[植物演化] 新化石暗示了被子植物根的多樣性。 20191031 Current Biology

Evolution: Diversification of Angiosperm Rooting Systems in the Early Cretaceous

Alexander J. Hetherington and Liam Dolan

A monocot from the Early Cretaceous developed a cluster of anatomically similar roots from the base of a stocky stem. This discovery indicates that angiosperm rooting systems were more diverse than previously thought at this time.

一株從白堊紀早期的被子植物從它結實的莖底發展出一叢解頗構造上相似的根。這一個發現顯示了在這個時期，被子植物的根系遠比之前所想的要多元許多

Most fossils of angiosperm roots from the Early Cretaceous that have been described to date are described as adventitious and developed from horizontal shoot axes called rhizomes (Figure 1) [1–8]. Rhizomes of many extant herbaceous plants grow horizontally through the surface of soil and develop roots that supply water and nutrients as well as tethering the axis to its growth substrate [9]. Leaves, vertically growing (orthotropic) shoots and often roots develop along rhizomes forming ramifying branched systems. The prevalence of this bauplan — root-bearing rhizomes — in the Early Cretaceous fossil record supports the hypthosis that early angiosperms developed a rhizotamoushabit [10]. The recent paper by Coiffard et al. [11] describes a new Early Cretaceous monocot species, Cratolirion bognerianum, revealing that there was at least another type of rooting system present in the Early Cretaceous. The description of the fossil C. bognerianum

demonstrates that rooting systems were more diverse than previously thought in

the late Aptian to early Albian just over 100 million years ago [11].

到目前為止，多數白堊紀早期被子植物化石的根都是不定根，並且是從水平生長的地下莖發育而成。（圖一左側）許多生長在土表的草本植物，地下莖會與土壤表面水平生長，並且生成根來攝取水分也固定自己在生長的表面上。這些植物的葉子則垂直於土壤表面生長，而根的部分則沿著地下莖生長並且形成分支系統。這種類似機翼的雙向生長方式，在早期白堊紀的化石中常被發現，這一個現象支持了早期被子植物是以地下莖方式生長的假說。最近由Coiffard與同事們所發表的論文中，描述了一個新發現的單子葉植物Cratolirion bognerianum化石，顯示了至少在白堊紀早期，另一種根系的存在。這個化石的發現也說明了在大約一千萬年前的白堊紀早期，植物根的系統遠比以前所想的多元許多。

圖一

C. bognerianum (Figures 1, 2A,B) develops a rosette of leaves from a stocky stem, with roots developing from the lower portion of the stem. The roots are classified as adventitious because they develop from the base of the stem (Figure 2B). These adventitious roots form a cluster, and this type of root system where roots initiate from a local point in bundles are classified as fascicular (fasciculus is the Latin word for bundle) (Figure 2B) [11]. Only one type

of root developed from these fascicles in C. bognerianum; roots are the same diameter where they attach to the base of the stem and with similar branching architectures (Figure 2B). Fasiculate rooting systems of this structure were assigned as Type VII of Cannon’s rooting system scheme [12] and characterise the roots of many extant monocots today, such as the rooting system of Oryza sativa (rice) (Figure 2C,D) [13]. C. bognerianum represents to our knowledge the earliest example of this type of rooting system preserved in angiosperms.

C. bognerianum （圖二Ａ、Ｂ）從健壯的莖部發展出環狀發育的葉子，而根的部分則從莖的下部發育出來。這些跟由於是從根的基部發育出來，所以被定義為不定根（圖二Ｂ）。這些不定根形成一束，而這類型從一個局部的點發育出來而形成一束的根系，被分類為"fascicular 小束“ （fasciculus是拉丁文的束的意思）（圖二Ｂ）。C. bognerianum只會從這些小束中形成一種根。這些跟有相似的直徑，也都從根的幾部生長出來，並且有相似的分支型態（圖二Ｂ）。這種形態的小束根細被定義為Cannon的第七型態（TypeVII）根系，有非常多今日的單子葉植物也被歸類在這型態的根系中，例如水稻的根系（圖二Ｃ、Ｄ）。C. bognerianum 是至今我們所發現這類型態根系植物中最早的代表植物。

圖二

The discovery that C. bognerianum

developed a Type VII root system is

important because this system has not been described in other Early

Cretaceous angiosperms that developed a rhizotamous habit with adventitious roots (Figure 1). Furthermore, since C. bognerianum is one of the oldest crown group monocots, it suggests that this type of rooting system may have been an evolutionary innovation of the monocots and therefore a synapomorphy of the group [11]. Although an interesting hypothesis, further work on this character is necessary as the development of fasiculate roots has not been surveyed extensively in extant angiosperms. Regardless of whether a fasciculate root system is a synapomorphy of crown group monocots, the discovery of C. bognerianum suggests that the earliest time this type of rooting system is found is within the monocots and adds a new type of rooting system to our understanding of early angiosperms.

