Third party funded individual grant
Start date : 01.08.2024
End date : 31.07.2027
We are investigating how sucrose is distributed in deciduous and coniferous trees. The focus is on the process of phloem loading and the storage of sucrose in the trunk. The influence of seasons and drought stress on carbohydrate distribution in trees is analyzed.
Trees transport sugars over long distances from photosynthetically active leaves (source organs) to roots and other sink organs. The regulation of sugar production, storage and transport in the phloem is a way for plants to respond to seasonal or other environmental changes. This is particularly important for long-lived organisms such as trees and in the context of ongoing global warming. A crucial step in the distribution of carbon compounds in the plant is the loading of the phloem with sucrose, the most important transport form of carbohydrates. In general, sucrose can pass through plasmodesmata from mesophyll cells to phloem cells without leaving the symplast. The prerequisite for this is that all cells are connected via plasmodesmata and that the sucrose concentration in the mesophyll cells is higher than that in the phloem. However, if not all cell types are sufficiently connected via plasmodesmata, sucrose must first be exported from the mesophyll cells or the phloem parenchyma cells into the apoplast before it can be loaded from there into the phloem. It is known from herbaceous plants that two types of proteins are required for this transport: SUGARS WILL EVENTUALLY BE EXPORTED TRANSPORTER (SWEET), which catalyze the export of sucrose into the apoplast, and sucrose uptake transporters (SUT/SUC), which load the phloem with sucrose from the apoplast. This active loading process leads to higher sucrose concentrations in the phloem than in the cytosol of the mesophyll cells. The mechanisms of phloem loading with sucrose have been intensively studied in herbaceous plants, but not yet in trees. The aim of this project is therefore to investigate the mechanisms of phloem loading in Fagus sylvatica and Picea abies. The aim is to elucidate these important processes in deciduous trees as well as in conifers. Our project focuses on the following four aspects: I) determination of sucrose concentrations in mesophyll cells and the phloem; II) identification and characterization of sucrose transporters of the SWEET and SUT type, the analysis of their expression by in situ hybridization as well as their cell type-specific localization by immunohistochemistry; III) analysis of cell-cell connectivity and the expression of sucrose transporters during leaf development (sink-source transition) and IV) the influence of seasons and drought stress on the phloem loading strategies. The knowledge gained about the loading mechanisms of phloem in trees and the influence of different environmental conditions will help to understand the importance of these physiologically important processes for the ecological fitness of different tree species.