TY - BOOK

T1 - Gene expression analysis in apple roots on soils with replant disease

AU - Rohr, Annmarie-Deetja

N1 - Doctoral thesis

PY - 2021

Y1 - 2021

N2 - Apple replant disease (ARD) is a serious economic risk for tree nurseries and fruit growers world-wide. ARD causes root damage and stunting in young apple trees if replanted on a spot previously occupied by the same or closely-related species, which negatively impacts yield and quality of trees and fruits. No feasible and sustainable counteraction is available up to date but understanding the apple plant’s role in this complex phenomenon can help in the development of novel reme-dies and the early diagnosis to aid risk assessment. In this thesis, the reaction of apple rootstock plants to ARD was investigated in terms of ARD indicator gene expression in root and leaf material to identify an early and universal transcriptional marker for ARD. Firstly, the expression of a set of previously identified candidate genes was investigated in two growth-based biotests with untreated and disinfected ARD soil. The first test included the apple genotypes M26, M9, B63 and Malus × robusta MAL0595 with differing ARD sensitivity, two ARD soils of different origin and a set of 108 candidate genes. The second test included M26, three ARD soils and seven candidate genes. These experiments revealed an early prominent induction of phytoalexin biosynthesis under ARD conditions. Two of the involved genes, biphenyl synthase 3 (BIS3) and biphenyl 4-hydroxylase b (B4Hb), as well as the ethylene-responsive transcription factor 1B-like (ERF1B) were chosen for further characterization by applying a set of abiotic stressors, which revealed a response of ERF1B to heat stress, while BIS3 and B4Hb were not affected. All three genes were induced by grass soil but to a significantly lower degree compared to ARD soil. In the second step, we analyzed BIS3, B4Hb and ERF1B expression in a split-root system to simulate the plant growing in ARD and non-ARD soil at the same time. Induction of gene expression and increase of specific phenolic compounds was present in roots growing in ARD soil, accompa-nied by the localized ARD symptoms of root discoloration and damage. Plants partly compensated shoot growth reduction if half of the root system was growing in healthy soil. The localized re-sponse was further confirmed in a split-column system without physical barriers between ARD and healthy soil, which indicated that ARD root symptoms, gene expression and phytoalexin con-tent depended on a direct or very close contact of ARD soil and the roots. The defense response of increased biphenyl, dibenzofuran, phloridzin and other aromatic com-pound contents was observed at significantly lower but detectable levels already in healthy soil. This may play a role in the shifts observed in rhizosphere and bulk soil microbial communities leading to ARD by a gradual enrichment in specialized detrimental communities. The immobile nature of ARD enables old trees to cope with this situation by growing into soil regions not yet affected by ARD. Young trees placed in this situation, however, suffer from severe growth depres-sion known as ARD.

AB - Apple replant disease (ARD) is a serious economic risk for tree nurseries and fruit growers world-wide. ARD causes root damage and stunting in young apple trees if replanted on a spot previously occupied by the same or closely-related species, which negatively impacts yield and quality of trees and fruits. No feasible and sustainable counteraction is available up to date but understanding the apple plant’s role in this complex phenomenon can help in the development of novel reme-dies and the early diagnosis to aid risk assessment. In this thesis, the reaction of apple rootstock plants to ARD was investigated in terms of ARD indicator gene expression in root and leaf material to identify an early and universal transcriptional marker for ARD. Firstly, the expression of a set of previously identified candidate genes was investigated in two growth-based biotests with untreated and disinfected ARD soil. The first test included the apple genotypes M26, M9, B63 and Malus × robusta MAL0595 with differing ARD sensitivity, two ARD soils of different origin and a set of 108 candidate genes. The second test included M26, three ARD soils and seven candidate genes. These experiments revealed an early prominent induction of phytoalexin biosynthesis under ARD conditions. Two of the involved genes, biphenyl synthase 3 (BIS3) and biphenyl 4-hydroxylase b (B4Hb), as well as the ethylene-responsive transcription factor 1B-like (ERF1B) were chosen for further characterization by applying a set of abiotic stressors, which revealed a response of ERF1B to heat stress, while BIS3 and B4Hb were not affected. All three genes were induced by grass soil but to a significantly lower degree compared to ARD soil. In the second step, we analyzed BIS3, B4Hb and ERF1B expression in a split-root system to simulate the plant growing in ARD and non-ARD soil at the same time. Induction of gene expression and increase of specific phenolic compounds was present in roots growing in ARD soil, accompa-nied by the localized ARD symptoms of root discoloration and damage. Plants partly compensated shoot growth reduction if half of the root system was growing in healthy soil. The localized re-sponse was further confirmed in a split-column system without physical barriers between ARD and healthy soil, which indicated that ARD root symptoms, gene expression and phytoalexin con-tent depended on a direct or very close contact of ARD soil and the roots. The defense response of increased biphenyl, dibenzofuran, phloridzin and other aromatic com-pound contents was observed at significantly lower but detectable levels already in healthy soil. This may play a role in the shifts observed in rhizosphere and bulk soil microbial communities leading to ARD by a gradual enrichment in specialized detrimental communities. The immobile nature of ARD enables old trees to cope with this situation by growing into soil regions not yet affected by ARD. Young trees placed in this situation, however, suffer from severe growth depres-sion known as ARD.

U2 - 10.15488/10484

DO - 10.15488/10484

M3 - Doctoral thesis

CY - Hannover

ER -