理論提出植物根部的效率變高以及獨自運作的能力增加使它們能擴散至全世界
Morgan Kelly
一項關於植物演化的新理論提出4億年來驅使植物擴散至全世界的因素，或許不是地表之上我們能輕易看見的特徵，而是位在地底的適應構造讓植物能更有效率地獨自運作。
根據普林斯頓大學和北京中國科學院的研究人員於2月21日發表在期刊《科學》(Nature)的研究，當植物從它們最初發源、營養豐富的赤道地區往南北擴張時，它們的根尖變得更細且根系分布範圍也變得更廣，有助於植物從越發貧瘠的土壤中探求重要養分。
此外，隨著植物往環境變化越來越難以預測的地區擴張(像是乾燥的沙漠)，它們生長時對共生真菌――即「菌根菌」(mycorrhiza)――的依存程度也越來越低。菌根菌會寄居在植物根部並幫助宿主吸收植物必須的養分――氮和磷。
研究通訊作者Lars Hedin教授是普林斯頓大學環境學院生態學暨演化生物學系的主任，同時也是該校生物學的名譽教授。Hedin表示這項發現重新探討植物演化時如何適應新環境。他說科學家過去討論這項問題時著重於植物地面上的特徵，尤其是葉子的性質和植物進行光合作用時吸收陽光的效率。
反之，Hedin表示他和同事首度發現植物的根部直徑和對真菌的依賴程度――或者是缺乏真菌的程度，這兩項性質是各個生物相(biome)中的植物群集最為一致的特徵。所謂的生物相是由許多特定動植物組成的大型群集，像是沙漠、溫帶森林或者莽原生物相。
「隨著時間經過植物能逐漸征服全球必定有它們的秘訣。」Hedin表示，「我們的目標是要解開並了解這些策略，而我們的發現對植物演化提供了一個新的通用理論。殘酷的適者生存遊戲一直以來在地下悄悄上演著，我們有幸能當上有史以來首批解開這場遊戲背後科學法則的人。」
「這項研究對植物界的保育和管理工作來說具有很大的意義。」Hedin接著說明，「研究提供了某些隱藏在『地下』的法則指出哪些植物可以生存下來並散佈出去。當我們正需要放諸四海皆準的規則來建立氣候模型並深入了解生物圈時，這項研究提供了一個關於植物演化的通用觀點。」
研究第一作者，Hedin研究團隊的普林斯頓研究生Mingzhen Lu表示，如果一種植物忍受特定環境的能力確實取決於根部的性質，當我們要保育瀕危物種或者推測植物可能會如何適應氣候變遷時，這些發現便相當有價值。
「在實際用途上，這項發現可以使我們簡單扼要地表達一種植物獲取養分的策略。」Lu表示，「瞭解植物獲取養分的基本策略有助於我們找出保護它們的方法，或者讓我們得知在哪些情況下它們會存活下來還是邁向滅亡。」
愛達荷大學自然資源學院的院長Kurt Pregitzer表示，在這個物種流動高度頻繁的世界中，入侵種對生物多樣性造成的威脅越加增長，而研究成果在對抗入侵種時或許相當有用。
「入侵種可以大範圍地取代原生植物並在全球造成重大的經濟損失。」Pregitzer表示，「這項研究或許可以開拓新的科學研究方向，讓我們更加瞭解入侵種植物的根系是如何幫助這些外來物種淘汰掉原生物種。」
Pregitzer表示這篇發表於《自然》(Nature)的研究的獨到之處在於規模及精心設計的研究方法。他說：「這是首度有研究的範圍涵蓋如此多樣的陸域環境。結果顯示植物根部演化出的策略僅能在同一科、屬或種身上看到。這些根部特徵或許能讓植物在自然情況下競爭激烈的生態系中成功立足。」
研究人員花費兩年時間仔細研究一座關於根部特徵的資料庫，其內容之多為世上獨有，包含的369個物種分別來自七種生物相：沙漠、草原、地中海型、寒帶、溫帶、亞熱帶和熱帶。
資料由最後一位共同通訊作者，中國科學院地理科學與資源研究所Dali Guo教授的實驗室在長達十年的時間中逐步匯整而成。第一作者Zeqing Ma是Guo教授實驗室的研究人員；共同作者Xiangliang Xu則和Guo教授任職於同一單位。論文其他共同作者包括英國曼徹斯特大學的生態學教授，研究植物根部的專家Richard Bardgett；賓州州立大學木本植物生理學的教授David Eissenstat；以及明尼蘇達大學的研究人員M. Luke McCormack。
就植物用來搜索養分的根尖來說，研究人員發現熱帶和亞熱帶生物相中植物根尖的最小直徑呈現出最大的分布範圍，從小於0.25釐米到1厘米以上都有。研究人員稱呼這些根部較粗的植物採用的策略為「保守型」。它們和地球最早的陸地植物相似，依賴在暖濕的熱帶亞熱帶土壤中常見的土壤真菌來提供養分。研究人員認為它們採用保守策略的原因是常年處於濕熱環境、富含養分的土壤對植物來說是「一成不變的」。
另一方面，在以土壤貧瘠、冬季酷寒或/和降雨稀少為環境特色的生物相中，植物細部根尖的直徑則落在相對狹隘的範圍中，這在「反覆無常」的環境中來說是較為理想的。舉例來說，研究中生活在沙漠和草原的植物種類根部直徑皆小於0.25釐米。在這些生物相中，植物的根尖往更細的方向演化，如此它們才能將耗費的每一分碳都用來更有效率地探索土壤，同時它們對共生真菌的依賴程度也比較低。
Lu表示研究人員所用的大量資料使他們能以前所未有的詳細程度來探討植物根部的演化過程。「由於資料不足的關係，植物生態系統在地下的樣貌一直未被好好研究。」他表示，「因此，在地下進行的事物受到什麼法則掌管，我們仍然所知甚少。」
Hedin補充：「迄今，所有研究學者在試著去瞭解組織管理植物的構造時都相當自然地著眼在地表以上能看到的特徵。但我們的發現卻違背了地表以上理論――這確實令人感到十分驚訝。」
Pregitzer表示研究顯示根和葉是依循著不同路徑來演化。他說植物生態學家已知葉片的型態和構造對一種植物能否成功來說至關重要，但是「我們並不瞭解呈現出龐大多樣性的植物根系是否也有同樣作用。」
「有趣的是，過去我們不太瞭解植物的根部是如何演化來幫助它們在原本的棲地成功生長。」Pregitzer表示，「現在我們知道葉片和根部會對演化過程中不同的選擇性壓力產生回應，這讓我們可以開始更深入瞭解根部的型態和功能，如何促成我們現今在地球上可以見到如此千變萬化的植物。」
Hedin表示這項發現也和另一個由普林斯頓大學的研究人員探討出的概念相符，其認為植物並非是被動地由環境產生出來的特徵，而是可以主動適應並形塑它們所處的環境。在2015年發表在《自然―植物》(Nature Plants )，Hedin為主要作者的論文中，他們認為生態系之所以呈現出各式各樣的形貌，是因為植物運作的方式不僅可以圖利它們自身，也決定了它們所處棲地的生產力和成份。
Hedin表示：「從演化的時間尺度來看，植物就像是會主動探索出最佳策略來保衛自己以存活下來。」他和Lu將此觀點運用到由中國科學院的共同研究人員所彙整而成的資料庫當中。
