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After decades of seeing plants as passive recipients of fate, scientists have found them capable of behaviors once thought unique to animals. Some plants even appear to be social, favoring family while pushing strangers from the neighborhood.
Research into plant sociality is still young, with many questions unanswered. But it may change how people conceive of the floral world, and provide new ways of raising productivity on Earth's maxed-out farmlands.
"When I was in school, researchers assumed that some plants were better or worse than others at getting resources, but they were blind to the whole social situation," said Susan Dudley, a McMaster University biologist. "I went looking for it, and to my shock, found it. And we've found more of it since."
In a paper published in the November American Journal of Botany, Dudley describes how Impatiens pallida, a common flowering plant, devotes less energy than usual to growing roots when surrounded by relatives. In the presence of genetically unrelated Impatiens, individuals grow their roots as fast as they can.
Acknowledging relatives in this way is an example of kin recognition. It's common in the animal world, and is a precursor to kin selection, in which animals help their familial group, not just themselves. Dudley thinks plants have kin selection, too. It's a controversial idea, but that it's even being debated shows how far research into plant sociality has come.
When Dudley was in school in the 1980s, the very idea of plant sociality was practically taboo among scientists. It had burst into popular consciousness a decade earlier with the publication of The Secret Life of Plants, a New Age classic which also discussed orgones and dowsing. Later studies on "talking trees" went unreplicated, and the idea fell into disrepute.
But even if full-blown sentience was a silly idea, research on plant communication gathered. Much of it described how plants defended themselves, producing toxins and concentrating resources on their immune systems when unrelated neighboring plants were eaten. That clearly involved some sort of chemical signaling. Further studies conclusively showed plants were able to recognize themselves. Whether plants might respond to their relatives became a legitimate and intriguing question.
The answer isn't only of concern to people with imaginations stirred by thoughts of chatting flora. It could provide a whole new perspective on plant behavior and evolution. By providing insights that improve agricultural productivity, studies of kin recognition could literally bear fruit.
"We know that in the animal world, kin recognition and selection plays a very important role for family structure, altruistic behavior and those kinds of things," said Hans de Kroon, a plant ecologist at Radboud University in the Netherlands. "It's so prominent in the animal literature. Once we start to discover that plants can recognize their kin, there's a whole set of hypotheses we can apply to studying plants, that nobody ever thought to."
The field's landmark paper came from Dudley's laboratory in 2007, when she showed how American searocket plants accelerated their root growth when placed in pots of strangers, but slowed it down when potted with siblings. Were they animals, they'd be described as sharing water and food.
In a Communicative and Integrative BiologyArabidopsis thalianaproved to be in the roots - and just as Dudley had seen, growth patterns varied according to whether secretions came from genetically unrelated plants, or family. paper published in October, University of Delaware biologists Harsh Bais and Meredith Biedrzycki tried to isolate the means of recognition by exposing seedlings, each in its own pot, to root secretions from other Arabidopsis plants. The signal indeed
Intriguingly, the plants in Dudley's latest study were potted separately and unexposed to each others' secretions, suggesting that their leaves emit chemical signals, as well as their roots. That's supported by the research of University of California, Davis ecologist Richard Karban, who in a June Ecology Lettersboosts its immune system when exposed to the damaged cuttings of a related plant [pdf]. It seems to hear warnings from its kin. study showed that sagebrush
More studies are needed to show exactly what sort of benefits are provided by these signaling and response systems. De Kroon said kin recognition doesn't necessarily mean kin selection: maybe the plants are communicating, but it doesn't do them much good in practice.
One of Dudley's students, Amanda File, is now studying whether some trees favor their own progeny, which might grow best near their parents. Dudley and graduate student Guillermo Murphy, a co-author of the American Journal of Botany paper, are looking for for kin selection in invasive plants.
"We're testing the hypothesis that invasive plants evolve greater altruism within their populations, allowing them to be better invaders of their new habitats," said Dudley.
For plants used in agriculture, Dudley recommends kin recognition studies to see whether certain arrangements of relatives and strangers would be especially productive. De Kroon is looking at multi-species mixes. Karban hopes to use communication insights to engineer natural defense systems against pests.
"Maybe we thought before that only humans could do certain things, or vertebrates, or animals," said Karban. "Plants are capable of much more sophisticated behavior than we assumed."
幾十年以來，科學(xué)家一直把植物看成是被動(dòng)接受命運(yùn)安排的生物，但是日前科學(xué)家發(fā)現(xiàn)植物也有一些主動(dòng)行為，而在此之前科學(xué)家一直以為那些只是動(dòng)物特有的行為�？茖W(xué)家們甚至還發(fā)現(xiàn)，從表面看有些植物好像還有社交活動(dòng)，熱愛自己的家族，以及會(huì)排斥附近出現(xiàn)的陌生"人".
