Beans use an immune receptor to call in airstrikes on caterpillars
Beans use an immune receptor to call in airstrikes on caterpillars
豆类利用免疫受体召唤“空中打击”来对付毛毛虫
For decades, scientists have understood that plants can release volatile organic compounds—essentially airborne chemical signals—to attract the natural enemies of the things that eat them, like caterpillars. What we didn’t know was exactly how a plant translates the physical act of being eaten into a specific, predator-summoning distress signal. “[One] thing we didn’t know is how the plant detects the caterpillar in the first place,” says Adam Steinbrenner, a biologist at the University of Washington. Now, after years of experimenting with common bean plants in the lab and in the agricultural fields of Oaxaca, Mexico, Steinbrenner’s team pinpointed a single immune receptor that orchestrates its anti-caterpillar defense system.
几十年来，科学家们一直了解植物可以释放挥发性有机化合物（本质上是空气传播的化学信号），以吸引那些以它们为食的生物（如毛毛虫）的天敌。但我们并不清楚植物究竟是如何将“被啃食”这一物理行为转化为特定的、能够召唤捕食者的求救信号的。华盛顿大学的生物学家亚当·斯坦布伦纳（Adam Steinbrenner）表示：“我们此前不知道的一点是，植物最初是如何探测到毛毛虫的。”如今，经过在实验室以及墨西哥瓦哈卡州农田中对菜豆植株多年的实验，斯坦布伦纳的团队终于确定了一种能够协调其抗毛毛虫防御系统的单一免疫受体。
Drooling caterpillars
流口水的毛毛虫
When an herbivorous insect like a caterpillar feeds on a plant, it introduces its saliva straight into the plant’s damaged tissues. This saliva contains biological clues called HAMPs: herbivore-associated molecular patterns. One of the HAMPs molecules is a peptide called inceptin, and there’s an 11-amino acid fragment of inceptin named In11, as well. Both of them turn out to be a fragment of the ATP synthase found in chloroplasts—basically a piece of one of the plant’s own proteins. As the caterpillar ingest the leaf, its gut enzymes chop up the plant’s cellular engines and their pieces, including In11, are regurgitated back onto the leaf’s surface, albeit at extremely small concentrations.
当毛毛虫等植食性昆虫取食植物时，会将唾液直接注入植物受损的组织中。这种唾液中含有被称为 HAMPs（植食性昆虫相关分子模式）的生物学线索。其中一种 HAMPs 分子是一种名为“inceptin”的肽，此外还有一个由 11 个氨基酸组成的 inceptin 片段，被称为 In11。事实证明，它们都是叶绿体中 ATP 合酶的片段——本质上是植物自身蛋白质的一部分。当毛毛虫吞食叶片时，其肠道酶会分解植物的细胞引擎，这些碎片（包括 In11）会被反刍回叶片表面，尽管浓度极低。
Over millions of years, plants like the common bean have evolved a specialized cell-surface receptor called the inceptin receptor just to detect In11. When this receptor interacts with In11, it sets off a signaling cascade in the plant’s cells, initiating immune responses. Proving that this specific receptor is responsible for releasing predator-summoning signals, though, was extremely tricky. “We were excited to do that, but we needed the perfect comparison plants—plants lacking the receptor versus ones that have the intact receptor,” Steinbrenner says. The problem was that common bean plants are notoriously difficult to genetically modify, so the usual modern techniques like gene silencing were off the table. Picking an easier-to-modify plant was off the table, too. “We were sort of limited to bean because this receptor we were studying is only present in certain bean species,” Steinbrenner explains. To get around it, his team had to introduce the modifications they needed the old-fashioned way—through selective breeding.
经过数百万年的进化，菜豆等植物进化出了一种专门的细胞表面受体，即“inceptin 受体”，专门用于探测 In11。当该受体与 In11 相互作用时，会引发植物细胞内的信号级联反应，从而启动免疫应答。然而，要证明这种特定的受体确实负责释放召唤捕食者的信号极其困难。斯坦布伦纳说：“我们对此感到兴奋，但我们需要完美的对照植株——即缺乏该受体的植株与拥有完整受体的植株进行对比。”问题在于，菜豆以难以进行基因改造而闻名，因此基因沉默等常见的现代技术无法使用。选择一种更容易改造的植物也不可行。斯坦布伦纳解释道：“我们只能局限于研究豆类，因为我们正在研究的这种受体仅存在于某些豆类物种中。”为了解决这个问题，他的团队不得不通过老式的方法——选择性育种——来引入所需的遗传修饰。
Breeding siblings
培育“同胞”植株
The first step was to find a common bean plant with a muted In11 receptor. What the team needed was a natural mutant that was unable to detect the caterpillar’s saliva. They screened a massive panel of Mesoamerican beans, looking for varieties that failed to produce ethylene gas, a classic plant stress indicator, when exposed to In11. Out of 89 varieties tested, they found two that completely ignored the peptide. Of these two, they picked a Honduran strain called W6 13807. When the researchers sequenced the genome of this insensitive bean, they found it had a naturally occurring 103-base-pair deletion in the gene that encodes the inceptin receptor. This mutation, they found, deletes a crucial chunk of the receptor, resulting in a truncated, non-functional protein.
