Aloni B, Wyse R E, Griffith S
United States Department of Agriculture, Agricultural Research Service, Utah State University, UMC 63, Logan, Utah 84321.
Plant Physiol. 1986 Jun;81(2):482-6. doi: 10.1104/pp.81.2.482.
Stems of Vicia faba plants were used to study phloem unloading because they are hollow and have a simple anatomical structure that facilitates access to the unloading site. After pulse labeling of a source leaf with (14)CO(2), stem sections were cut and the efflux characteristics of (14)C-labeled sugars into various buffered solutions were determined. Radiolabeled sucrose was shown to remain localized in the phloem and adjacent phloem parenchyma tissues after a 2-hour chase. Therefore, sucrose leakage from stem segments prepared following a 75-minute chase period was assumed to be characteristic of phloem unloading. The efflux of (14)C assimilates from the phloem was enhanced by 1 millimolar p-chloromercuribenzene sulfonic acid (PCMBS) and by 5 micromolar carbonyl cyanide m-chlorophenly hydrazone (CCCP). However, PCMBS inhibited and CCCP enhanced general leakage of nonradioactive sugars from the stem segments. Sucrose at concentrations of 50 millimolar in the free space increased efflux of [(14)C]sucrose, presumably through an exchange mechanism. This exchange was inhibited by PCMBS and abolished by 0.2 molar mannitol. Increasing the osmotic concentration of the efflux medium with mannitol reduced [(14)C]sucrose efflux. However, this inhibition seems not to be specific to sucrose unloading since leakage of total sugars, nonlabeled sucrose, glucose, and amino acids from the bulk of the tissue was reduced in a similar manner. The data suggest that phloem unloading in cut stem segments is consistent with passive efflux of sucrose from the phloem to the apoplast and that sucrose exchange via a membrane carrier may be involved. This is consistent with the known conductive function of the stem tissues, and contrasts with the apparent nature and function of unloading in developing seeds.
蚕豆植株的茎被用于研究韧皮部卸载，因为它们是中空的，且具有简单的解剖结构，便于进入卸载部位。在用¹⁴CO₂对源叶进行脉冲标记后，切取茎段，并测定¹⁴C标记的糖类向各种缓冲溶液中的流出特性。在2小时的追踪期后，放射性标记的蔗糖显示仍局限于韧皮部和相邻的韧皮部薄壁组织中。因此，在75分钟追踪期后制备的茎段中蔗糖的泄漏被认为是韧皮部卸载的特征。1毫摩尔对氯汞苯磺酸盐（PCMBS）和5微摩尔羰基氰化物间氯苯腙（CCCP）增强了¹⁴C同化物从韧皮部的流出。然而，PCMBS抑制了茎段中非放射性糖类的总体泄漏，而CCCP增强了这种泄漏。自由空间中50毫摩尔浓度的蔗糖增加了[¹⁴C]蔗糖的流出，推测是通过交换机制。这种交换被PCMBS抑制，并被0.2摩尔甘露醇消除。用甘露醇增加流出介质的渗透浓度降低了[¹⁴C]蔗糖的流出。然而，这种抑制似乎并非蔗糖卸载所特有的，因为来自大部分组织的总糖、未标记蔗糖、葡萄糖和氨基酸的泄漏也以类似方式减少。数据表明，切割茎段中的韧皮部卸载与蔗糖从韧皮部被动流出到质外体一致，并且可能涉及通过膜载体的蔗糖交换。这与茎组织已知的传导功能一致，并且与发育种子中卸载的明显性质和功能形成对比。
