Universality of phloem transport in seed plants

Since Münch in the 1920s proposed that sugar transport in the phloem vascular system is driven by osmotic pressure gradients, his hypothesis has been strongly supported by evidence from herbaceous angiosperms. Experimental constraints made it difficult to test this proposal in large trees, where the distance between source and sink might prove incompatible with the hypothesis. Recently, the theoretical optimization of the Münch mechanism was shown to lead to surprisingly simple predictions for the dimensions of the phloem sieve elements in relation to that of fast growing angiosperms. These results can be obtained in a very transparent way using a simple coupled resistor model. To test the universality of the Münch mechanism, we compiled anatomical data for 32 angiosperm and 38 gymnosperm trees with heights spanning 0.1-50 m. The species studied showed a remarkable correlation with the scaling predictions. The compiled data allowed calculating stem sieve element conductivity and predicting phloem sap flow velocity. The central finding of this work is that all vascular plants seem to have evolved efficient osmotic pumping units, despite their huge disparity in size and morphology. This contribution extends the physical understanding of phloem transport, and will facilitate detailed comparison between theory and field experiments.     

Interactions among tobacco sieve element occlusion (SEO) proteins

Angiosperms transport their photoassimilates through sieve tubes, which comprise longitudinally-connected sieve elements. In dicots and also some monocots, the sieve elements contain parietal structural proteins known as phloem proteins or P-proteins. Following injury, P proteins disperse and accumulate as viscous plugs at the sieve plates to prevent the loss of valuable transport sugars. Tobacco (Nicotiana tabacum) P-proteins are multimeric complexes comprising subunits encoded by members of the SEO (sieve element occlusion) gene family. The existence of multiple subunits suggests that P-protein assembly involves interactions between SEO proteins, but this process is largely uncharacterized and it is unclear whether the different subunits perform unique roles or are redundant. We therefore extended our analysis of the tobacco P-proteins NtSEO1 and NtSEO2 to investigate potential interactions between them, and found that both proteins can form homomeric and heteromeric complexes in planta.     

Seasonal dynamics of phloem and xylem formation in silver fir and Norway spruce as affected by drought

The dynamics of phloem growth ring formation in silver fir (Abies alba Mill.) and Norway spruce (Picea abies Karst.) at different sites in Slovenia during the droughty growing season of 2003 was studied. We also determined the timing of cambial activity, xylem and phloem formation, and counted the number of cells in the completed phloem and xylem growth rings. Light microscopy of cross-sections revealed that cambial activity started on the phloem and xylem side simultaneously at all three plots. However, prior to this, 1–2 layers of phloem derivatives near the cambium were differentiated without previous divisions. The structure of the early phloem was similar in silver fir and Norway spruce. Differences in the number of late phloem cells were found among sites. Phloem growth rings were the widest in Norway spruce growing at the lowland site. In all investigated trees, the cambium produced 5–12 times more xylem cells than phloem ones. In addition, the variability in the number of cells in the 2003 growth ring around the stem circumference of the same tree and among different trees was higher on the xylem side than on the phloem side. Phloem formation is presumably less dependent on environmental factors but is more internally driven than xylem formation.     

拟南芥花蜜腺筛分子及蜜腺组织发育过程中的细胞学研究

应用高压冷冻和低温替代技术,以拟南芥（Arabidopsis thalanaL.)花蜜腺发育过程中细胞的超微结构变化进行了研究。蜜腺组织中深色细胞的超微结构与筛分子早期分化的超微结构十分相似;细胞核中染色质逐渐出现凝集并且边缘化;细胞器分布异常;细胞质浓稠,这些超微结构特征与近年来报道的动植物细胞程序性死亡的超微结构相似,在筛分子和深色细胞分化中,细胞核及一些细胞器的逐渐解体与原蜜汁的运输,加工和蜜汁的分泌有直接联系,这反映了蜜腺发育过程中筛分子和蜜腺组织的细胞学变化是与蜜腺的生长,发育和生理功能的完善联系在一起的。     

