Bourquin S, Bonnemain J L, Delrot S
Centre National de la Recherche Scientifique (UA 574), Station Biologique de Beau-Site, 25 rue du Faubourg Saint-Cyprien, 86000 Poitiers, France.
Plant Physiol. 1990 Jan;92(1):97-102. doi: 10.1104/pp.92.1.97.
The apoplast of mature leaves excised from broadbean (Vicia faba L.) plants was infiltrated with 2 millimolar p-chloromercuribenzenesulfonic acid (PCMBS) via the transpiration stream, and the ability of the tissues to take up sugars was tested. An infiltration time of 75 minutes was sufficient to obtain a maximal (75%) inhibition of exogenous [(14)C]sucrose (1 millimolar) uptake. This infiltration affected neither CO(2) assimilation nor the transmembrane potential difference of leaf cells but strongly inhibited phloem loading of endogenous [(14)C] assimilates. The study of the symplastic relations between the different cell types of the mature leaf showed that the density of the plasmodesmata is generally very low in comparison with other species investigated so far, particularly when considering the mesophyll/bundle sheath and the bundle sheath/phloem cells connections, as well as the connections of the transfer cell-sieve tube complex with the surrounding cells. These three successive barriers therefore strongly limit the possibilities of symplastic transit of the assimilates to the conducting cells. The comparison of the densities of plasmodesmata in an importing and an exporting leaf suggests that the maturation of the leaf is characterized by a marked symplastic isolation of the phloem, and, within the phloem itself, by the isolation of the conducting complex. As a consequence, these physiological and cytological data demonstrate the apoplastic nature of loading in the mature leaf of Vicia faba, this species undoubtedly presenting a typical model for apoplastic loading.
通过蒸腾流将2毫摩尔对氯汞苯磺酸盐（PCMBS）渗入从蚕豆（Vicia faba L.）植株上切下的成熟叶片的质外体中，并测试组织吸收糖分的能力。75分钟的渗入时间足以使外源[¹⁴C]蔗糖（1毫摩尔）的吸收受到最大程度（75%）的抑制。这种渗入既不影响CO₂同化作用，也不影响叶细胞的跨膜电位差，但强烈抑制了内源[¹⁴C]同化物的韧皮部装载。对成熟叶片不同细胞类型之间共质体关系的研究表明，与迄今为止研究的其他物种相比，胞间连丝的密度通常非常低，特别是考虑到叶肉/维管束鞘以及维管束鞘/韧皮部细胞之间的连接，以及传递细胞-筛管复合体与周围细胞的连接时。因此，这三个连续的屏障极大地限制了同化物共质体转运到传导细胞的可能性。对输入叶和输出叶中胞间连丝密度的比较表明，叶片成熟的特征是韧皮部明显的共质体隔离，并且在韧皮部内部，是传导复合体的隔离。因此，这些生理和细胞学数据证明了蚕豆成熟叶片中装载的质外体性质，该物种无疑是质外体装载的典型模型。
