Paiva Elder Antônio Sousa
Departamento de Botânica, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, 31270-901, CP 486, Brazil
Ann Bot. 2016 Apr;117(4):533-40. doi: 10.1093/aob/mcw012. Epub 2016 Feb 29.
In plants, the products of secretory activity leave the protoplast and cross the plasma membrane by means of transporters, fusion with membranous vesicles or, less commonly, as result of disintegration of the cell. These mechanisms do not address an intriguing question: How do secretory products cross the cell wall? Furthermore, how do these substances reach the external surface of the plant body? Such diverse substances as oils, polysaccharides or nectar are forced to cross the cell wall and, in fact, do so. How are chemical materials that are repelled by the cell wall or that are sufficiently viscous to not cross passively released from plant cells?
I propose a cell-cycle model developed based on observations of different secreting systems, some unpublished results and an extensive literature review, aiming to understand the processes involved in both the secretory process and the release of secretion products. In the absence of facilitated diffusion, a mechanical action of the protoplast is necessary to ensure that some substances can cross the cell wall. The mechanical action of the protoplast, in the form of successive cycles of contraction and expansion, causes the material accumulated in the periplasmic space to cross the cell wall and the cuticle. This action is particularly relevant for the release of lipids, resins and highly viscous hydrophilic secretions. The proposed cell-cycle model and the statements regarding exudate release will also apply to secretory glands not elaborated upon here. Continuous secretion of several days, as observed in extrafloral nectaries, salt glands and some mucilage-producing glands, is only possible because the process is cyclical.
在植物中，分泌活动的产物离开原生质体，并通过转运蛋白、与膜泡融合的方式穿过质膜，或者在不太常见的情况下，由于细胞解体而穿过质膜。这些机制并未解决一个有趣的问题：分泌产物是如何穿过细胞壁的？此外，这些物质又是如何到达植物体的外表面的？像油、多糖或花蜜等多种物质都被迫穿过细胞壁，而实际上它们也确实做到了。那些被细胞壁排斥或粘性足够大而无法被动穿过的化学物质是如何从植物细胞中释放出来的呢？
我基于对不同分泌系统的观察、一些未发表的结果以及广泛的文献综述，提出了一个细胞周期模型，旨在了解分泌过程以及分泌产物释放过程中所涉及的机制。在没有易化扩散的情况下，原生质体的机械作用对于确保某些物质能够穿过细胞壁是必要的。原生质体以连续的收缩和扩张循环的形式产生的机械作用，会使积累在周质空间中的物质穿过细胞壁和角质层。这种作用对于脂质、树脂和高粘性亲水性分泌物的释放尤为重要。所提出的细胞周期模型以及关于渗出物释放的观点也将适用于此处未详细阐述的分泌腺。如在花外蜜腺、盐腺和一些产生黏液的腺体中观察到的持续数天的连续分泌，之所以可能，只是因为这个过程是周期性的。