「我們從植物學的觀點來看，瞭解到他們獨有的全球資料庫可以應用在演化相關的問題。」Hedin表示，「這項完美的合作將新概念跟歷經多年辛勤的野外工作結合而產生如此漂亮的成果，缺少任何一方都無法達成。」
Theory suggests root efficiency, independence drove global spread of
flora
A new theory of plant evolution
suggests that the 400 million-year drive of flora across the globe may not have
been propelled by the above-ground traits we can see easily, but by underground
adaptations that allowed plants to become more efficient and independent.
As plant species spread
north and south from their nutrient-rich tropical beginnings, the fine tips of
their roots became narrower and more widespread to help them explore
increasingly poor soil for vital nutrients, according to a Feb. 21 study in the journal Nature led by researchers from Princeton
University and the Chinese Academy of Sciences (CAS) in Beijing.
In addition, as plants
spread into unpredictable environments such as arid deserts they grew less
dependent on the symbiotic fungi — or mycorrhiza — that colonize roots and help
host plants obtain the essential plant nutrients nitrogen and phosphorous.
The findings reconsider how
plants adapted to new environments as they evolved, said corresponding
author Lars Hedin, the
George M. Moffett Professor of Biology and chair and professor of ecology and evolutionary biology and
the Princeton Environmental Institute. Scientists have in the past focused on
above-ground characteristics, primarily leaf traits and the efficiency with
which plants absorb sunlight for photosynthesis, he said.
Instead, Hedin said, he and
his colleagues have found for the first time that root diameter and reliance on
fungi — or the lack thereof — are the traits that most consistently
characterize the plant community across entire biomes, which are large distinct
communities of animals and plants such as a desert, temperate forest or
savanna.
“These are the secret
strategies that plants have used over time to take over the world,” Hedin said.
“Our goal was to unlock the understanding of those strategies, and our findings
offer a new global theory for plant evolution. Hidden underground there has
been a tremendous game of survival-of-the-fittest and we are fortunate to have
the first-ever view of the science of that game.
“This work has major
implications for conservation and our stewardship of the plant world,” Hedin
continued. “It provides some of the hidden, below-ground rules by which plants
survive and spread. It’s a global view of plant evolution at a time when global
rules are essential for building climate models and understanding the
biosphere.”
Mingzhen Lu, first
Princeton author and a graduate student in Hedin’s research group, said that if
root traits do in fact determine a plant’s ability to withstand a particular
environment, these findings could be valuable in conserving endangered species
or projecting how plants might adapt to climate change.
