Detoxification of host plant phenolic aglycones by the spruce budworm

This study examines the post-ingestive fate of two host-plant derived small-molecule phenolics (the acetophenones piceol and pungenol) that have previously been shown to be toxic to the outbreaking forest pest, spruce budworm (Choristoneura fumiferana). We test first whether these compounds are transformed during passage through the midgut, and second whether the budworm upregulates activity of the detoxification enzyme glutathione-s-transferase (GST) in response to feeding on these compounds. Insects were reared on either foliage or artificial diet to the fourth instar, when they were transferred individually to one of two treatment diets, either control or phenolic-laced, for approximately 10 days, after which midguts were dissected out and used for Bradford soluble protein and GST enzyme activity analysis. Frass was collected and subjected to HPLC-DAD-MS. HPLC showed that the acetophenones do not autoxidize under midgut pH conditions, but that glucose- and glutathione- conjugates are present in the frass of insects fed the phenolic-laced diet. GST enzyme activity increases in insects fed the phenolic-laced diet, in both neutral pH and alkaline assays. These data show that the spruce budwom exhibits counter-adaptations to plant phenolics similar to those seen in angiosperm feeders, upregulating an important detoxifying enzyme (GST) and partially conjugating these acetophenones prior to elimination, but that these counter-measures are not totally effective at mitigating toxic effects of the ingested compounds in the context of our artifical-diet based laboratory experiment.

本研究旨在探讨两种宿主植物来源的小分子酚类化合物（即香草酮类化合物 piceol 和 pungenol）在入侵性森林害虫——云杉卷叶蛾（Choristoneura fumiferana）体内的代谢命运。研究首先验证了这些化合物在通过中肠时的转化情况，其次检测了卷叶蛾在摄食这些化合物后，是否上调了解毒酶谷胱甘肽-S-转移酶（GST）的活性。昆虫在叶子和人工饲料上饲养至第四龄期，然后单独转移至两种处理饲料之一，分别为对照组或酚类化合物添加组，持续约10天，之后解剖中肠用于Bradford可溶性蛋白和GST酶活性分析。粪便被收集并经过高效液相色谱-二极管阵列-质谱（HPLC-DAD-MS）分析。高效液相色谱结果显示，在云杉卷叶蛾中肠的pH条件下，香草酮类化合物不会发生自氧化，但在摄食酚类化合物添加饲料的昆虫粪便中存在葡萄糖和谷胱甘肽的共轭物。摄食酚类化合物添加饲料的昆虫中，GST酶活性在中性pH和碱性条件下均有所增加。这些数据表明，云杉卷叶蛾对植物酚类化合物的适应机制与被子植物食植者相似，通过上调重要的解毒酶（GST）并在排除前部分共轭这些香草酮类化合物，但这些对策并不能完全有效地减轻摄入化合物在基于人工饲料的实验室实验中的毒性效应。