PARENT SESSION

Symposium S1C Photosynthate transport and transporters
Monday August 30th, 2004 10:20 AM-12:20 PM Room 510A
Chair: Andreas Weber
Co-Chair: Marinus Pilon

Role of phloem loading type in the plasticity of photosynthetic acclimation to light environment. William Adams^*,1, Véronique Amiard¹, Kristine Bachmann¹, Robert Turgeon², Barbara Demmig-Adams¹, ¹ Department of Ecology & Evolutionary Biology, Boulder, Colorado, USA² Department of Plant Biology, Ithaca, New York, USA

ABSTRACT- Three species representing different phloem loading types, pea (Pisum sativum), Arabidopsis thaliana, and pumpkin (Cucurbita pepo), were grown under low light (LL), high light (HL), or transferred from LL to HL. Pea exports sugars apoplastically via H+/sucrose symporters, Arabidopsis employs a mixed mode of export (apoplastic/symplastic), and pumpkin exports sugars exclusively through plasmodesmata and the trapping of raffinose sugars in the phloem. Upon transfer from LL to HL, fully expanded leaves of pea and Arabidopsis upregulated photosynthetic capacity to the level observed in plants grown in HL, whereas fully expanded leaves of pumpkin upregulated this capacity to only half that of HL-grown leaves. The frequency of plasmodesmata in pumpkin intermediary cells was the same irrespective of growth light. Pumpkin vein density per leaf area was higher in HL-acclimated leaves compared to those acclimated to LL, and thus HL-acclimated leaves possessed a greater number of plasmodesmata per leaf area compared to LL-acclimated leaves. Upon transfer, mature LL-grown leaves did not increase the number of veins per leaf area, and the limited acclimation of photosynthesis upon transfer is thus consistent with this inability to increase vein/plasmodesmatal density in a fully mature leaf and a limited carbon export capacity. For pea, vein density did not differ between leaves acclimated to LL versus HL. However, the extent of cell wall invaginations in the specialized transfer cells of this apoplastic loader were considerably greater in leaves acclimated to HL compared to LL. Furthermore, these invaginations increased in mature leaves transferred from LL to HL to the same level observed in HL-grown leaves. Since sucrose export proteins are located on the cell membrane of these transfer cells, this increased total surface area provides a plausible explanation for the ability of pea to upregulate carbon export capacity upon transfer from LL to HL, allowing full photosynthetic acclimation to HL. As with pea, transfer of Arabidopsis from LL to HL resulted in increased cell wall invaginations of transfer cells adjacent to sieve elements. Jasmonic acid may be involved in signaling this response, since methyl jasmonate-treated plants in LL developed levels of transfer cell wall invagination comparable to HL-acclimated leaves.

KEY WORDS: Phloem loading, Jasmonic acid, Sun-shade acclimation, Transfer cells