Data from: Diversity and abundance of phyllosphere bacteria are linked to insect herbivory

Simultaneous or sequential attack by herbivores and microbes is common in plants. Many seed plants exhibit a defence trade-off against chewing herbivorous insects and leaf-colonizing ('phyllosphere') bacteria, which arises from cross-talk between the phytohormones jasmonic acid (JA, induced by many herbivores) and salicylic acid (SA, induced by many bacteria). This cross-talk may promote reciprocal susceptibility in plants between phyllosphere bacteria and insect herbivores. In a population of native bittercress (Cardamine cordifolia, Brassicaceae), we tested whether simulating prior damage with JA or SA treatment induced resistance or susceptibility (respectively) to chewing herbivores. In parallel, we conducted culture-dependent surveys of phyllosphere bacteria to test the hypothesis that damage by chewing herbivores correlates positively with bacterial abundance in leaves. Finally, we tested whether bacterial infection induced susceptibility to herbivory by a major chewing herbivore of bittercress, Scaptomyza nigrita (Drosophilidae). Overall, our results suggest that reciprocal susceptibility to herbivory and microbial attack occurs in bittercress. We found that JA treatment reduced and SA treatment increased S. nigrita herbivory in bittercress in the field. Bacterial abundance was higher in herbivore-damaged vs. undamaged leaves (especially Pseudomonas syringae). However, Pedobacter spp. and Pseudomonas fluorescens infections were negatively associated with herbivory. Experimental Pseudomonas spp. infections increased S. nigrita herbivory in bittercress. Thus, plant defence signalling trade-offs can have important ecological consequences in nature that may be reflected in a positive correlation between herbivory and phyllosphere bacterial abundance and diversity. Importantly, the strength and direction of this association varies within and among prevalent bacterial groups.植食性生物与微生物对植物的同时或先后攻击在植物界中十分普遍。许多种子植物会针对咀嚼式植食性昆虫与叶际定殖（phyllosphere）细菌演化出防御权衡机制，该机制源于植物激素茉莉酸（jasmonic acid, JA，可被多数植食生物诱导）与水杨酸（salicylic acid, SA，可被多数细菌诱导）之间的交叉互作。这种激素交叉互作可能会在植物体内介导叶际细菌与昆虫植食者之间的双向易感性。我们以本土种群的心叶碎米荠（Cardamine cordifolia，十字花科Brassicaceae）为研究对象，测试了通过JA或SA处理模拟植食生物先期损伤，是否会分别诱导植物对咀嚼式植食生物产生抗性或易感性。与此同时，我们开展了基于培养法的叶际细菌调查，以验证“咀嚼式植食生物造成的叶片损伤与叶内细菌丰度呈正相关”这一假说。最后，我们验证了细菌侵染是否会诱导心叶碎米荠对其主要咀嚼式植食者——暗脉细果蝇（Scaptomyza nigrita，果蝇科Drosophilidae）产生取食易感性。综合来看，我们的研究结果表明，心叶碎米荠确实存在植食性取食与微生物侵染之间的双向易感性。田间试验结果显示，JA处理会降低心叶碎米荠被暗脉细果蝇取食的程度，而SA处理则会增强其被取食的概率。植食生物损伤的叶片内细菌丰度显著高于未损伤叶片（尤其是丁香假单胞菌（Pseudomonas syringae））。但Pedobacter属（Pedobacter spp.）与荧光假单胞菌（Pseudomonas fluorescens）的侵染则与植食取食呈负相关。人工接种假单胞菌属（Pseudomonas spp.）的试验表明，其会增强心叶碎米荠被暗脉细果蝇取食的程度。因此，植物防御信号通路的权衡机制在自然生态系统中具有重要的生态学效应，这种效应可通过植食取食与叶际细菌丰度、多样性之间的正相关关系得以体现。值得注意的是，这种关联的强度与方向在不同的优势细菌类群内部及类群之间均存在差异。