Data from: Root and shoot glucosinolate allocation patterns follow optimal defence allocation theory

1. Optimal defence allocation theory (ODT) is one of the most prominent theoretical frameworks to explain the allocation of defence compounds within plants. It predicts that the most valuable and vulnerable plant organs have the highest levels of chemical defence. The ODT has been well worked out and experimentally tested for shoot defences, but not for root defences. To assess if ODT principles apply similarly to roots and shoots, we examined glucosinolates in aboveground and belowground organs of nine plant species belonging to two families. 2. In order to evaluate whether ODT equally applies to shoot and root organs, we designed a conceptual model in which aboveground and belowground organs were assigned to orders of importance to plant performance. We hypothesized that organs constituting the plant’s core structure are better protected than more distal organs. 3. The nine plant species were cultivated, and their roots and shoots were harvested and divided in three orders for glucosinolate analysis. Using a specialist (Delia radicum) and a generalist (Amphimallon solstitiale) root herbivore, we also experimentally tested the hypothesis that the generalist herbivore prefers to feed on fine roots with a low glucosinolate concentration, while the specialist prefers taproots with a high glucosinolate concentration. 4. We found that both in roots and shoots the higher-ordered core structural organs (taproots and stems) had the highest levels of glucosinolates. Belowground, taproots and lateral roots were better protected than the more distal, and less costly, fine roots in seven out of nine species tested. The specialist root herbivore preferred feeding on the highly defended taproots, which is in line with what has been found for aboveground specialist herbivores. Moreover, the glucosinolate concentration in roots overall was significantly higher than that in shoots. 5. Synthesis. These results support the hypothesis that ODT generally applies to glucosinolate allocation in aboveground and belowground organs and may mainly serve to maintain the integrity of the main plant structure. Moreover, it suggests that aboveground and belowground insect herbivores independently exert similar selection pressures on defence allocation patterns in roots and shoots.

1. 最优防御分配理论（Optimal defence allocation theory, ODT）是阐释植物体内防御化合物分配机制的核心理论框架之一。该理论预测，植物中经济价值最高且最易受侵害的器官，其化学防御物质含量也最高。目前，ODT理论已在植物地上部分防御领域得到充分推演与实验验证，但尚未针对根系防御开展相关研究。为验证ODT原理是否同样适用于根系与地上组织，本研究对隶属于2个科的9种植物的地上与地下器官中的硫代葡萄糖苷（glucosinolates）进行了检测分析。
2. 为评估ODT是否可同等适用于植物地上与地下器官，我们构建了概念模型，将地上与地下器官按照其对植物生存与生长性能的重要性进行分级排序。我们提出假说：构成植物核心结构的器官，其防御水平优于更远端的外围器官。
3. 我们对9种供试植物进行栽培，收获其根系与地上组织，并按重要性分级划分为3个层级以开展硫代葡萄糖苷含量分析。此外，我们分别利用专食性根植食性昆虫萝卜蝇（Delia radicum）与广食性根植食性昆虫夏至鳃金龟（Amphimallon solstitiale）开展实验，验证如下假说：广食性植食者偏好取食硫代葡萄糖苷含量较低的细根，而专食性植食者则偏好取食硫代葡萄糖苷含量较高的主根。
4. 研究结果显示，无论是根系还是地上组织，处于较高层级的核心结构器官（主根与茎秆）的硫代葡萄糖苷含量均最高。在地下部分，9个供试物种中有7个的主根与侧根防御水平高于更远端、构建成本更低的细根。专食性根植食性昆虫偏好取食防御能力较强的主根，这与已有针对地上专食性植食性昆虫的研究结果一致。此外，根系整体的硫代葡萄糖苷含量显著高于地上组织。
5. 综合分析：本研究结果支持"ODT理论普遍适用于植物地上与地下器官的硫代葡萄糖苷分配"这一假说，且该防御分配模式主要用于维持植物主体结构的完整性。此外，本研究结果还表明，地上与地下昆虫植食者可独立对根系与地上组织的防御分配模式施加相似的选择压力。