Endodermis, root

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. 

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... 

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. 

Several different tissue types - epidermis, endodermis, laticifers, idioblasts, pericycle, and cortex — have now been implicated in the biosynthesis and/or accumulation of various alkaloids in plants. Recently, we have localized berberine in the endodermis of Thalictrum flavum roots at the onset of secondary growth.150 Rather than being sloughed off, the endodermis was found to undergo extensive anticlinal division leading to an expanding cellular cylinder that ultimately displaced all external tissues. Endodermal-specific berberine accumulation continued throughout root development, but was extended to include 3 to 4 layers of smaller pericycle cells in the oldest roots near the base of the stem. The cell type-specific accumulation of an antimicrobial alkaloid and the unusual development of the endodermis and pericycle in T. flavum roots are consistent with the putative role of berberine in plant defense. 

Sedimentation of amyloplasts within the cell has been correlated with the capacity of the plant to perceive gravity. The buoyant mass of amyloplasts present in specialized cells in the center of the root cap and in the stem (depending on the plant species, in the endodermis, the bundle sheath, or in the parenchyma to the inside of the vascular bundle) would allow the amyloplasts to sediment inside the cell, where the cytosol would have a relatively low viscosity. This sedimentation would translate into a signal of an unknown nature, maybe through pressure onto a sensitive part of the cell or acting as a mechano transducer, etc. Whatever the nature of the signal, it eventually results in the asymmetry of the organ and its curvature. The isolation of starchless mutants of Arabidopsis thaliana and Nicotiana sylvestris has made... 

Immediately inside the endodermis is the pericycle, which is typically one cell thick in angiosperms. The cells of the pericycle can divide and form a meristematic region that can produce lateral or branch roots in the region just above the root hairs. Radially inside the pericycle is the vascular tissue. The phloem generally occurs in two to eight or more strands located around the root axis. The xylem usually radiates out between the phloem strands, so water does not have to cross the phloem to reach the xylem of a young root. The tissue between the xylem and the phloem is the vascular cambium, which through cell division and differentiation produces xylem (to the inside in stems and older roots) and phloem (to the outside in stems and older roots). 

In the roots of many species a siibepidennal layer or layers of hypodermis occur. Radial walls of hypodennal cells can also be blocked with a waxy material analogous to the Casparian strip in the endodermis, in which case the layers are often termed an exodermis. 

C. Suppose that the root xylem is arranged concentrically in a ring 500 pm below the root surface. If the average conductivity of the epidermis, cortex, endodermis, and xylem cell walls in the root is like that of the dry soil, what is the decrease in hydrostatic pressure across them ... 

Nicotiana alkaloids, which serve as chemical defence compounds, are synthesized in the roots and are transported to other plant organs, such as aerial parts, via the xylem. These alkaloids accumulate in vacuoles. PMT and A622 oxidoreductase are strongly expressed in the endodermis and outer cortex cells of tobacco root tips and to a lesser degree in other parts of the cortex and parenchyma cells surrounding the xylem (Shoji et ah, 2002). The localization of nicotine biosynthesis in the parenchyma cells surrounding the xylem may aid the loading of the xylem with nicotine. 

Genes of protoberberine biosynthesis are abundantly expressed in rhizomes of Thalictrum flavum, but were also active in roots and other organs (Samanani et al. 2005). In roots, transcripts were localized in the immature endodermis and root pericycle. In rhizomes transcripts were found in the protoderm of leaf primordial. As known from other plants, these data show that the sites of synthesis are not identical with the sites of accumulation. In many instances, a long-distance transport must occur. If this is the case, alkaloids have to pass several biomembranes. ABC-transporters and H+-alkaloid antiporters can be involved (see Chapter 1). 

Pig. 55.—Cross-section through a portion o a root of A corns calamus. A, Cortical parenchyma Bt endodermis C, pericycle phloem F, xylem. At F Y, are large tracheal tubes, which were formed last, the narrow tubes near the periphery of the xylem being formed first. At the center of the root, within the circle of the radial vascular bundle, occur thin-walled parenchymatous pith cells. (From Sayre after Frank.)... 

Pig. 61.—Photomicrograph of a transverse section of a California Privet root of primary growth showing epidermis (e) hypodermis (A) cortex (c) endodermis (en) pericambium (p) a xylem arm of the radial bundle (f) and pith (m). 

Note that this is somewhat larger in diameter. Observe the root hairs starting from the epidermis a broad cortex a large clear and open looking endodermis then pericambium next, a central patch of xylem showing a faint pentarch relation. Pushed out are five... 

Fig. 63.—Transverse section of California Privet root made about an inch and a half above the section shown in Pig. 6l and showing secondary structure. Note that epidermis, primary cortex and endodermis have completely disappeared. Cork (cfe) phellogen -ph) secondary cortex (rc) protophloem p ) secondary phloem cambium (c) secondary xylem ( ) and protoxylem ( ).

D. Make a permanent mount of a fourth T . S. cut through the same root some distance above the third. Note that the epidermis, primary cortex and endodermis have completely peeled off. Cork is found as the external bounding layer and underneath it, cork... 

Bark, wound callus, and specialized tissues such as the endodermis that controls entry into the root vascular system, have walls lined with suberin. Suberin, like cutin, is a... 

Fig. 2. Part of a longitudinal section of young root of Solanum tuberosum, at the point of origin of a lateral root p pericycle end endodermis pc inner layer of cortical parenchyma. Shading indicates the presence of solanine in the cells. From Molle (6).

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