Root apoplast

Enstone, D. E. and Peterson, C. A. 1992. A rapid fluorescence technique to probe the permeability of the root apoplast. Canadial Journal of Botany - Revue Cana-dienne de Botanique, 70 1493-1501. 

Release of organic acids for complexing ions outside the root apoplast plays a role in Ah" -tolerant cultivars of maize and wheat. The excreted (or exuded) malic and/or citric acid incorporates Ah" into negatively charged stable anion complexes which are harmless for roots and which are not taken up (Jones etal. 1996). 

Nishiyama, H. Ohya, X Xanoi, K. Nakanishi, X. M. A simple measurement of the pH of root apoplast by the fluorescence ratio method. Plant Root 2008, 2, 3-6. 

Apoplastic transport of sodium across the root Plant vigour (degree of dwarfing)... 

Yeo, A.R., Yeo, M.E. Flowers, T.J. (1987). The contribution of an apoplastic pathway to sodium uptake by rice roots in saline conditions. Journal of Experimental Botany, 38, 1141-53. 

Some divalent cations such as Cu and Pb form very stable complexes with pectate, but are unlikely to be present at sufiScient concentration in the apoplast of plants to form a major fraction of the counterions associated with the pectic fraction in vivo. The Al ion may deserve closer examination, as it is certainly able to displace Ca from cell walls and reaches substantial concentrations in plant roots under some conditions [60,61]. aluminium is not usually considered to be freely translocated, however. Basic peptides with their negative charges spaced at a similar interval to galacturonans (0.43 nm or a small multiple thereof) can in principle have a very high afiBnity for pectate [62,63], but the extensins that are associated with the most insoluble pectic fractions [M] do not appear to have this type of structure. The possibility that the non-extractable pectic polymers in most cell walls are very strongly complexed with some cation other than Ca " cannot be ruled out, but there is little evidence to support it at present. 

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. 

Bonfante et al. (73) used monoclonal antibodies and enzyme-gold complexes to reveal pectins and cellulose at the interface between the fungal wall and the host plasma membrane in AM roots (Fig. 6), and additional wall components have been investigated with other molecular probes (74-76). These studies indicate that the interface is an apoplastic space of high molecular complexity where the boundaries of the partners are defined. The examination of other endomycorrhizal systems has demonstrated that their interface is morphologically similar but different in composition. Cellulose and pectins are present at the interface... 

In contrast to the exterior localization of cutin, suberin can be deposited in both external and internal tissues. External deposition occurs in the periderm of secondary roots and stems and on cotton fibers, whereas internal deposition occurs in the root endodermis and the bundle sheath of monocots. The Casparian strip of the root en-dodermis contains suberin, which produces a barrier isolating the apoplast of the root cortex from the central vascular cylinder. Suberin also produces a gas-impermeable barrier between the bundle sheath and mesophyll cells in C4 plants. The bark of trees contains periderm-derived cork cells that have a high suberin content. 

Vasquez MD, Poschenreider C, Corrales I, Barcelo J. Change in apoplastic aluminum during the initial growth response to aluminum by roots of a tolerant maize variety. Plant Physiol 1999 119 435-444. 

The acquisition of iron in plant roots has been described in Chapter 7. Once in the apoplast of the root, the iron must be transported through the roots to the xylem and thence to the leaves. In order to ensure that the iron does not precipitate or generate oxygen radicals during its transport, the iron is bound to an intracellular transporter of iron (both Fe2+and Fe3+),... 

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

The secretion of secondary metabolites to the apoplast is another way to circumvent the toxic effects of some plant natural products. Plants use this mechanism especially in root cells. Then, they can interact with neighboring plants (allelopathy) or modify soil environment to promote... 

APase in onion roots. Enzyme activity was mainly extracellular with the heaviest concentration in corner spaces between the epidermal and hypo-dermal layers. He suggested the possibility of a subcutaneous pore through which the enzyme could be released to the root surface. Bieleski and co-workers (Reid Bieleski, 1970 Bieleski Johnson, 1972) studied the psi induction and location of APase in duckweed (Spirodela oligorrhiza). APase in control plants was located primarily in and around the vascular strands. In P-deficient plants psi-APase activity was 10-20 times the control value. Enzyme activity was primarily located in the epidermis of the root and undersurface of the frond, the tissue locations most likely to provide access to phosphate esters in the medium. These workers further demonstrated that hydrolysis of organic phosphates occurred in the external medium and/or the apoplast followed by Pi uptake into the cell. 

Hammond-Kosack K E, Atkingson H J, Bowles D J. 1989. Systemic accumulation of noval proteins in the apoplast of the leaves of potato plants following root invasion by the cyst nematode Globodera rostochiansis. Plant Pathol 35 495-506. 

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. 

Petersen, C.A., Emanuel, M.E., Humphreys, G.B. Pathway of movement of apoplastic fluorescent dye tracers through the endodermis at the site of secondary root formation in corn (Zea mays) and broad bean (Vicia... 

Agrichemicals which travel mainly in the apoplast characteristically accumulate at the leaf tips and margins of mature leaves, whereas compounds that travel in the phloem accumulate at growing regions (i.e., new leaves, buds, root tips, and storage organs). 

Intracellular distribution of essential transition metals is mediated by specific metallochaperones and transporters localized in endomembranes. In other words, the major processes involved in hyperaccumulation of trace metals from the contaminated medium to the shoots by hyperaccumulators as proposed by Yang et al. (2005) include bioactivation of metals in the rhizosphere through root-microbial interaction enhanced uptake by metal transporters in the plasma membranes detoxification of metals by distributing metals to the apoplasts such as binding to cell walls and chelation of metals in the cytoplasm with various ligands (such as PCs, metallothioneins, metal-binding proteins) and sequestration of metals into the vacuole by tonoplast-located transporters. 

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