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Andrew Leakey's Laboratory

Phloem Loading as a Driver of Plant Photosynthetic Responses to Carbon Supply

PI: Elizabeth Ainsworth (USDA/ARS)

Co-PIs: Daniel Bush (Colorado State University), Andrew Leakey (University of Illinois)

Funding Source: USDA-NIFA

Species have different strategies for loading sugars into the phloem, which vary in the route that sugars take to enter the phloem and the energetics of accumulation.  Meta-analysis of previous FACE experiments indicates that species with apoplastic versus symplastic phloem loaders differ significantly in their photosynthetic responses to elevated [CO2].

This project will test the broad hypothesis that species with different phloem loading strategies have fundamentally different responses to elevated [CO2] via differential abilities to enhance export capacity and by differential effects of elevated sugar levels on photosynthesis. Species with symplastic loading are believed to have less flexibility in response to changes in environmental conditions because sucrose export from mesophyll cells is dependent on fixed anatomical plasmodesmatal connections. Furthermore, plants with different phloem loading strategies have distinct foliar sugar signatures and this could interact with the sugar-mediated acclimation of photosynthetic capacity that is a key element of plant responses to elevated [CO2]. Field and controlled environment CO2 fumigation experiments integrating molecular, biochemical and physiological analyses will be done on:

(1) three pairs of agricultural species with established differences in phloem loading strategy
(2) transgenic sugar beet and soybean over-expressing sucrose transporters to enhance photoassimilate export capacity.

Generating mechanistic understanding of the functional significance and operation of distinct phloem loading strategies will be key to adapting crops to future, elevated [CO2]. Improved fundamental knowledge about the mechanisms controlling photoassimilate export and sink-source balance is also needed to insure the success of ongoing efforts to engineer improved photosynthetic efficiency in crops.