ARD Splitroot

Apple replant disease (ARD) is a threat to apple fruit and tree production worldwide caused by shifts in the soil microbiome. Chemical and physical measures of soil disinfection are the most employed countermeasures but inter-row plantings demonstrate that ARD is widely immobile in the soil, which causes the plant to face ARD affected and healthy soil simultaneously in the field. We simulated this situation in a split-root approach in two experiments using rhizoboxes, soil from the ORDIAmur ARD references sites Heidgraben and Ellerhoop and the apple rootstock M26. Root growth was monitored via the transparent front panel of the rhizoboxes and shoot growth was measured weekly. After two weeks, root systems from a subset of plants were sampled for expression analysis of ARD candidate genes. After eight weeks, masses of roots and shoots were recorded and contents of selected phenolic compounds were analyzed. Root and shoot growth were severely reduced whenever both compartments of the rhizobox were filled with ARD soil in comparison to both compartments filled with disinfected ARD soil. Plants with only half of the root system growing in ARD soil showed a local reduction in root growth with an intermediate shoot growth. In ARD affected roots, the expression of the three ARD biomarker genes, biphenyl synthase 3, biphenyl 4-hydroxylase and ethylene-responsive transcription factor 1B-like, was locally induced after two weeks in comparison to disinfected ARD or control soil. Phenolic compound profiles in the roots showed a high variation between samples and soils. The total phenolic content as well as the contents of epicatechin and an unknown phenolic compound were locally increased in roots grown in ARD soil in comparison to disinfected ARD soil after eight weeks. Our results show how apple plants focus their molecular defense towards ARD locally in the roots. The local nature of ARD presents both challenges in the assessment of ARD severity in the field as for possible management strategies involving soil tillage. Research domain: Plant Breeding Research question: The objectives of this study were (i) to link the previously observed localized growth response to a local response of the selected genes and phenolic metabolites and (ii) identify the phenolic compounds that can be useful in unraveling ARD.