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

树木将碳水化合物从叶子长距离运输到根和其他库组织。调节糖的生产，其在韧皮部中的储存和运输是植物应对季节或其他环境变化的一种选择。这对于树木等长寿生物尤其重要，并且考虑到全球变暖的持续性。植物体内碳分配的一个关键步骤是将主要的碳运输形式蔗糖装载到韧皮部。一般来说，蔗糖可以通过胞间连丝从一个细胞移动到另一个细胞，进入传导筛分子而不离开共质体。为此，叶肉细胞中的蔗糖浓度必须高于韧皮部。在传导细胞与蔗糖递送细胞没有充分连接的区域，蔗糖需要输出到质外体中并输入到传导细胞中。从草本植物中已知，这种运输需要两种类型的蛋白质：糖最终将被输出转运蛋白（SWEET）-家族的成员催化蔗糖输出到质外体中，蔗糖摄取转运蛋白（SUT/SUC）-家族的成员在韧皮部的细胞中积累蔗糖。这种主动的装载过程导致韧皮部中的蔗糖浓度高于叶肉中的蔗糖浓度。韧皮部装载蔗糖的策略在草本植物中已被深入研究，但在树木中还没有。因此，我们的目的是分析的韧皮部加载策略在水青冈和欧洲云杉，以更好地了解这一过程中的落叶树木，以及在针叶树。本研究的主要内容包括：（1）叶肉细胞和传导细胞中蔗糖浓度的测定：（2）SWEET和SUT型蔗糖转运蛋白的鉴定和表征，原位杂交分析其表达，免疫组织化学分析其细胞类型特异性定位; III）分析叶发育过程中细胞-细胞连接性和蔗糖转运蛋白表达的差异，（4）一年中季节变化的影响和干旱胁迫对韧皮部装载策略的影响。了解树木的韧皮部装载过程和不同环境条件的影响，将有助于了解这些重要树种的生态适应性，这些过程的相关性。

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.