Scaling phloem transport: water potential equilibrium and osmoregulatory flow

In this work, the common assumption that phloem sap is in water potential equilibrium with the surrounding apoplast was examined. With a dimensionless model of phloem translocation that scales with just two dimensionless parameters (R?and F?), a ‘map’ of phloem behaviour as a function of these parameters was produced, which shows that the water potential equilibrium assumption (R?F? >> 1) is valid for essentially all realistic values of the relevant scales. When in water potential equilibrium, a further parameter reduction is possible that limits model dependence to a single parameter (F?), which describes the ratio of the solution's osmotic strength to its axial pressure drop. Due to the locally autonomous nature of individual sieve element/companion cell complexes, it is argued that long‐distance integrative control is most efficient when F? is large (that is, when the pressure drop is relatively small), permitting the sieve tube to regulate solute loading in response to global changes in turgor. This mode of transport has been called ‘osmoregulatory flow.’ Limitations on the pressure drop within the transport phloem could require that sieve tubes be shorter than the long axis of the plant, and thus arranged in series and hydraulically isolated from one another.     

黄瓜韧皮部的类血影蛋白

以黄瓜(Cucumis sativus L.)叶柄为实验材料,应用胶体金免疫电镜技术证明类血影蛋白存在于韧皮部的筛管-伴胞复合体中,广泛分布于筛分子中的韧皮蛋白纤丝以及筛分子网络结构上,并且分布在伴胞的细胞质和线粒体膜以及筛分子与伴胞之间的分支状胞间连丝上,表明该蛋白可能由伴胞合成并经由二者之间的胞间连丝运输到筛分子中.用免疫印迹技术证明,黄瓜韧皮部汁液蛋白中存在类血影蛋白,其分子量约为260 kD,与动物细胞中血影蛋白的分子量接近.     

难溶于二甲亚砜多糖的甲基化方法研究

本文对难溶于二甲亚砜(DMSO)但易溶于水的多糖——羊栖菜褐藻糖胶的甲基化方法进行了研究。10mg多糖用0．1mL水溶解后,加和3mL DMSO,使多糖转溶于DMSO,然后加人2mL 3A分子筛脱除水分。将DNSO溶液过滤后,滤液中加入50mg NaOH粉末,室温下反应10min。加入0．5mL碘甲烷,室温下反应60min。加1mL水终止反应,加1mol／L HAc中和、透析、冻干。该法简便易行,所得产物的甲基化程度比较高。     

Scaling phloem transport: information transmission

Sieve tubes are primarily responsible for the movement of solutes over long distances, but they also conduct information about the osmotic state of the system. Using a previously developed dimensionless model of phloem transport, the mechanism behind the sieve tube's capacity to rapidly transmit pressure/concentration waves in response to local changes in either membrane solute exchange or the magnitude and axial gradient of apoplastic water potential is demonstrated. These wave fronts can move several orders of magnitude faster than the solution itself when the sieve tube's axial pressure drop is relatively small. Unlike the axial concentration drop, the axial pressure drop at steady state is independent of the apoplastic water potential gradient. As such, the regulation of whole‐sieve tube turgor could play a vital role in controlling membrane solute exchange throughout the translocation pathway, making turgor a reliable source of information for communicating change in system state.     

筛管内同化物运输所需压力梯度的流体动力学模拟实验

本文根据流体力学的相似原理,进行了同化物在筛管中的压力流动模拟实验,测出了压力梯度与筛孔直径、传导速度间的关系曲线,并与按 Poiseuille 公式计算出的理论值进行比较。结果证明,同化物传导所需的压力梯度与速度呈线性关系,与 Poiseuille 公式一致。但是,筛板对传导的阻力大大超过以往的理论估计。即使在正常的传导速度和典型的筛管尺寸下,流动所需的压力梯度为1 bar/m 左右。随着筛孔直径的减小,压力梯度大大增加。在本实验的五种筛孔直径下,压力梯度的实测值与按 Poiseuille 公式计算的理论值之比约为2—4。本文还进行了筛孔部分堵塞的模拟实验,测出的压力梯度远超过在植物体内可能存在的范围。