Tissue Distributions of Dhurrin and of Enzymes Involved in Its Metabolism in Leaves of Sorghum bicolor

Abstract

The tissue distributions of dhurrin lphydroxy-(S)-mandelonitrhie8-Dplucosidel and ofenzymes involved in its metaboUsm have been investigated in leaf blades of light-grown Sorghu bicolor seedlngs. Enzymk digestion of these leaves using celulase has enabled preparations of epidermal and mesophyli protoplasts and bundle sheath strands to be isolated with only minor cross-contamination. Dhurrin was located entirely in the epidermal layers of the leaf blade, whereas the two enzymes responsible for its catabolism, namely dhurrin ,-glucosidase and hydroxynitrile lyase, resided almost exclusively in the mesophyli tissue. The final enzyme of dhurrin biosynthesis, uridine diphosphate glucose;p-hydroxymandelonitrile glucosyltransferase, was found in both mesophylH (32% of the total activity of the leaf blade) and epidermal (68%) tissues. The bundle sheath strands did not contain significant amounts of dhurrin or of these enzymes. It was concluded that the separation of dhurrin and its catabolic enzymes in different tissues prevents its large scale hydrolysis under normal physiological conditions. The well documented production of HCN (cyanogenesis), which occurs rapidly on crushing Sorghum leaves, would be expected to proceed when the contents of the ruptured epidermal and mesophyll cells are allowed to mix.

Variability of expression of cyanogenesis in white clover (Trifolium repens L.)

“…It is known that the linamarase is located in the cell walls of epidermal cells in the leaves of T. repens (Boersma et a!., 1983;Kakes, 1985). It is only surmise from other work that the cyanoglucosides may be stored in the vacuoles of these cells (Kojima et a!., 1979;Cutler and Conn, 1982).…”

Tissue Distribution and Subcellular Localization of Prephenate Aminotransferase in Leaves of Sorghum bicolor

“…Protoplasts were obtained using a modification ofpreviously described procedures (11,14). Leaves from 6-d-old seedlings were abraded with carborundum, then incubated for 3 h at 30°C in a solution of 1.5% (w/v) Cellulysin (Calbiochem) in Mes-mannitol buffer (20 mM Mes [pH 5.6] and 0.6 M mannitol).…”

“…Bundle sheath strands were prepared by previously described procedures (11). Strands of bundle sheath were lightly homogenized with a Teflon pestle to remove mesophyll contaminants.…”

Chemical Defence and Toxins of Plants

“…In the shoot of sorghum, Sorghum bicolor, dhurrin is contained in the vacuoles of epidermal cells, whereas -glucosidase is kept in the chloroplasts of mesophyll cells and hydroxynitrile lyase is kept in the cytosol of mesophyll cells. 37 In cassava, Manihot esculenta, linamarin and lotaustralin are widely distributed among tissues, 38 whereas linamarase is concentrated in laticifer and latex 39 and hydroxynitrile lyase is concentrated in the cell wall of leaf tissues but absent in the root. 40 As hydrogen cyanide is highly toxic to most animals and insects, cyanide glycoside-borne defense systems are effective against most herbivores.…”

“…Such compartmentation and enzymatic activation of precursors are commonly observed in several different groups of secondary metabolites, 103 which includes the activation of cyanide glycosides by -glucosidase and hydroxynitrile lyase that produce toxic hydrogen cyanide in many plants such as sorghum, clover, and almond 34,35,[37][38][39][40] (Figure 4), the activation of glucosinolates by thioglucosidase that produce toxic isothiocyanates or nitriles in plants belonging to the order Brassicales [43][44][45] (Figure 5), the activation of O-coumaric acid by -glucosidase that produces coumarin in white melilot (Meliotus alba), 103 the activation of phenolics by PPO that produce highly reactive quinones in many plants 55 (Figure 7), and the activation of an iridoid glycoside, oleuropein, by -glucosidase 73 (Figure 12).…”