第一步是找到一种 In11 受体发生突变的菜豆植株。团队需要的是一种无法探测毛毛虫唾液的天然突变体。他们筛选了大量中美洲豆类品种，寻找那些在接触 In11 后无法产生乙烯（一种经典的植物胁迫指标）的品种。在测试的 89 个品种中，他们发现了两个完全“无视”该肽的品种。在这两者中，他们选择了名为 W6 13807 的洪都拉斯品系。当研究人员对这种不敏感的豆类进行基因组测序时，发现其编码 inceptin 受体的基因中存在一段 103 个碱基对的天然缺失。他们发现，这种突变删除了受体的一个关键部分，导致产生了一种截短的、无功能的蛋白质。
To test the effect of this dysfunctional receptor on the plant’s defenses, the team began breeding the plants for their experiment. Through a series of genetic crosses and backcrosses between the mutant and a standard bean variant that was responsive to In11, they created sibling plants that were nearly identical genetically except for the presence or absence of the functional inceptin receptor. “We were just being breeders and that took several years”, Steinbrenner recalls. When these two siblings were put side by side in the lab and in the field, it turned out the consequences of having a broken inceptin alarm were rather grave for the bean plants.
为了测试这种功能失调的受体对植物防御的影响，团队开始培育用于实验的植株。通过在突变体与对 In11 有反应的标准豆类品种之间进行一系列的杂交和回交，他们培育出了“同胞”植株，这些植株在遗传上几乎完全相同，唯一的区别在于是否拥有功能性的 inceptin 受体。斯坦布伦纳回忆道：“我们当时就像育种员一样，这花了几年时间。”当这两株“同胞”植株在实验室和野外并排种植时，结果显示，inceptin 警报系统失效对豆类植株来说后果相当严重。
The cost of silence
沉默的代价
First, the researchers examined direct defenses—the chemical and physical changes the plant undergoes to make its leaves less palatable for caterpillars and thus hamper their growth. When caterpillars fed on the mutant beans with inactive inceptin receptors, though, they had a field day. Over a five-day feeding period, their growth rate was over 70 percent higher than on the plants with a functional receptor. More detailed analysis revealed exactly why this was the case. In plants that could detect the In11 peptide, a feeding caterpillar triggered the rapid up-regulation of 527 genes, including the ones responsible for anti-herbivore defenses. The plants that were oblivious to the In11 in the caterpillar spit failed to mount this targeted response. Instead, they reacted as if they were just being mechanically wounded by the wind or a passing animal. Without the receptor, they entirely missed that a live, hungry insect was actively eating them.
首先，研究人员检查了直接防御机制——即植物为了使叶片不再那么适口从而阻碍毛毛虫生长而发生的化学和物理变化。然而，当毛毛虫取食那些 inceptin 受体失活的突变豆类时，它们简直如鱼得水。在五天的取食期内，它们的生长速度比在拥有功能性受体的植株上快了 70% 以上。更详细的分析揭示了原因：在能够探测到 In11 肽的植物中，毛毛虫的取食触发了 527 个基因的快速上调，其中包括负责抗植食性防御的基因。而那些对毛毛虫唾液中的 In11“视而不见”的植物，则无法启动这种针对性的反应。相反，它们的反应就像只是被风或路过的动物机械性地擦伤了一样。没有了受体，它们完全没有意识到有一只活生生、饥肠辘辘的昆虫正在啃食它们。
Another consequence for In11 insensitive beans was that they were unable to summon predatory wasps.
对于 In11 不敏感的豆类，另一个后果是它们无法召唤捕食性黄蜂。
Calling air support
呼叫空中支援
When a normal bean plant detects In11, it begins synthesizing and emitting a highly specific blend of volatile organic chemicals. To a predatory wasp, this blend of scents signals not just “a plant is damaged,” but specifically “a caterpillar is actively feeding here right now.” Lab tests showed that the plants without the active inceptin receptor failed to emit this volatile blend when exposed to either the synthetic In11 peptide or actual caterpillar oral secretions. To see how much this lack of chemical signaling mattered in the wild, the researchers packed up their sibling bean lines and headed to an experimental agricultural field in Oaxaca, Mexico. There, they placed pairs of bean plants—one with the active receptor and one without it—out in the open. They treated the plants with either water…
当正常的菜豆植株探测到 In11 时，它会开始合成并释放一种高度特异性的挥发性有机化学物质混合物。对于捕食性黄蜂来说，这种气味混合物不仅传达了“植物受损”的信号，更具体地传达了“现在有一只毛毛虫正在这里取食”的信息。实验室测试表明，当暴露于合成的 In11 肽或真实的毛毛虫口腔分泌物时，缺乏活性 inceptin 受体的植株无法释放这种挥发性混合物。为了观察这种化学信号的缺失在野外有多大影响，研究人员带上他们的“同胞”豆类品系，前往墨西哥瓦哈卡州的一个实验农田。在那里，他们将成对的豆类植株（一株有活性受体，一株没有）放置在露天环境中。他们用清水处理了这些植株……