發現C. bognerianum擁有第七型態根系這件事情非常的重要，因為在白堊紀早期，從來沒有除了具有地下莖以外，擁有不定根的植物被報導過。此外，C. bognerianum是最早的單子葉植物群中的一種，這說明了這類型的跟可能是單子葉植物在演化上的創新，因此是單子葉植物所共同擁有的演化特徵。雖然這是一個有趣的假說，但是猶豫小束根細在多數被子植物中的發育仍未被全面觀察過，所以針對這個這個特徵的近一步研究仍是非常必須的。不論這個小束根細是否是冠狀單子葉植物的共同演化特徵，C. bognerianum的發現仍然表示這類型的根系，在早期的被子植物中就已經存在了。

Within the bigger picture of angiosperm root evolution, it also tells us, albeit from a

small sample size, that the roots developed in these early angiosperms only account for a small subset of the types of roots that characterise living angiosperms. The roots of living angiosperms were divided into ten major categories by Cannon [12]. Adventitious roots developing from horizontal rhizomes only account for two of these root types (Type IX and X of Cannon [12]). This means that it is currently unknown when angiosperms evolved the other 8 rooting system types. The discovery that fasiculate roots (Type VII [12]) existed in early crown group monocots indicates that a rooting system of this type had evolved by the Early Cetaceous [11] and it begs the question of how many more extant rooting systems had evolved by this time.

更深的來看被子植物根的演化，這個發現也告訴了我們，雖然只是從很少數的例子，但是在這些早期被子植物跟的發育性態，只佔了現今被描繪的被子植物根系特徵中的一小部分。在Cannon的系統中，根被劃分成十種不同型態。從水平地下莖生長出不定根的型態，只佔了十種之中的兩種（第九與第十類）。這也表示了我們仍然無法得知被子植物在何時演化出剩下八種型態的根。在早期冠狀單子葉植物中，小束根系的發現（第七類）代表著至少白堊紀早期，這種根系就已經演化出來了。這也令我們很想再次詢問，在這個時間點，究竟多少種根系已經演化出來了呢？

Rooting systems have been largely overlooked in the debate on angiosperm origins — Darwins abominable mystery. Understanding the evolution of angiosperm rooting systems has a key part to play in reconstructing the growth habits, symbioses, ecology and physiology of early angiosperms. These are all essential characteristics for reconstructing the evolutionary steps that led to the remarkable success of extant angiosperms. Furthermore, describing root evolution in early angiosperms is an essential starting place for understanding the rich diversity of angiosperm rooting systems alive today.

在討論被子植物的起源（達爾文所留下來令人不悅的謎團之一）時，根系的發展一直都被過度的忽視。暸解被子植物根系的起源，可以讓我們更容易重新建構早期被子植物可能的棲地型態、共生物種、環境以及生理。這些都是要傳新建構，被子植物為何能如此成功的演化路徑時，所不可或缺的資訊。此外，要了解現今被子植物繁盛的根系時，暸解早期被子植物根的演化是一個必要的起始點。

The discovery of C. bognerianum [11], a crown group monocot from the Early Cretaceous, represents a step forward in our understanding of rooting system evolution. C. bognerianum is the earliest species to be preserved with a fasciculate rooting system that differs in its structure to the rhizotamous growth habit of many other Early Cretaceous angiosperms. It therefore represents a key fossil discovery for investigating rooting system evolution, and rooting character homologies at the base of the angiosperms.

C. bognerianum的發現，代表著我們對於植物根演化又向前了一步。 C. bognerianum與多數白堊紀早期，以地下莖生長的被子植物不同。是具有小束根系的植物中保存最好的化石。也因此這化石是研究植物根部演化中，在被子植物的起源時，跟已經有相似的特徵的重要發現。

原文出處： https://www.sciencedirect.com/science/article/abs/pii/S0960982219310796