Leisso Rachel, Rudell Dave, Mazzola Mark
Montana State University Western Agriculture Research Center, Corvallis, MT, United States.
Physiology and Pathology of Tree Fruits Research, Agricultural Research Service (ARS), United States Department of Agriculture (USDA), Wenatchee, WA, United States.
Front Plant Sci. 2018 Sep 21;9:1336. doi: 10.3389/fpls.2018.01336. eCollection 2018.
Previous reports regarding rhizodeposits from apple roots are limited, and complicated by microbes, which readily colonize root systems and contribute to modify rhizodeposit metabolite composition. This study delineates methods for collection of apple rhizodeposits under axenic conditions, indicates rootstock genotype-specific differences and validates the contributions of vegetative activity to rhizodeposit quantity. Primary and phenolic rhizodeposit metabolites collected from two apple rootstock genotypes, G935 and M26, were delineated 2 months after root initiation by utilizing gas chromatography/liquid chromatography-mass spectrometry (GC/LC-MS), respectively. Twenty-one identified phenolic compounds and 29 sugars, organic acids, and amino acids, as well as compounds tentatively identified as triterpenoids were present in the rhizodeposits. When adjusted for whole plant mass, hexose, erythrose, galactose, phloridzin, kaempferol-3-glucoside, as well as glycerol, and glyceric acid differed between the genotypes. Phloridzin, phloretin, epicatechin, 4-hydroxybenzoic acid, and chlorogenic acid were among the phenolic compounds found in higher relative concentration in rhizodeposits, as assessed by LC-MS. Among primary metabolites assessed by GC-MS, amino acids, organic acids, and sugar alcohols found in relatively higher concentration in the rhizodeposits included L-asparagine, L-cysteine, malic acid, succinic acid, and sorbitol. In addition, putative ursane triterprenoids, identified based on accurate mass comparison to previously reported triterpenoids from apple peel, were present in rhizodeposits in high abundance relative to phenolic compounds assessed via the same extraction/instrumental method. Validation of metabolite production to tree vegetative activity was conducted using a separate set of micropropagated trees (genotype MM106) which were treated with a toxic volatile compound (butyrolactone) to inhibit activity/kill leaves and vegetative growth. This treatment resulted in a reduction of total collected rhizodeposits relative to an untreated control, indicating active vegetative growth contributes to rhizodeposit metabolites. Culture-based assays indicated an absence of bacterial or fungal endophytes in roots of micropropagated G935 and M26 plants. However, the use of fungi-specific primers in qPCR indicated the presence of fungal DNA in 30% of the samples, thus the contribution of endophytes to rhizodeposits cannot be fully eliminated. This study provides fundamental information for continued research and application of rhizosphere ecology driven by apple rootstock genotype specific rhizodeposition.
先前关于苹果根系根际沉积的报道有限，且因微生物而变得复杂，微生物很容易在根系中定殖并促使根际沉积代谢物组成发生改变。本研究描述了在无菌条件下收集苹果根际沉积的方法，指出了砧木基因型特异性差异，并验证了营养活性对根际沉积量的贡献。分别利用气相色谱/液相色谱-质谱联用仪（GC/LC-MS），在发根2个月后，确定了从两种苹果砧木基因型G935和M26收集的初级和酚类根际沉积代谢物。根际沉积中存在21种已鉴定的酚类化合物、29种糖、有机酸和氨基酸，以及初步鉴定为三萜类的化合物。当根据整株植物质量进行调整时，己糖、赤藓糖、半乳糖、根皮苷、山奈酚-3-葡萄糖苷，以及甘油和甘油酸在基因型之间存在差异。通过LC-MS评估，根皮苷、根皮素、表儿茶素、4-羟基苯甲酸和绿原酸是在根际沉积中相对浓度较高的酚类化合物。在通过GC-MS评估的初级代谢物中，根际沉积中相对浓度较高的氨基酸、有机酸和糖醇包括L-天冬酰胺、L-半胱氨酸、苹果酸、琥珀酸和山梨醇。此外，基于与先前报道的苹果果皮三萜类化合物的精确质量比较鉴定出的假定乌苏烷三萜类化合物，相对于通过相同提取/仪器方法评估的酚类化合物，在根际沉积中含量丰富。使用另一组微繁殖树（基因型MM106）进行代谢物产生与树木营养活性的验证，这些树用有毒挥发性化合物（丁内酯）处理以抑制活性/杀死叶片和营养生长。相对于未处理的对照，这种处理导致收集到的根际沉积总量减少，表明活跃的营养生长有助于根际沉积代谢物。基于培养的分析表明，微繁殖的G935和M26植物根系中不存在细菌或真菌内生菌。然而，在qPCR中使用真菌特异性引物表明30%的样品中存在真菌DNA，因此内生菌对根际沉积的贡献不能完全排除。本研究为基于苹果砧木基因型特异性根际沉积驱动的根际生态学的持续研究和应用提供了基础信息。
