Collaborative Research: Photosynthetic Acclimation, Photoprotection, and Phloem Loading

作为太阳能的有效收集器，当光线特别强烈时，或者当恶劣的环境条件阻止能量被有效利用时，植物就会受到光线的损害。常绿植物使用两种截然不同的策略来应对冬季的强光。一些物种通过完全关闭光合作用和重新排列它们的光收集装置来无害地消散吸收的光来保护自己。其他物种能够适应并保持生产力，在短暂的温暖期利用太阳能进行光合作用和生长。采用这两种策略的植物有时会在田间并排生长。有推测认为，这些对环境变化的不同反应涉及光合作用产物（糖）从产生它们的叶细胞被装载到长距离运输组织（韧皮部）的机制的差异。根据这一观点，在冬季维持光合能力的物种可能是那些使用运输蛋白进行韧皮部负荷的物种，而下调光合作用的物种可能通过连接这些细胞类型的孔（胞间连丝）负荷。通过后一条路径加载可能对冷破坏更敏感。每种植物类型的一个代表，菠菜（有运输蛋白）和毛蕊花（有气孔），将被用来研究在田间自然条件和生长室内受控条件下对低温的反应。这些物种有相似的生长形式（莲座），在科罗拉多州越冬时保留叶子。它们，以及每种类型的其他物种（有运输蛋白的豌豆和有气孔的南瓜）也将被用来研究植物从弱光到强光的反应，这也会使叶子受到潜在的破坏性，多余的能量。糖转运蛋白的抑制剂将用于确定通过气孔输出糖的物种是否通过改变其糖输出策略和添加转运蛋白成分来适应寒冷或强光。通过描述一系列与碳输出能力相关的额外结构和超微结构特征，包括输出韧皮部元素的数量和横截面积、孔连接的密度以及与运输蛋白相关的膜特征，将扩大对这两种糖输出策略的关注。在寒冷或强光条件下，部分或全部这些结构特征可能会被增强，以增加碳输出能力，并且假设下调的物种在调节这些结构特征的能力方面表现出更少的灵活性。通过这些研究，对糖输出特性作为光合驯化的潜在关键决定因素的更好理解将开始出现。至于这项工作的更广泛的影响，该项目将(1)让研究生参与科学发现的过程，(2)促进植物生物学学科之间和两个机构研究者之间的合作，以及(3)使人们更好地了解作物和本地植物对环境胁迫的适应性/抗性的潜在机制。这将进一步有助于更好地预测响应全球变化的植物生产力的变化，并为通过基因工程提高作物对增加的压力的抵抗力提供必要的基础知识。

As efficient collectors of solar energy, plants are subject to damage by light when it is particularly intense, or when harsh environmental conditions prevent the energy from being used productively. Evergreen plants use one of two contrasting strategies to cope with strong light during the winter. Some species protect themselves by completely shutting down photosynthesis and rearranging their light-collecting apparatus to harmlessly dissipate absorbed light. Other species are able to acclimate and remain productive, using solar energy for photosynthesis and growth during brief warm spells. Plants using these two strategies sometimes grow side-by-side in the field. There is speculation that these contrasting responses to environmental change involve differences in the mechanisms by which the products of photosynthesis (sugars) are loaded into the long-distance transport tissue (the phloem) from the leaf cells where they are made. According to this view, species that maintain photosynthetic capacity in winter may be those that use transport proteins for phloem loading, whereas species that downregulate photosynthesis may load via the pores (plasmodesmata) that connect these cell types. Loading via the latter path may be more sensitive to disruption by cold. One representative of each plant type, spinach (with transport proteins) and Mullein (with pores), will be used to study the response to cold temperature under natural conditions in the field and controlled conditions in growth chambers. These species have similar growth forms (rosettes) and retain leaves while overwintering in Colorado. They, as well as additional species of each type (pea, with transport proteins, and pumpkin, with pores) will also be used to study the response of plants when they are transferred from low to high light, which also subjects leaves to potentially damaging, excess energy. An inhibitor of the sugar transport proteins will be used to determine whether species known to export sugars via pores acclimate to cold or high light by altering their sugar export strategy and adding a transport protein component. The focus on these two sugar export strategies will be broadened by characterizing a range of additional structural and ultrastructural features related to carbon export capacity, including the number and cross-sectional area of exporting phloem elements, the density of pore connections, and membrane features associated with the transport proteins. Some or all of these structural features may be augmented to increase carbon export capacity under cold or high light conditions, and it is hypothesized that the downregulating species will exhibit less flexibility in their ability to modulate these structural features. Through these studies a better understanding of sugar export characteristics as potential pivotal determinants of photosynthetic acclimation will begin to emerge. As to the broader impact of this work, this project will (1) involve graduate students in the process of scientific discovery, (2) facilitate a collaboration between disciplines within plant biology and between investigators at two institutions, and (3) allow a better understanding of the underlying mechanisms of the adaptability/resistance of crops and native plants to environmental stress. This will furthermore contribute to a better prediction of changes in plant productivity in response to global change, as well as providing the underlying knowledge necessary for potential increases in the resistance of crops to increased stress through genetic engineering.