Pericycle, root

The first pair of examples we would like to discuss occurs in a field which lends itself naturally to be conquered by theory. Indeed, the past three decades have seen the exploration of mechanistic details of pericyclic reactions as one of the major success stories of computational chemistry. Rooted in qualitative molecular orbital theory, the key concept of... 

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. 

SUZUKI, K YAMADA, Y, HASHIMOTO, T Expression of Atropa belladonna putrescine jV-methyltransferase gene in root pericycle. Plant Cell Physiol., 1999, 40,289-297. 

HASHIMOTO, T., HAYASHI, A., AMANO, Y, KOHNO, J., IWANARI, H., USUDA, S., YAMADA, Y., Hyoscyamine 6P-hydroxylase, an enzyme involved in tropane alkaloid biosynthesis, is localized at the pericycle of the root. J. Biol. Chem., 1991,266, 4648-4653. 

SUZUKI, K., YUN, D.-J., CHEN, X.Y., YAMADA, Y., HASHIMOTO, T., An Atropa belladonna hyoscyamine 6-P-hydroxylase gene is differentially expressed in the root pericycle and anthers. Plant Mol. Biol, 1999, 40, 141-152. 

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). 

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. 60.—Cross-section of a young root of Phaseolus muHi-florus. A, pr, cortex m, pith X, stele or central cylinder—all tissue within the pericycle, inclusive g, primary xylem bundles b, primary phloem bundles. B, cross-section of older portion of root lettered as in A b, secondary phloem, k, cork. (Stevens, after Vines.)... 

Putrescine A-methyltransferase (PMT, EC 2.1.1.53) catalyses the first specific step in the biosynthesis of tropane alkaloids, cocaine and nicotine [123]. Putrescine is methylated by PMT via SAM (S-adenosylmethionine) transferring the methyl group from SAM to an amino group of putrescine. Fig. (1). This enzyme has been isolated from roots of both Nicotiana tabacum and D. stramonium [124], and the activity of this enzyme is restricted to the roots of Solanaceous species corroborated by results describing a root pericycle-specific activity in A. belladonna [125]. Nevertheless, more recently, a low mRNA pmt transcript level in leaves of N. tabacum, with a rise in transcript level after mechanical wounding has been detected [126]. 

In resistant hosts, pericycle cells near the head of the sedentary animal become necrotic, and a marked reduction in egg laying is seen (both in numbers of egg masses and in eggs/mass). Thus, reductions in eggs (or egg masses) per gram of root is normally used as evidence of resistance (47). 

Concentrations of constitutive terpenoids in the root epidermis of cotton are unrelated to differences in resistance. But concentrations of terpenoid aldehydes formed in the vicinity of the pericycle, near the head of the animal, act as phytoalexins and are closely correlated with levels of resistance (11, 48). Little or no phytoalexin is formed in the pericycle of susceptible cultivars. Mixtures of terpenoid phytoalexins are more toxic to the nematode than gossypol alone, and mixtures containing methylated terpenoid phytoalexins (from 6. hirsutum) are more toxic than those that contain only nonmethyTated phytoalexins (from 6. arboreum) (49). Thus, the structure of terpenoid phytoalexins also is importan for resistance to root knot nematode. 

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).

Fig. 5. Vertical and horizontal sections of a pea nodule approximately 4 weeks old and 3 mm in length attached to a root which is also sectioned. Root vascular strand, RV root peiicycle, RP root endodermis, RE root cortex, RC nodule cortex, NC nodule vascular strand, NV nodule pericycle, NP nodule vascular strand endodermis, NVE nodule endodermis, NE nodule meristem, M infection zone, I early symbiotic zone, ES late symbiotic zcme, LS senescent zone, Sn nodule surface, NS. (Drawn after Bond, 1948, and from sections from Newcomb, 1976, and Syono et al., 1976.)...

PMT and H6H, which catalyze the first and last steps, respectively, in the biosynthesis of the tropane alkaloid scopolamine (Scheme 3), were localized to the pericycle in the roots of A. belladonna and Hyoscyamus muticus (Fig. 2A) 132,145). PMT also catalyzes the first step in nicotine biosynthesis (Scheme 3) and has been localized to the endodermis, outer cortex, and xylem in N. sylvestris (244,245). In contrast, TR-I, an intermediate enzyme in the tropane alkaloid pathway, resides in the endodermis and nearby cortical cells (Fig. 2A) (135) thus, intermediates of tropane alkaloid metabolism must also be transportai between cell types. The biosynthesis and storage of acridone alkaloids were also associated with endodermis in Ruta graveolens (246). 

The use of undifferentiated cultures proved to be unsuccessful for tropane alkaloids production. These alkaloids are produced in normal and transformed roots [42, 51]. Several lines of evidence suggest that the differentiation of the tissue is necessary for the synthesis of these metabolites [42, 51]. Different studies suggest that this is probably related to the localization of key biosynthetic enzymes [6]. Among them, Suzuki et al. [52, 53] demonstrated that the h6h and pmt genes were expressed specifically in root pericycle of Atropa belladonna plants. In addition, Nakajima et al. [54, 55] pointed out that Tropinone reductase enzymes were accumulated in lateral roots of Hyoscyamus niger. 

Tropane alkaloids are synthesized in root pericycle and translocated to leaves [41]. For this reason, the investigations on scopolamine production were focused on the hairy root cultures of several Solanaceous plants [31, 42-45]. However, the yields obtained to date were not suitable for a commercial production in order to replace the extraction from the natural producer plants [46]. 

PMT expression occurs in the root pericycle of A. belladonna L. and H. muticus L. [2, 36, 37] and in endodermis, outer cortex, and xylem in Nicotiana sylvestris L. [36, 37]. The localization of PMT to the pericycle would facilitate the access to arginine 12 or ornithine 8 [2,6]. In the same way, A -methylputrescine 13 or some biosynthetic intermediate could then be transported to the endodermis for further elaboration into tropine 27 [6]. 

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