Reaching for the sky: exploring massive convergent evolution towards woodiness in Brassicaceae

木本植物（树木和灌木）覆盖了地球陆地面积的30%，提供了至关重要的生态系统服务，例如通过储存大气中的碳来缓冲气候变化。然而，目前还不清楚为什么有些植物是木本的，有些是草本的。这个看似简单的问题引起了查尔斯·达尔文的兴趣，当他在19世纪30年代访问海洋岛屿时，他使用了“岛屿木质性”这个术语（后来改为更广泛的术语“衍生木质性”）来描述木质开花植物物种，这些植物是从草本亲缘植物进化而来的，而草本亲缘植物又起源于祖先木质物种。直到今天，科学家们还没有给出令人满意的答案，为什么衍生木质在开花植物的进化史上发生了数百次，可能是因为木材的形成是一个复杂的过程，可以通过不同的方式触发，而且在很大程度上还没有得到充分的研究。解决这个长期存在的问题需要一种创新的多学科方法，将进化和生态模型联系起来。世界性的芥菜科（芸苔科，4000种）可以作为一个很好的模型群来理解木材的形成，因为许多作物和模式物种的出现，以及向衍生木本生长形式的显著进化转变。10%的芸苔科植物是木本植物，最高可达9米。在岛屿和大陆的不同环境中，木本植物可能已经进化了100多次，这对一个单一的科来说是非同寻常的。然而，深入的系统发育关系在芸苔科仍然知之甚少，从而阻碍了进化问题在家庭水平。本研究主要有三个目标：(1)通过对1000个精心挑选的物种进行杂交测序，基于（时间校准的）十字花科系统发育，准确评估十字花科向木质化过渡的数量；(2)测试木质化作为包括木质十字花科在内的100多个分支物种多样化的潜在驱动因素的影响；(3)确定这些分支中木质化与（古）气候和土壤变量之间的相关性。该项目的成果将彻底改变我们对木材形成的理解，弥合进化生物学家和生态生理学家之间的差距，并提供一个强大的系统发育框架，将在未来几年为整个十字花科群落服务。此外，提出的木材进化工作可以为发育遗传学家开辟新的机会，他们将能够在许多与拟南芥密切相关的木草本姐妹对中寻找开启木材途径的关键调控基因。

Woody plants (trees and shrubs) cover 30 percent of the planet's land area and offer crucial ecosystem services, such as buffering climate change by storing carbon from the atmosphere. It is unclear, however, why some plants are woody and others herbaceous. This seemingly simple question intrigued Charles Darwin when he visited oceanic islands in the 1830s, and he used the term ‘insular woodiness’ (later rephrased to the broader term ‘derived woodiness’) for woody flowering plant species that have evolved from herbaceous relatives that in turn originated from ancestrally woody species. To this day, scientists have failed to provide satisfying answers why derived woodiness have occurred hundreds of times during evolutionary history of flowering plants, probably because wood formation is a complex process that can be triggered in different and largely understudied ways. Solving this long-standing question requires an innovative multidisciplinary approach, linking evolutionary and ecological models. The cosmopolitan mustard family (Brassicaceae, 4000 species) serves as an excellent model group to understand wood formation due to the occurrence of many crops and model species, and the remarkable number of evolutionary transitions towards the derived woody growth form. Ten percent of the Brassicaceae species are woody – reaching up to 9m tall – and woodiness has evolved probably more than 100 separate times in contrasting environments on both islands and continents, which is extraordinary for a single family. However, deep phylogenetic relationships within Brassicaceae remain poorly understood, thereby impeding evolutionary questions at the family level. This proposal focuses on three main objectives: (1) accurately assess the number of transitions towards woodiness in Brassicaceae based on a (time-calibrated) family phylogeny using hybridization sequencing of 1000 carefully selected species, (2) test the impact of woodiness as potential driver for species diversification across the 100+ clades including woody Brassicaceae, and (3) determine the correlation between woodiness versus (palaeo)climate and soil variables in these clades. Outcomes of this project will revolutionize our understanding of wood formation, bridge the gap between evolutionary biologists and ecophysiologists, and provide a robust phylogenetic framework that will serve the entire Brassicaceae community for the coming years. Moreover, the proposed work on wood evolution can open up new opportunities for developmental genetists who will be able to search for the key regulatory genes that turn on the wood pathway in many woody-herbaceous sister pairs that are all closely related to Arabidopsis.