Beyond photosynthesis: overturning source-centric plant growth paradigms

木材是一种具有独特性能的非凡材料，占所有生物质的60%左右。它负责封存每年约18%的化石燃料碳排放，降低大气中二氧化碳的增长速度，从而减缓气候变化。虽然对控制木材形成的因素进行了许多研究，这些研究已经量化了模式和关系，但我们对潜在生理过程的了解非常少。这极大地限制了我们对全球碳循环、气候和大气二氧化碳对木材解剖的控制以及利用树木年轮解释过去环境的理解。木材的形成是通过树皮下的新细胞的产生，然后经历扩大和壁增厚，在死亡之前成为具有功能的木质部，木质部是负责水运输和结构支持的主要组织。该项目旨在揭示木材形成的控制因素，因此最终的木材解剖和碳含量在很大程度上是未知的。我们将通过测量实验操作对克隆树模型杂交杨树的广泛解剖、生化和基因组特征的影响来做到这一点，并利用这些知识，通过开发木材形成和全树生长的机制模型，在我们对陆地碳循环的理解方面取得重大进展。动态全球植被模型（dgvm）是研究全球陆地碳动态的主要工具，但并未明确考虑生长。相反，dgvm是围绕植物生长等同于光合作用和呼吸平衡的范式构建的，然而有相当多的证据表明，生长是独立于整体碳平衡控制的。如果这是真的，这将在很大程度上使这些模型的基本前提失效。当前方法的局限性在于，虽然dgvm平均再现了历史陆地碳汇的大小，但它们具有非常不同的潜在气候和二氧化碳敏感性，因此不能被认为为这种不平衡背后的过程提供机制解释。以增长为主导的DGVM有可能解决这一问题，从而大大提高我们预测未来全球碳循环行为的能力。到目前为止，由于缺乏基本的生理知识，这是不可能的。我们对杂交杨树的实验操作将产生将显性生长纳入dgvm所需的知识，推翻其最基本的范式，从而在理解和可预测性方面取得重大突破。我们高度创新的合作将一个研究小组聚集在木材形成的分子生理学的前沿，一个在植物生长模型和dgvm开发方面经验丰富的研究小组。这种独特的合作有可能在我们对直接针对陆地碳循环模型需求的木材形成的理解方面取得重大进展。要考虑的非生物因素包括温度、大气CO2、日长和土壤湿度。我们将研究碳水化合物供应如何与温度相互作用，控制细胞数量、大小和壁厚。在另一系列实验中，我们将利用日长和温度的变化，在加速的年生长周期下产生年轮，并分析这些因素如何通过变化分化阶段的速率和持续时间来实现对年轮解剖的影响。土壤湿度控制将通过对细胞扩大、增殖和碳水化合物供应的影响来量化干旱对木材解剖的影响，杂交杨树将在整个项目期间在田间条件下生长，为我们对树木生长的新认识提供有力的测试。

Wood is a remarkable material with unique properties, and accounts for c.60% of all living biomass. It is responsible for sequestering c.18% of fossil fuel carbon emissions annually, reducing the growth rate of atmospheric CO2 and hence mitigating climate change. While there have been many investigations into the factors controlling wood formation, which have quantified patterns and relationships, our knowledge of the underlying physiological processes is very poor. This greatly limits our understanding of the global carbon cycle, climatic and atmospheric CO2 controls on wood anatomy, and the interpretation of past environments using tree rings.Wood formation occurs through the production of new cells just under the bark, which then undergo enlargement and wall thickening, before dying and becoming functioning xylem, the main tissue responsible for water transport and structural support. This project aims to uncover the controls, currently largely unknown, on wood formation, and hence final wood anatomy and carbon content. We will do this by measuring the effects of experimental manipulations on a wide range of anatomical, biochemical, and genomic characteristics in a clonal tree model, hybrid poplar, and use this knowledge to produce a major advance in our understanding of terrestrial carbon cycling through the development of mechanistic models of wood formation and whole-tree growth.Dynamic Global Vegetation Models (DGVMs) are our main tools for studying global terrestrial carbon dynamics, but do not explicitly consider growth. DGVMs are instead constructed around the paradigm that plant growth is equivalent to the balance of photosynthesis and respiration, whereas there is considerable evidence that growth is controlled independently of the overall carbon balance. If true, this would largely invalidate the basic premise of these models. The limitations of the current approach are evident in the finding that while DGVMs, on average, reproduce the magnitude of the historical terrestrial carbon sink, they have very different underlying climate and CO2 sensitivities and therefore cannot be considered to provide a mechanistic explanation of the processes behind this imbalance. A growth-led DGVM has the potential to resolve this problem and thereby greatly improve our ability to predict the behaviour of the future global carbon cycle. This has so far not been possible due to the lack of fundamental physiological knowledge. Our experimental manipulations on hybrid poplar will produce the knowledge necessary for the incorporation of explicit growth into DGVMs, overturning their most fundamental paradigm, and so enable a major breakthrough in understanding and predictability.Our highly innovative collaboration brings together a research group working at the cutting edge of the molecular physiology of wood formation, with one highly experienced in the development of plant growth models and DGVMs. This unique collaboration has the potential to produce a major advance in our understanding of wood formation directly targeted at the needs of terrestrial carbon cycling models.The abiotic factors to be considered are temperature, atmospheric CO2, daylength, and soil moisture. We will investigate how carbohydrate supply interacts with temperature in controlling cell numbers, sizes, and wall thicknesses. In another series of experiments, we will produce growth rings under an accelerated annual growth cycle using changing daylength and temperature, and analyse how the influences of these factors on ring anatomy are achieved through changes in the rates and durations of differentiation phases. Soil moisture manipulations will be used to quantify the influence of drought on wood anatomy through effects on cell enlargement, proliferation, and carbohydrate supply, and hybrid poplar will be grown under field conditions for the entire length of the project to provide a strong test of our new understanding of tree growth.