Apoplastic chemicals

Edgington and Peterson (4) have subdivided apoplastic xeno-biotics into two classes. Euapoplastic (only transported in the apoplast) and pseudoapoplastic (transport occurs mainly in the xylem but entry into the symplast occurs). Most traditional "apoplastic" chemicals are now known to really be pseudoapoplastic chemicals, e.g., atrazine, diuron, oxamyl, etc. The unresolved question is why don t these pseudoapoplastic chemicals which cross the cell membranes and enter the symplast remain in the symplasm of the phloem There have been numerous studies focusing on the molecular requirements for phloem mobility (1-5), In general, there is not a good correlation between phloem mobility and water solubility, metabolism of the xenobiotic, or the presence of various substitution groups in a molecule. 

Phenolics as major components of apoplastic chemical protection... 

In soil, the chances that any enzyme will retain its activity are very slim indeed, because inactivation can occur by denaturation, microbial degradation, and sorption (61,62), although it is possible that sorption may protect an enzyme from microbial degradation or chemical hydrolysis and retain its activity. The nature of most enzymes, particularly size and charge characteristics, is such that they would have very low mobility in soils, so that if a secreted enzyme is to have any effect, it must operate close to the point of secretion and its substrate must be able to diffuse to the enzyme. Secretory acid phosphatase was found to be produced in response to P-deficiency stress by epidermal cells of the main tap roots of white lupin and in the cell walls and intercellular spaces of lateral roots (63). Such apoplastic phosphatase is safe from soil but can be effective only when presented with soluble organophosphates, which are often present in the soil. solution (64). However, because the phosphatase activity in the rhizo-sphere originates from a number of sources (65), mostly microbial, and is much higher in the rhizosphere than in bulk soil (66), it seems curious that plants would have a need to secrete phosphatase at all. 

Further progress may derive from a more accurate definition of the chemical and physical properties of the humic substances present at the rhizosphere and how they interact with the root-cell apoplast and the plasma membrane. An interaction with the plasma membrane H -ATPase has already been observed however this master enzyme may not be the sole molecular target of humic compounds. Both lipids and proteins (e.g., carriers) could be involved in the regulation of ion uptake. It therefore seems necessary to investigate the action of humic compounds with molecular approaches in order to understand the regulatory aspects of the process and therefore estimate the importance of these molecules as modulators of the root-soil interaction. 

Based on the overall distribution pattern in plants, chemical transport historically has been characterized as being apoplastic or symplastic. Since the mid-1970 s it has been increasingly clear that many compounds are ambimobile (4), in that these chemicals travel in both the apoplast and symplast depending on the physical characteristics of the molecule. In fact, most of the chemicals that were previously characterized as moving only in the apoplast or xylem are now regarded as ambimobile because they penetrate membranes quite readily (4). 

Although the weak acid hypothesis appears to explain the mobility of compounds such as chlorophenoxy derivatives, there are several exceptions to the weak-acid hypothesis (4, 1A> 15). For example, some xenobiotics are phloem mobile but are not weak acids and do not appear to be converted to a weak acid prior to transport (e.g., amitrole, oxamyl). Also, some xenobiotics (e.g., glyphosate) which have an ionizable COOH group are loaded into the phloem independently of apoplast pH. These should lose their mobility under pH conditions which ionize the chemical in the free space. Furthermore, accumulation of the weak acid glyphosate against a concentration gradient does not occur (14). 

Most organics can be readily taken up by plant roots and foliage. The chemicals can be transported in living plant tissue (symplast) and nonliving tissue (apoplast). Enzymes in the symplasts may metabolize the chemicals into less toxic compounds. Toxic organics are translocated in cell walls and xylem (apoplast) that form on interconnected continuum within plants. 

Schreiber L., Franke R., Hartmann K.D., Ranathunge K., Steudle E., The chemical composition of suberin in apoplastic barriers affects radial hydraulic conductivity differently in the roots of rice (Oryza sativa L. cv. IR64) and com (Zea mays L. cv. Helix), J. Exp.. Bot., 56(415), 2005, 1427-1436. 

A unique feature of plants is the presence of a system of cell walls which is called apoplast. For many years, apoplast has not been perceived as an important part of plant organisms. At present, it is no longer a dead part of the plant body but a temporally and spatially changing extracellular matrix. It is well-known that many processes depend not only on changes in the chemical and structural composition of the cell wall, but that the cell wall is a place where signal transduction takes place (Fry et al., 1993). If so, also process of SE was investigated from that point of view. 

Selmar, D., Apoplastic occurrence of cyanogenic P-glucosidases and consequences for the metabolism of cyanogenic glucosides, in p-Glucosidases Biochemistry and Molecular Biology, ACS Symposium Series 533,190-203, American Chemical Society, Washington, DC, 1993. 

Some apoplastic phenolics play constitutive chemical defense as it has been demonstrated in bark tissues of woody plants (Franceschi et al, 2005). In leaf mesophyll of Quercus robur L., immunohistological studies revealed that cell wall is the main site of common accumulation of pentagalloylglucose, the precursor of the esterification enzyme (galloyltransferase)... 

The leaching of cations, in particular of K, Ca and Mg, plays a crucial role in leaf ionic relations. As a regular part of the biogeo-chemical nutrient cycling, the flux of leached cations needs to be balanced by an equivalent uptake and translocation. The plant s ability to adjust an increased flux due to acidic deposition determines the extent and duration of leaching-induced, altered ionic relations, first of all within the apoplastic space. - Basic principles and implications of cation leaching will be presented in order to elucidate this kind of an indirect effect of acid rain. 

Schreiber, L., K. Hartmann, M. Skrabs, and J. Zeier. 1999. Apoplastic barriers in roots Chemical composition of endodermal and hypodermal cell walls. J. Exp. Botany 50(337) 1267-1280. 

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