Root growth, function and rhizosphere microbiome analyses show local rather than systemic effects in apple plant response to replant disease soil

Apple replant disease (ARD) is the phenomenon of soil decline occurring after repeated planting of apple trees at the same site. This study aimed to elucidate whether ARD is systemic, i.e. whether the contact of parts of the root system with ARD soil causes the whole plant to show poor shoot and root growth. A split-root experiment was conducted with seedlings of ‘M26’, offering the same plant for its root system the choice between the substrates ARD soil (+ARD), γ-sterilized ARD soil (-ARD) or soil from a grass parcel (Control) with the following combinations: +ARD/+ARD, -ARD/-ARD; +ARD/-ARD; +ARD/Control. Root growth was analysed throughout the 34-day growing period. Samples from bulk, rhizosphere and rhizoplane soil were collected separately for each compartment, and analysed by fingerprints of 16S rRNA gene or ITS fragments amplified from total community (TC) DNA. The response of the plant to +ARD was not systemic as root growth in -ARD compartment was always superior to root growth in +ARD soil. Crosswise 15N-labelling of the N-fertilizer applied to the split-root compartments showed that nitrate-N uptake efficiency was higher for roots in -ARD soil compared to those in +ARD. Bacterial and fungal community composition in the rhizoplane and rhizosphere of the same plants differed significantly between the compartments containing +ARD/-ARD or +ARD/Control. The strongest differences between the bacterial fingerprints were observed in the rhizoplane and rhizosphere. Bacterial genera with increased abundance in response to ARD were mainly Streptomyces but also Sphingobium, Novosphingobium, Rhizobium, Lysobacter and Variovorax. The strongest differences between the fungal fingerprints were observed in bulk soil. Our data showed that the response of the apple plant to ARD soil is local and not systemic.

苹果连作障碍（Apple Replant Disease, ARD）指在同一地块重复种植苹果树后出现的土壤衰退现象。本研究旨在阐明苹果连作障碍是否具有系统性，即根系部分区域接触ARD土壤是否会导致整株植株地上部与根系生长不良。本研究以‘M26’苹果实生苗为材料开展分根试验（split-root experiment），为该植株的根系提供三种供试基质：ARD土壤（标记为+ARD）、γ射线灭菌ARD土壤（标记为-ARD）以及草地原生土壤（标记为对照组），共设置四种组合：+ARD/+ARD、-ARD/-ARD、+ARD/-ARD、+ARD/对照组。在为期34天的生长周期内全程分析根系生长情况。分别从每个分根室采集非根际土壤（bulk soil）、根际土壤（rhizosphere）与根面土壤（rhizoplane）样品，通过对从总群落（Total Community, TC）DNA中扩增得到的16S核糖体RNA基因（16S rRNA gene）或内转录间隔区（Internal Transcribed Spacer, ITS）片段进行指纹图谱分析，以解析微生物群落结构。植株对+ARD组的响应并非系统性的——-ARD分根室的根系生长始终优于+ARD土壤中的根系。对施加于各分根室的氮肥采用交叉15N标记法后发现，-ARD组根系的硝态氮吸收效率显著高于+ARD组根系。同一植株不同分根室（+ARD/-ARD组合或+ARD/对照组组合）的根面与根际微生物群落组成存在显著差异。细菌指纹图谱的最大群落差异出现在根面与根际环境中。对ARD响应后丰度升高的细菌属主要为链霉菌属（Streptomyces），此外还包括鞘氨醇单胞菌属（Sphingobium）、新鞘氨醇单胞菌属（Novosphingobium）、根瘤菌属（Rhizobium）、溶杆菌属（Lysobacter）以及贪食菌属（Variovorax）。真菌指纹图谱的最大群落差异则出现在非根际土壤中。本研究数据表明，苹果树对ARD土壤的响应具有局部性，而非系统性。