目前，植物社交行為的研究才剛起步，還有很多問題尚未解決。不過，這項(xiàng)研究有可能改變?nèi)藗儗?duì)植物世界的看法，同時(shí)還有望提出新方法以解決目前地球上農(nóng)田嚴(yán)重超支、農(nóng)產(chǎn)品產(chǎn)量難以提高的問題。
麥克馬斯特大學(xué)的生物學(xué)家蘇珊 達(dá)德利說："我上學(xué)那會(huì)兒，研究斷定植物植株吸收養(yǎng)分的能力是有差別的，有些強(qiáng)有些弱。但是當(dāng)時(shí)他們卻沒有發(fā)現(xiàn)整個(gè)植物社交環(huán)境的存在。后來我開始研究這個(gè)領(lǐng)域，讓我震驚的是，我發(fā)現(xiàn)了植物存在社交環(huán)境！到目前為止，在這一領(lǐng)域我們又有了更多研究成果。"
《美國植物學(xué)期刊》11月份那一期上發(fā)表了Dudley的一篇文章，文中Dudley描繪了一種名為淡紫鳳仙花的普通開花植物，這種植物如果和親屬植株長在一起，根部生長就不如以往狂盛。但是，一旦周圍有非親屬基因的鳳仙花植株存在，它們的根就使勁長。
這種承認(rèn)親屬存在的情況也屬于親屬識(shí)別的例子。動(dòng)物不僅保護(hù)自身也會(huì)保護(hù)自己的家族，這些現(xiàn)象在動(dòng)物世界里是司空見慣的，而且也是動(dòng)物進(jìn)行親屬選擇的表現(xiàn)。Dudley認(rèn)為植物同樣會(huì)進(jìn)行親屬選擇。該觀點(diǎn)目前還存在爭議，不過人們對(duì)這一觀點(diǎn)的爭論正好可以反映出植物社交研究現(xiàn)已走到了哪一步。
上世紀(jì)80年代Dudley還在讀書的那個(gè)時(shí)候，植物社交這個(gè)觀點(diǎn)在科學(xué)界簡直是個(gè)忌諱。但是十年前"植物的秘密生活"一文的發(fā)表，讓全社會(huì)開始關(guān)注這個(gè)事情，這篇同時(shí)探討了orgones和dowsing的文章可謂是一篇新的時(shí)代經(jīng)典之作。但是，之后關(guān)于"會(huì)說話的樹"的研究卻沒有得到人們的跟風(fēng)，至此這個(gè)觀點(diǎn)就開始不受重視了。
全民跟風(fēng)雖然不是好事，但是至少植物交流的研究慢慢多起來了。絕大多數(shù)的此類研究寫的都是植物在自己附近的非親屬植株被吃掉后是如何反應(yīng)的，包括它們怎么保護(hù)自己、怎么制造毒液、怎么在免疫系統(tǒng)儲(chǔ)備資源等等。很明顯，這里涉及了一些化學(xué)方面的信號(hào)。最后，進(jìn)一步研究又表明植物能認(rèn)出它們自己那一伙的植株。所以，親屬植株之間可能會(huì)有反應(yīng)是合乎情理的，但是它本身又是個(gè)相當(dāng)復(fù)雜的問題。
對(duì)該問題給出解答并不僅僅是因?yàn)殛P(guān)心人民，怕聊天植物引的他們胡思亂想，還因?yàn)樗転槲覀兝斫庵参镄袨椋私庵参镞M(jìn)化提供了一個(gè)全新的視角。親屬識(shí)別研究為提高農(nóng)作物產(chǎn)量提供了不少想法，所以這項(xiàng)研究的確有可能結(jié)出豐碩的成果。
荷蘭Radboud大學(xué)的植物生態(tài)學(xué)家Hans de Kroon說："我們知道在動(dòng)物世界里，親屬識(shí)別和親屬選擇在動(dòng)物的家族結(jié)構(gòu)、利他行為及其他各方面都發(fā)揮著至關(guān)重要的作用。同時(shí)，在動(dòng)物文學(xué)作品中的表現(xiàn)也非常明顯。一旦我們發(fā)現(xiàn)植物也能識(shí)別親人，那么就將有一整套的假設(shè)可以用到植物研究中，而這是我們之前從未想到的。"
2007年，這一領(lǐng)域具有里程碑意義的文章從Dudley的實(shí)驗(yàn)室里誕生了。Dudley在實(shí)驗(yàn)室里發(fā)現(xiàn)American searocket這種植物如果跟陌生植株同盆種植，根系生長就會(huì)加快，相反的，如果和兄弟植株種在一起就會(huì)減慢。假如它們是動(dòng)物，那就表現(xiàn)為共同分享水和食物。
到底這些化學(xué)信號(hào)和反應(yīng)系統(tǒng)給植物提供什么好處呢，這還有待更多研究證實(shí)。De Kroon說親屬識(shí)別并不完全等同于親屬選擇，可能植株之間存在交流，但是這并不能給它們帶來實(shí)際的好處。
Dudley的學(xué)生Amanda File發(fā)現(xiàn)離母株最近的幼苗生長最好，因此他現(xiàn)在正在研究一些樹種看它們是否真的自己關(guān)心下一代。目前，Dudley正和自己的研究生Guillermo Murphy--《美國植物學(xué)期刊》的合著者，研究入侵植物中的親屬選擇情況。
Dudley說："目前我們正在驗(yàn)證一個(gè)假設(shè)，就是入侵植物在進(jìn)化過程中整個(gè)種族的利己性可能會(huì)變得越來越強(qiáng)，從而能讓自己在現(xiàn)今的入侵地生活的更好。"
針對(duì)農(nóng)業(yè)作物，Dudley建議通過親屬識(shí)別研究看如何安排親屬植株或者謀生植物之間的種植位置，以提高農(nóng)作物產(chǎn)量。De Kroon正在研究多品種植株套種。Karban希望能利用有關(guān)植物交流的各種觀點(diǎn)來改造植物對(duì)害蟲的自然防御系統(tǒng)。
Karban說："也許在此之前我們認(rèn)為只有人、脊椎動(dòng)物或者牲畜才有某些行為，但是也許我們低估了植物，它們的行為可以比我們想像的要復(fù)雜。"
