Transport symplast

Metalaxyl and most of its active analogues are chiral molecules. Chirality is caused by the asymmetric carbon atom in the alkyl side chain of the alanine moiety. The two optically pure enantiomers S (+) and R (-) differ widely in their biological activity both in vitro and in vivo. In all experiments, the R (-) enantiomer was more active than its antipode S (+) (22, 24, 30). The main characteristics of metalaxyl have been discussed in detail by several authors (J, 21, 28, 29, 32> 38). Of particular value is the rapid uptake of metalaxyl by the plant tissue, especially under the wet conditions that favor foliar Oomycete diseases. Acylalanines are easily translocated in the vascular system of the plant after foliar, stem or root treatment (35, 47). The predominant route of transport is the transpiration stream, thus apoplastic (12, 35). Symplastic transport occurs but is much less evident (35, 47). In potatoes treated by foliar sprays of metalaxyl concentrations (0.02-0.04 ppm), Bruin et al. (SO were able to demonstrate protection of harvested tubers from late blight. 

Petty, H.R. 1993. Molecular Biology of Membranes. Structure and Function. Plenum, New York. Preston, R.B. 1974. The Physical Biology of Plant Cell Walls. Chapman Hall, London. Roberts, A.G., and Oparka, K.J. 2003. Plasmodesmata and the control of symplastic transport. 

Gunning B E S, Hughes J 1976 Quantitative assessment of symplastic transport of prenectar into the trichomes of Abutilon nectaries. Aust J Plant Physiol 3 619-637... 

The pyridazinone herbicides, of which norflurazon and pyrazon (chloridazon) are marketed, are applied pre- or early postemergence. The compounds are of modest to intermediate lipophilicity, and consequently they are readily taken up by the roots and translocated to the shoots, though essentially no symplastic transport has been observed. 

Bentazon, mefluidide, and perfluidone are of intermediate lipophilicity with pKa = 3.5, 4.6, and 2.5, respectively. Mefluidide is phloem translocated, but the contact herbicide bentazon is not moved symplastically perfluidone moves in the xylem following uptake by roots, but experiments testing its symplastic transport have not been reported. 

Dimethylarsinic acid (cacodylic acid) is a weak acid of pKa 6.3, while methylarsonic acid (MAA) is a weak dibasic acid (pKa 4.1 and 9.1). Being very polar, both compounds are transported in the apoplast and in the symplast, though symplastic transport in the case of dimethylarsinic acid is weak perhaps due to rapid phytotoxic action in the treated leaves. 

Benzoylprop and flamprop differ by only a single substituent and have pKa — 3.5 and log Xqw 3. They are well transported in the xylem, but phloem transport is very weak. The poor symplastic transport of flamprop in castor bean observed by Rigitano et could not be readily explained by these authors. However, the poor accumulation of flamprop in barley roots could indicate that the anion moves through membranes with unexpected ease, and this would limit retention in the phloem. 

Jachetta, J., A.P. Appleby, and L. Boersma. 1986. Apoplastic and symplastic pathways of atrazine and glyphosate transport in shoots of seedling sunflower. Plant Physiol. 82 1000-1007. 

Example of Ester-Loading Methods for Symplastic Tracing Intra- and Intercellular Transport in the Motile Tubule-Vacuole System... 

Figure 6.9 Idealized structure of a root showing apoplastic and symplastic pathways for solute transport to the xylem...

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). 

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. 

Fig. 2.6 Molecular mechanisms hold to explain accumulation of transition metal ions by and in plants. Letters (a) to (e) are to be taken in the same vertical arrangement in both plant and this picture, e.g. a = mobilization around the root, c = transport within the xylem. (a) metal ions get mobilized by secretion of chelators which in addition acidify the rhizosphere. (b) uptake of hydrated metal ions or (rather) their chelate complexes is augmented by various systems bound to the plasma membrane, (c) transport of transition metals from roots to shoot occurs via the xylem. Presumably the larger share is transported by means of the root symplast an apoplastic passage in the root tips is also conceivable. After exchange (oxidative destruction) of the original ligands metals which made it into the xylem are other kinds of chelator complexes or else aquated ions, (d) After getting into the leaf apoplast several metals are bound to the...

The acquisition of iron, copper, and zinc in plant roots has been described in Chapter 7. Once within the root epidermal cell, the iron must be transported through the roots to the xylem and thence to the leaves, and this intercellular metal transport is illustrated for dicots in Fig. 8.8 and for monocots in Fig. 8.9. In dicots, Fe, Zn, and Cu are taken up into the symplast by transporters in the epidermis. Reduction of Fe and possibly of Cu by FR02 and acidification of the soil by an Arabidopsis ATPase contribute to increased metal uptake. Metals can then travel through the symplastic space to the vasculature. Transport into the xylem is still not fully characterised. In the case of Fe, it is probably as citrate, and the citrate transporter FRD3 has been shown to efflux citrate into the xylem and is required for Fe transport to the shoot. Zn and Cu are thought to be effluxed into the xylem by... 

In monocots, Fe and Zn are taken up as phytosiderophore chelates by YSL transporters in the epidermis. Fe can also be taken up by OsIRTI. Metals move through the symplastic space to the vasculature. The citrate effluxer FRDLl is important for loading of citrate into the xylem and subsequent Fe transport to the shoot through the transpiration stream. YSL transporters also may play a role in unloading the xylem into the shoot and the phloem. Fe is unloaded from the phloem by OsYSL2 and OsIRTI into shoot and seed tissue. 

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. 

FIGURE 1. A basic transport unit that has been successively constructed beginning with the bathing media, Bq and and a bicellular symplast with a plasmodesma. See text for further details. 

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