Root apex

The role of the secretion from the root apex of organic acids such as citric and malic in the resistance of maize and wheat, respectively, to Al toxicity (81,82) has emerged recently as one with plausibility (83). These studies have been carried out in solution cultures, but how does the suggestion hold up in soil The first and probably greatest difficulty is that the toxic species of Al, probably hydrated Al ", must diffuse to some site in the root apex and stimulate the produc-... 

The boundary conditions for solving Equation (6.1) are (a) at the root base, [02]g is the ambient value in the atmosphere and (b) at the root apex, [02]g is the minimum value required for root respiration 30ttmoldm (gas space)]. The equations are solved numerically. 

North Carolina State University, Crop Sciences Department Raleigh, NC Research on the mechanism of aluminum toxicity in plants. This study will determine the extent of aluminum accumulation inside cells at the root apex using microanalytical techniques and will define associated effects on cell division, cell expansion, and cellular accumulation of calcium and magnesium. U.S. Department of Agriculture... 

The root apex is covered with a calyptra extending proximally up to about 600 pm from the tip. The apical meristem is of the closed type, with each of the three root tissue systems, i.e. the vascular cylinder, the cortex, and the calyptra/epidermis, arising from its own independent groups of initials.16 Cellular differentiation had already started at a distance of about 600 pm from the tip in the 16 h-control while very little differentiation could be seen in roots from treated seeds. 

Examination of the root apex showed much the same effects as reported above (Fig. 15.6a-d). The calyptra was a particularly sensitive tissue, showing dramatic mitochondrial swelling at all dilutions tested (Fig. 15.6b-d). A conspicuous effect of the treatment with the RO fraction on the root apex was the inhibition of amyloplast development as statoliths in the columella cells of the calyptra (Fig. 15.6b-c). The almost complete lack of starch and of lipid-associated microbodies in... 

Radish seeds were treated with RO fraction at dilutions 1 8, 1 10 and 1 14 as described above and were processed for electron microscopy 46 h after the start of the treatment. Roots from control seeds, moistened with distilled water, were fixed after 2, 16 and 46 h in order to get several developmental stages as references for treated seeds. The root apex, about 1 mm long, was cut under a dissecting microscope from at least 10 seeds and fixed in the primary fixative for 2 h at room temperature under mild vacuum. Since the control seeds had not yet started germination after 2 h imbibition and nearly 100% of the seeds treated with 1 8-diluted had not germinated after 46 h, in these two cases the seed integument was cut open with a razor blade in order to dissect the root. 

Secondary metabolites can accumulate in the same cell and tissue in which they are formed, but intermediates and end-products can also be transported to other locations for further elaboration or accumulation. For example, TAs and nicotine are typically produced near the root apex, but mostly accumulate within leaf cell vacuoles. Even TA biosynthesis itself involves intercellular transport of several pathway intermediates (Fig.7.9A). P-Glucuronidase (GUS) localization in A. belladonna roots transformed with a PMT promoter-GUS fusion showed that PMT expression is restricted to the pericycle.144 Immunolocalization and in situ RNA hybridization also demonstrated the pericycle-specific expression of H6H.145,146 In contrast, TR-I was immunolocalized to the endodermis and outer root cortex, whereas TR-II was found in the pericycle, endodermis, and outer cortex.85 The localization of TR-I to a different cell type than PMT and H6H implies that an intermediate between PMT and TR-I moves from the pericycle to the endodermis (Fig.7.9A). Similarly, an intermediate between TR-I and H6H must move back to the pericycle. The occurrence of PMT in the pericycle provides the enzyme with efficient access to putrescine, ornithine, and arginine unloaded from the phloem. In the same way, scopolamine produced in the pericycle can be readily translocated to the leaves via the adjacent xylem. 

Becker GL, Cohen S, Borer R. The sequelae of accidentally injecting sodium hypochlorite beyond the root apex. Report of a case. Oral Surg Oral Med Oral Pathol 1974 38(4) 633-8. 

Histology of Root Apex.—Microscopic examination shows this... 

Aluminum ions (Al3+) have toxic effect on plants, and number of studies documented the toxic impact of Al3+ on roots,1 hypocotyls,2 and germinating pollen.3,4 It has been proposed that early effects of Al3+ toxicity at the root apex, such as those on cell division, cell extension or nutrient transport, involve the direct intervention of A1 on cell function.5 Model mechanisms of Al3+ toxicity has been proposed that A1 stimulates the NADPH oxidase and induces the generation of superoxide (02j that triggers the influx of calcium ion (Ca2+). The resultant reactive oxygen species (ROS) and cytosolic free Ca2+ concentration elevation may lead to development of phytotoxicity.6... 

Fig. 2. MTs in cells of ev. Alarik maize roots after their treatments either with the auxin transport inhibitor naphthylphthalamic acid (NPA) (100 pM, 6 h) (a-g) and with lAA (100 pM, 24 h) (h-j). After NPA treatment, periclinal divisions were induced in the outer cortex (a, thin arrows indicate young cell walls, thick arrow indicates pre-prophase band of MTs). Cells of the epidermis preserved well-ordered transverse CMTs both near the root apex (b) and in the transition zone (g). On the other hand, cells of the root cap (c), outer cortex (d), inner cortex (e) and of the stele parenchyma (f) showed disturbances to their general appearance and ordering (compare with Figs. 3c, d, g, h). Similarly to NPA treatment, relatively well-ordered transverse CMT arrays are still preserved in postmitotic cells of the epidermis (h) as well as in all cells of the former meristem (i,j) after the auxin treatment. Abbreviations EP, epidermis IC, inner cortex OC, outer cortex RC, root cap.

Auxin-transport inhibition was recently reported to interfere with the normal course of cellular development in the root apex. Roots of the tir3 mutant of Arabidopsis are believed to be affected in this way [79]. The primary root of the mutant has a meristem twice the length of wild-type (500 p,m compared to 250 p,m), suggesting that an altered auxin distribution within the root apex permitted more cells than normal to remain in a... 

Huang JW, Share JE, Grunes DL and Kochian LV (1992) Aluminum effects on ccdciumfluxes at the root apex of aluminum-tolerant and aluminum-sensitive wheat cultivars. Plant Physiol 98 230-237. 

Assuming 1 1 FePS uptake, daily peak uptake rates of Fe, and hence PS, just behind the root apex in barley have been found to be —15% of the PS... 

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