“Our findings simplify how we
can practically characterize a plant’s strategy for obtaining nutrients,” Lu
said. “Knowing their underlying nutrient strategy will help us know how to
preserve them, or know the conditions under which they could or could not
survive.”
Kurt Pregitzer, the Thomas
Reveley Professor and dean of the College of Natural Resources at the
University of Idaho, said this work could be especially useful in combating
invasive species, which, in a highly mobile world, increasingly threaten
biodiversity. Pregitzer is familiar with the research but had no role in it.
“Invasive species cause
widespread displacement of native plants and tremendous economic impacts across
the globe,” Pregitzer said. “This study may open entirely new lines of
scientific investigation that help us better understand how invasive-plant root
systems help these exotic species outcompete native plants.”
The Nature paper is unique for its scale and careful application of
scientific methods, Pregitzer said. “This study is the first conducted across a
wide range of terrestrial environments and it demonstrates that plant species
have evolved root strategies that are conserved within corresponding families,
genera and species,” he said. “These root traits likely facilitate plant
success in highly competitive natural ecosystems.”
The researchers spent two
years examining a uniquely large database of root traits consisting of 369
species from seven biomes: desert, grassland, Mediterranean, boreal, temperate,
subtropical and tropical.
These data were compiled
over the course of a decade in the lab of late co-corresponding author Dali
Guo, a professor at CAS’ Institute of Geographic Sciences and Natural Resources
Research. First author Zeqing Ma is a research associate in Guo’s lab and
co-author Xiangliang Xu is a colleague of Guo’s. The paper’s co-authors also
included root experts Richard Bardgett, professor of ecology at the University
of Manchester in the U.K.; David Eissenstat, professor of woody plant
physiology at Pennsylvania State University; and M. Luke McCormack, a research
associate at the University of Minnesota.
The researchers found that
plants in tropical and subtropical biomes exhibited the largest diameter range
for the finest root tips that forage for nutrients, from less than 0.25
millimeters up to 1 millimeter. These thicker-rooted plants employ what the
authors call a “conservative” strategy — similar to that of Earth’s
earliest land plants — that relies on the soil fungi prevalent in wet, warm
tropical and subtropical soils to provide nutrients. The researchers refer to
nutrient-rich soil in consistently sultry environs as “predictable.”
Meanwhile, fine-root
diameters in “unpredictable” biomes characterized by poor soil, cold winters
and/or infrequent precipitation fall within a narrower range ideal for that
environment. For instance, the desert and grassland species studied all had
root diameters of less than 0.25 millimeters. Root tips in these biomes evolved
to be thinner so they could more efficiently explore soil for every unit of
carbon the plant expends, and they have less dependence on symbiotic fungi.
The extensive data the
researchers used allowed them to explore the evolution of plant roots to an
extent never before possible, Lu said. “Below-ground plant ecology has been
understudied, limited by a paucity of data,” he said. “Because of that, the
governing rule of what’s going on below ground has been very poorly known.”
“Thus far,” Hedin added,
“everybody has quite naturally tried to understand how plants are organized by
looking at above-ground traits. But our findings do not follow the above-ground
theories — that was a surprise.”
The study reveals that root
and leaf evolution have followed different paths, Pregitzer said. Plant
ecologists have known that the form and function of leaves are essential to a
plant species’ success, but “we did not understand if this was true across the
tremendous diversity of plant root systems,” he said.
“Interestingly, little was
known about how plant roots have evolved to facilitate success in their native
habitats,” Pregitzer said. “Now we know that leaves and roots have responded to
different evolutionary selective pressures, and we can start building a better
understanding of how root form and function drive plant success within the
tremendous biological diversity we see on Earth.”
The findings align with
ideas explored at Princeton that suggest that plants — rather than being
passive features of their environment — have actively adapted to and shaped
their environments, Hedin said. He was senior author of a 2015 paper in Nature
Plants that
suggested that ecosystems take their various forms because plants behave in
ways that not only benefit themselves but also determine the productivity and
composition of their habitats.
“Over evolutionary time,
it’s as if plants have actively explored the best strategies to safeguard their
own survival,” Hedin said. He and Lu brought this perspective to the database
put together by their colleagues at CAS.
“We understood from a plant
perspective how to bring evolutionary questions to their unique global dataset,”
Hedin said. “It was this great collaboration where we combined new ideas with
years of painstaking fieldwork to produce this great result. It couldn’t have
happened without both sides.”
原始論文：Zeqing Ma, Dali Guo, Xingliang Xu, Mingzhen Lu, Richard D. Bardgett,
David M. Eissenstat, M. Luke McCormack, Lars O. Hedin. Evolutionary
history resolves global organization of root functional traits. Nature,
2018; DOI: 10.1038/nature25783
引用自：Princeton University. "Theory suggests
root efficiency, independence drove global spread of flora."
