalexins in ARD roots. The goal of the present project is to unravel the role of ARD induced phytoalexins on the ARD syndrome and to elucidate molecular reactions in ARD affected plants. The focus will be on deducing their role in ARD considering further interacting genes/proteins. Toxicity and localization of the compounds will be analyzed as well as detoxification mechanisms, e.g. transport from the cytoplasm. Furthermore, additional toxic byproducts in cyanide metabolism as well as energy supply will be investigated more closely to achieve a detailed overview of molecular mechanisms of ARD. Fluorescence in situ hybridization, microscopy, gene expression studies and metabolic analyses will be employed to achieve this goal. Comparisons between a sensitive and a less sensitive genotype will provide knowledge for the early prediction of markers of ARD severity in soils and will help in breeding programs to select ARD tolerant apple plants.