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

树木长距离地将碳水化合物从树叶输送到根和其他下沉组织。调节韧皮部中糖的生产、储存和运输是植物应对季节或其他环境变化的一种选择。鉴于全球持续变暖，这对树木等长寿生物尤其重要。植物碳素分配的关键步骤是将主要的碳素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，进入传导筛分元件，而不离开共生体。为此，叶肉细胞的蔗糖浓度必须高于韧皮部的蔗糖浓度。在传导细胞没有充分连接到蔗糖输送细胞的区域，蔗糖需要输出到质外体并输入到传导细胞。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(甜蜜)家族的成员催化蔗糖输出到质外体，以及蔗糖吸收转运蛋白家族的成员在韧皮部的细胞中积累蔗糖。这种主动加载过程导致韧皮部中蔗糖浓度高于叶肉中蔗糖浓度。蔗糖负载韧皮部的策略在草本植物中得到了深入的研究，但在树木中还没有得到深入的研究。因此，我们的目的是为了更好地了解落叶乔木和针叶树的韧皮部装载策略。我们的项目主要集中在以下几个方面：i)叶肉细胞和传导细胞中蔗糖浓度的测定；ii)甜型和SUT型蔗糖转运蛋白的鉴定和鉴定，通过原位杂交分析它们的表达，以及通过免疫组织化学对它们的细胞型特异性定位；iii)分析叶片发育过程中细胞间连通性和蔗糖转运蛋白表达的差异；以及iv)一年中季节的影响和干旱胁迫对韧皮部装载策略的影响。了解树木的韧皮部加载过程以及不同环境条件的影响，将有助于理解这些过程与这些重要树种的生态适宜性的相关性。

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.