Zhang Wen-Hao, Zhou Yuchan, Dibley Katherine E, Tyerman Stephen D, Furbank Robert T, Patrick John W
Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, China.
School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2238, Australia.
Funct Plant Biol. 2007 May;34(4):314-331. doi: 10.1071/FP06271.
Interest in nutrient loading of seeds is fuelled by its central importance to plant reproductive success and human nutrition. Rates of nutrient loading, imported through the phloem, are regulated by transport and transfer processes located in sources (leaves, stems, reproductive structures), phloem pathway and seed sinks. During the early phases of seed development, most control is likely to be imposed by a low conductive pathway of differentiating phloem cells serving developing seeds. Following the onset of storage product accumulation by seeds, and, depending on nutrient species, dominance of path control gives way to regulation by processes located in sources (nitrogen, sulfur, minor minerals), phloem path (transition elements) or seed sinks (sugars and major mineral elements, such as potassium). Nutrients and accompanying water are imported into maternal seed tissues and unloaded from the conducting sieve elements into an extensive post-phloem symplasmic domain. Nutrients are released from this symplasmic domain into the seed apoplasm by poorly understood membrane transport mechanisms. As seed development progresses, increasing volumes of imported phloem water are recycled back to the parent plant by process(es) yet to be discovered. However, aquaporins concentrated in vascular and surrounding parenchyma cells of legume seed coats could provide a gated pathway of water movement in these tissues. Filial cells, abutting the maternal tissues, take up nutrients from the seed apoplasm by membrane proteins that include sucrose and amino acid/H symporters functioning in parallel with non-selective cation channels. Filial demand for nutrients, that comprise the major osmotic species, is integrated with their release and phloem import by a turgor-homeostat mechanism located in maternal seed tissues. It is speculated that turgors of maternal unloading cells are sensed by the cytoskeleton and transduced by calcium signalling cascades.
种子营养物质装载备受关注，因为它对植物繁殖成功和人类营养至关重要。通过韧皮部输入的营养物质装载速率，受源（叶、茎、生殖结构）、韧皮部途径和种子库中的运输及转运过程调控。在种子发育早期，大部分控制可能由为发育中的种子服务的分化韧皮部细胞的低传导途径施加。随着种子开始积累贮藏产物，根据营养物质种类不同，路径控制的主导地位会让位于源（氮、硫、微量矿物质）、韧皮部路径（过渡元素）或种子库（糖和主要矿物质元素，如钾）中的调控过程。营养物质和伴随的水分被输入母本种子组织，并从传导筛管分子卸载到广泛的韧皮部后共质体区域。营养物质通过尚未完全了解的膜转运机制从这个共质体区域释放到种子质外体中。随着种子发育的进行，越来越多输入的韧皮部水分通过尚未发现的过程被循环回母本植物。然而，集中在豆科植物种皮的维管束和周围薄壁细胞中的水通道蛋白，可能为这些组织中的水分运动提供一个门控途径。与母本组织相邻的子代细胞，通过包括蔗糖和氨基酸/氢离子同向转运体以及非选择性阳离子通道并行发挥作用的膜蛋白，从种子质外体吸收营养物质。子代对构成主要渗透物质的营养物质的需求，通过母本种子组织中的膨压稳态机制，与营养物质的释放和韧皮部输入整合在一起。据推测，母本卸载细胞的膨压由细胞骨架感知，并通过钙信号级联进行转导。
