Collaborative Research: Photosynthetic Acclimation, Photoprotection, and Phloem Loading

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

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.