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，使植物能够确定它们在空间和时间中的位置，并根据不断变化的光状态同步生长和发育。环境光条件对植物形态的控制，这一过程被称为光形态发生，是由光敏色素介导的，光敏色素吸收红光和远红光，以及不同的蓝光和紫外线吸收光感受器。陆生植物的光敏色素进化以一系列基因复制为标志，这些基因复制导致了独立进化和功能不同的品系。开花植物具有最多的光敏色素，也是最成功的陆地植物群。它们总共大约有25万种，数量超过了所有其他植物群的总和，除了两个陆地植被带外，它们支配着地球上所有的陆地植被带。它们的起源伴随着自三叠纪以来一直占统治地位的植物群的衰落，但它们的起源模式和它们惊人成功背后的因素仍然未知。本项目将DNA序列分析与早期分化开花植物幼苗的生理特性相结合，以验证光感机制的进化对开花植物早期成功的重要假设。具体来说，它验证了光敏色素A（一种开花植物特有的光敏色素）的进化增强了早期被子植物在茂密树荫下生存的能力的假设。序列分析将确定光敏色素A的功能分化是通过积极选择还是通过放松选择约束发生的，并将确定其与编码光敏色素c的姊妹基因区分开来的功能分化位点。生理实验将确定早期分化开花植物物种的幼苗是否表现出对稠密阴影的反应，这种反应已经在更多衍生物种（如拟南芥）中表现出来。并将确定是否在相关种子植物的幼苗中发现任何这些反应。总之，这些分析将验证光敏色素A的功能是一种适应性创新的假设，帮助开花植物在曾经由蕨类植物和裸子植物主导的森林冠层下获得立足点。比较分子分析是评价非模式物种功能多样化的有效机制。因此，在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，走向理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对光环境的多个方面独立地获得复杂但平行的生理反应。该数据库也将有助于识别基因的功能规范位点，这些基因的功能很难用传统的遗传学方法来表征。这种方法鉴定的位点的功能可以在诱变实验中进一步测试。田间和温室的生理试验将确定单种幼苗再生的最佳光照条件。因此，他们将深入了解植物群落的组成如何响应生态系统的干扰或变化而发生变化。

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.