A major cause of phytoplankton mortality is predation by zooplankton. Strategies to avoid grazers have probably played a major role in the evolution of phytoplankton and impacted bloom dynamics and trophic energy transport. Certain species of the genus Pseudo-nitzschia produce the neurotoxin, domoic acid (DA), as a response to the presence of copepod grazers, suggesting that DA is a defense compound. The biosynthesis of DA comprises fusion of two precursors, a C10 isoprenoid geranyl pyrophosphate and l-glutamate. Geranyl pyrophosphate (GPP) may derive from the mevalonate isoprenoid (MEV) pathway in the cytosol or from the methyl-erythritol phosphate (MEP) pathway in the plastid. l-glutamate is suggested to derive from the citric acid cycle. Fragilariopsis, a phylogenetically related but nontoxic genus of diatoms, does not appear to possess a similar defense mechanism. We acquired information on genes involved in biosynthesis, precursor pathways and regulatory functions for DA production in the toxigenic Pseudo-nitzschia seriata, as well as genes involved in responses to grazers to resolve common responses for defense strategies in diatoms. Several genes are expressed in cells of Pseudo-nitzschia when these are exposed to predator cues. No genes are expressed in Fragilariopsis when treated similarly, indicating that the two taxa have evolved different strategies to avoid predation. Genes involved in signal transduction indicate that Pseudo-nitzschia cells receive signals from copepods that transduce cascading molecular precursors leading to the formation of DA. Five out of seven genes in the MEP pathway for synthesis of GPP are upregulated, but none in the conventional MEV pathway. Five genes with known or suggested functions in later steps of DA formation are upregulated. We conclude that no gene regulation supports that l-glutamate derives from the citric acid cycle, and we suggest the proline metabolism to be a downstream precursor. Pseudo-nitzschia cells, but not Fragilariopsis, receive and respond to copepod cues. The cellular route for the C10 isoprenoid product for biosynthesis of DA arises from the MEP metabolic pathway and we suggest proline metabolism to be a downstream precursor for l-glutamate. We suggest 13 genes with unknown function to be involved in diatom responses to grazers.

中文翻译：

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对放牧的转录组反应揭示了导致软骨藻酸生物合成的代谢途径，并强调了硅藻中不同的防御策略。

浮游植物死亡的一个主要原因是浮游动物的捕食。避开食草动物的策略可能在浮游植物的进化中发挥了重要作用，并影响了水华动态和营养能量传输。拟菱形藻属的某些物种会产生神经毒素软骨藻酸 (DA)，作为对桡足类食草动物存在的反应，这表明 DA 是一种防御化合物。DA 的生物合成包括两种前体的融合，即 C10 类异戊二烯香叶基焦磷酸和 L-谷氨酸。焦磷酸香叶酯 (GPP) 可能源自细胞质中的甲羟戊酸类异戊二烯 (MEV) 途径或质体中的甲基赤藓糖醇磷酸 (MEP) 途径。L-谷氨酸被认为源自柠檬酸循环。脆杆藻是一种系统发育相关但无毒的硅藻属，似乎不具有类似的防御机制。我们获得了产毒拟菱形藻中参与生物合成、前体途径和 DA 产生调节功能的基因的信息，以及参与对食草动物反应的基因的信息，以解决硅藻防御策略的常见反应。当拟菱形藻细胞暴露于捕食者信号时，一些基因会在细胞中表达。当类似处理时，脆杆藻中没有基因表达，表明这两个类群进化出了不同的策略来避免捕食。参与信号转导的基因表明，拟菱形细胞接收来自桡足类的信号，转导级联分子前体，导致 DA 的形成。用于合成 GPP 的 MEP 途径中的 7 个基因中有 5 个上调，但传统 MEV 途径中没有上调。在 DA 形成的后续步骤中具有已知或暗示功能的五个基因被上调。我们的结论是，没有基因调控支持 ​​L-谷氨酸来源于柠檬酸循环，并且我们认为脯氨酸代谢是下游前体。拟菱形藻细胞接收桡足类信号并对其做出反应，但脆杆藻细胞则不然。用于 DA 生物合成的 C10 类异戊二烯产物的细胞途径源自 MEP 代谢途径，我们认为脯氨酸代谢是 L-谷氨酸的下游前体。我们建议 13 个功能未知的基因参与硅藻对食草动物的反应。