Phloem loading strategies in deciduous trees and conifers: analyses of Fagus sylvatica and Picea abies

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Trees transport carbohydrates from leaves to roots and other sink tissues over long distances. The regulation of production of sugars, its storage and transport in the phloem is one option for plants to respond to seasonal or other environmental changes. This is particularly important for long-lived organisms, such as trees, and in light of the ongoing global warming. A crucial step in carbon allocation in plants is the loading of sucrose, the main carbon transport form, into the phloem. Generally, sucrose can move from one cell to another through plasmodesmata into the conducting sieve elements without leaving the symplast. For this, the sucrose concentration in the mesophyll cells has to be higher than that in the phloem. In areas where the conducting cells are not sufficiently connected to the sucrose delivering cells, sucrose needs to be exported into the apoplast and imported into the conducting cells. It is known from herbaceous plants that two types of proteins are required for this transport: Members of the SUGARS WILL EVENTUALLY BE EXPORTED TRANSPORTER (SWEET)-family catalyze the export of sucrose into the apoplast, and members of the sucrose uptake transporters (SUT/SUC)-family accumulate sucrose in cells of the phloem. This active loading process leads to higher sucrose concentrations in the phloem than in the mesophyll of leaves. Strategies of phloem loading with sucrose have been intensively investigated in herbaceous plants, but not in trees. Therefore, our aim is to analyze the phloem loading strategies in Fagus sylvatica and Picea abies to better understand this process in deciduous trees as well as in conifers. Our project focuses the following aspects: I) determination of the sucrose concentrations in mesophyll cells and in conductive cells; II) identification and characterization of SWEET- and SUT-type sucrose transporters, the analyses of their expression via in-situ hybridization, as well as their cell-type specific localization by immuno-histochemistry; III) analyses of the differences in cell-cell connectivity and sucrose transporter expression during the development of leaves, and IV) the influence of the seasons over the course of a year and the influence of drought stress on the phloem loading strategies. An understanding of phloem loading processes in trees and the influence of different environmental conditions will help to understand the relevance of these processes for the ecological fitness of these important tree species.