Plasmodesmata: Symplastic Transport of Herbicides within the Plant

When studying herbicide absorption, translocation, metabolism, and mode of action, transport pathways are usually referred to as apoplast (dead cells) and symplast (living cells) as simple synonyms of xylem and phloem. However, the behavior of an herbicide within a plant greatly depends upon several factors and its movement accomplished by different routes and processes. If an herbicide takes too long to be absorbed after application, it will be more available for processes that would greatly reduce its absorption – rain, hot sun, and wind, among others. After the herbicide is absorbed, it needs to be quickly translocated from the point it was absorbed to the site of action. If it is not, chemical processes will take care of transforming the herbicide into non-toxic or less-toxic metabolites. For a quick and efficient translocation, several pathways act together in a relatively dependent manner – everything is connected at different degrees of the plant’s metabolic rate by the time reactions occur. For example, when a plant is under water stress, it may react differently to the same dose of herbicide usually applied to that species. In addition, phloem will only translocate an herbicide quickly if this compound is efficiently loaded into the phloem. From the leaf surface to the site of action, herbicide movement involves passage through the apoplast and symplast by several pathways, one of which is via plasmodesmata. In the classical concept of Munch (1930), plasmodesmata are considered to form simple cytoplasmic bridges between neighboring plant cells in order to create the symplasm. This concept has dominated, if not monopolized, the thinking of plant biologists and, in particular, plant physiologists over the last few decades. Recent advances in ultra-structural, physiological, and molecular studies on plasmodesmata indicate that this simple view is in need of revision (Lucas, 1993). Plasmodesmata are plasma channels connecting neighboring cells that allow the exchange of informational, functional, and structural molecules and xenobiotics among cells of the same "group" (domain) , both apoplastically and symplastically. Cells of the same domain behave as functional units, and substances are able to move between them at rates above the observed for trans-membrane movement. Plasmodesmata participate symplastically in long-distance movement, both by association with phloem and interchange between neighboring domains. When the plant is under stress, and xylem and phloem flux is slower, plasmodesmata could be more participative in