Instituto de Agrobiotecnología y Biología Molecular (IABIMO), Instituto Nacional de Tecnología Agropecuaria (INTA), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Hurlingham, Buenos Aires B1686IGC, Argentina.
Department of Forest Genetics and Plant Physiology, Umeå Plant Science Centre, Swedish University of Agricultural Sciences, Umeå 90183, Sweden.
Tree Physiol. 2022 Mar 9;42(3):458-487. doi: 10.1093/treephys/tpab123.
Plants constitute 80% of the biomass on earth, and almost two-thirds of this biomass is found in wood. Wood formation is a carbon (C)-demanding process and relies on C transport from photosynthetic tissues. Thus, understanding the transport process is of major interest for understanding terrestrial biomass formation. Here, we review the molecules and mechanisms used to transport and allocate C in trees. Sucrose is the major form in which C is transported in plants, and it is found in the phloem sap of all tree species investigated so far. However, in several tree species, sucrose is accompanied by other molecules, notably polyols and the raffinose family of oligosaccharides. We describe the molecules that constitute each of these transport groups, and their distribution across different tree species. Furthermore, we detail the metabolic reactions for their synthesis, the mechanisms by which trees load and unload these compounds in and out of the vascular system, and how they are radially transported in the trunk and finally catabolized during wood formation. We also address a particular C recirculation process between phloem and xylem that occurs in trees during the annual cycle of growth and dormancy. A search of possible evolutionary drivers behind the diversity of C-carrying molecules in trees reveals no consistent differences in C transport mechanisms between angiosperm and gymnosperm trees. Furthermore, the distribution of C forms across species suggests that climate-related environmental factors will not explain the diversity of C transport forms. However, the consideration of C-transport mechanisms in relation to tree-rhizosphere coevolution deserves further attention. To conclude the review, we identify possible future lines of research in this field.
植物构成了地球上 80%的生物量，而其中近三分之二的生物量存在于木材中。木材的形成是一个对碳（C）有高需求的过程，依赖于光合作用组织中 C 的运输。因此，了解运输过程对于理解陆地生物质的形成至关重要。在这里，我们回顾了用于在树木中运输和分配 C 的分子和机制。蔗糖是植物中 C 运输的主要形式，迄今为止，在所有研究过的树种的韧皮汁液中都能发现它。然而，在一些树种中，蔗糖伴随着其他分子，特别是多元醇和棉子糖家族的低聚糖。我们描述了构成这些运输组的分子，以及它们在不同树种中的分布。此外，我们详细介绍了它们合成的代谢反应、树木将这些化合物装载和卸载到血管系统内外的机制，以及它们在树干中如何径向运输，最终在木材形成过程中被分解。我们还讨论了树木在生长和休眠的年度循环过程中，韧皮部和木质部之间发生的一种特殊的 C 再循环过程。对树木中 C 携带分子多样性背后的可能进化驱动因素的搜索表明，被子植物和裸子植物树木之间的 C 运输机制没有一致的差异。此外，C 形式在物种间的分布表明，与气候相关的环境因素不会解释 C 运输形式的多样性。然而，考虑到 C 运输机制与树木-根际的共同进化，值得进一步关注。在综述的最后，我们确定了该领域未来可能的研究方向。
