Adaptive Evolution in the Photoreceptor Phytochrome A and its Role in the Ecological Success of the First Angiosperms

光信号的正确处理对所有植物的成功至关重要。因此，光感受器在植物发育中起着至关重要的作用，作为信息分子，植物可以确定它们在空间和时间上的位置，并根据不断变化的光环境来同步生长和发育。植物形态受环境光条件的控制，这一过程被称为光形态建成，是由吸收红光和远红光的光敏色素以及吸收不同的蓝色和紫外线的光受体介导的。陆地植物的光敏色素进化是以一系列基因复制为标志的，这些基因复制导致了独立进化和功能不同的线条。开花植物中光敏色素的数量最多，也是最成功的一类陆地植物。它们总共约有25万个物种，数量超过了所有其他植物群的总和，主导了地球上除两个以外的所有陆地植被带。它们的起源之后是自三叠纪以来占主导地位的植物群的衰落，但它们的起源模式和支撑它们壮观成功的因素仍然未知。该项目将DNA序列分析与早期开花植物幼苗的生理特性相结合，以验证光敏机制的进化对开花植物早期成功的重要性这一假说。具体地说，它测试了光敏色素A的进化增强了早期被子植物在浓荫下生存的能力的假说。光敏色素A是开花植物特有的光敏色素。序列分析将确定光敏色素A的功能差异是通过正选择还是通过放松选择限制而发生的，并将确定其与编码光敏色素C的姊妹基因不同的功能差异部位。生理实验将确定早期分叉的开花植物的幼苗是否表现出与更多衍生物种(如拟南芥)所表现的对密集遮荫的反应，并将确定是否在相关种子植物的幼苗中发现了这些反应。总而言之，这些分析将检验这样一个假设，即光敏色素A功能是一种适应性创新，帮助开花植物在曾经由蕨类和裸子植物主导的森林树冠下站稳脚跟。比较分子分析是评估非模式物种功能多样性的有效机制。因此，这个项目中进行的分析将进一步推动植物基因组学的一个目标，超越模型系统，了解多样性的分子基础。将产生的序列数据库将有助于确定其他种子植物是否可能独立地获得了对光环境的多个方面的复杂但并行的生理反应。这个数据库还将有助于识别基因中功能指定的位置，这些基因的功能很难用传统的遗传学方法来表征。这种方法确定的位点的功能可以在诱变实验中进一步测试。大田和温室的生理实验将确定个体树种幼苗更新的最适光照条件。因此，它们将为植物群落的组成如何变化提供洞察，以应对生态系统范围的干扰或变化。

The proper processing of light signals is crucial for the success of all plants. Thus, photoreceptors play a critical role in plant development, serving as informational molecules that allow plants to determine where they are in space and time and to synchronize growth and development to a changing light regime. The control of plant form by ambient light conditions, a process known as photomorphogenesis, is mediated by phytochromes, which absorb red light and far-red light, along with distinct blue and ultra-violet absorbing photoreceptors. Phytochrome evolution in land plants is marked by a series of gene duplications that have led to independently evolving and functionally distinct lines. Flowering plants have the highest number of phytochromes and also are the most successful group of land plants. They total approximately 250,000 species, outnumbering all other plant groups combined, and dominate all but two of the earth's terrestrial vegetation zones. Their origin was followed by the decline of plant groups that had dominated since the Triassic, but their mode of origin and the factors that underlie their spectacular success remain unknown. This project integrates DNA sequence analyses with physiological characterization of seedlings from early-diverging flowering plants to test the hypothesis that evolution in light-sensing mechanisms was important for the early success of flowering plants. Specifically, it tests the hypothesis that the evolution of phytochrome A, a phytochrome unique to flowering plants, enhanced the ability of early angiosperms to survive in dense shade. Sequence analyses will determine whether functional divergence of phytochrome A occurred by positive selection or by relaxation of selective constraints, and will identify sites of functional divergence that distinguish it from its sister gene, which encodes phytochrome C. Physiological experiments will determine whether seedlings of early-diverging flowering plant species exhibit the responses to dense shade that have been characterized in more derived species such as Arabidopsis, and will determine whether any of these responses are found in seedlings of related seed plants. Together these analyses will test the hypothesis that phytochrome A function was an adaptive innovation that helped flowering plants gain a foothold under the forest canopy once dominated by ferns and gymnosperms. Comparative molecular analyses are an efficient mechanism for evaluating functional diversification in nonmodel species. Thus, the analyses undertaken in this project will further one goal of plant genomics, to move beyond model systems toward understanding the molecular basis of diversity. The sequence database that will be generated will be useful for determining whether other seed plants might have independently attained complex but parallel physiological responses to multiple facets of the light environment. This database also will be useful for identifying sites of functional specification in genes whose function has been difficult to characterize using traditional genetics approaches. The function of sites identified this way can be further tested in mutagenesis experiments. The physiological experiments in the field and greenhouse will characterize light optima for seedling regeneration of individual species. Thus, they will provide insight into how the composition of plant communities might change in response to ecosystem-wide disturbance or change.