| Abstract: | Hippeastrum x hortorum bulbs produce red pigments upon wounding. Analysis by TLC, HPLC, MS and NMR revealed that this pigment, which is absent in unwounded tissue, is a mixture composed of an orange-coloured chalcone (3,2′4′-trihydroxy-4-methoxychalcone) and 3 flavans (7,4′-dihydroxy-8-methylflavan, 7,3′-dihydroxy-4′-methoxyflavan and 7-hydroxy-3′-4′-methylenedioxyflavan). The colourless flavans can be oxidised to red-coloured dimers or polymers. The induction of these “phytoalexins” is achieved by wounding and can be further amplified by biotic and abiotic elicitors; e.g. cell walls of yeast appeared to be a potent elicitor. Immediately after wounding or elicitation the apparent activities of phenylalanine ammonium lyase (PAL) and peroxidase, which are probably involved in the biosynthesis of the red pigments increase substantially (both enzymes are hardly measurable in unwounded tissue) and reach a transient maximum after 2 ? 3d whereas pigment formation becomes visible after 2d and reaches a maximum after 9 to 12 d. Since cycloheximide inhibits the formation of the phytoalexins, a de novo synthesis of the corresponding enzymes is likely. Total phytoalexin extracts showed an inhibition of bacterial growth (e.g. of Bacillus subtilis, B. megaterium) and of feeding by polyphagous larvae of the moth Syntomis mogadorensis. These data imply that the wounding- and elicitor-induced phytoalexins appear to function as defence compounds against microorganisms and herbivores. |
