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Phenolic metabolism in higher plants : I. Catechol biogenesis in Gaultheria, II. The biogenesis of rosmarinic acid in Mentha, III. Degradation of aromatic compounds by sterile plant tissues Ellis, Brian Edward

Abstract

I. Previous studies on biogenesis of simple phenols in plants have been restricted to hydroquinone. Among the other simple phenols, catechol is of particular interest because of its potential role as a ring-cleavage substrate. Tracer studies on the biogenesis of catechol in Gaultheria leaf discs showed that it was formed from salicylic acid by oxidative decarboxylation. Salicylate decarboxylating activity could be detected in buffered extracts of very young leaves. II. Among the numerous caffeic acid esters presently known in plants, only 3-0-caffeoylquinic acid (chlorogenic acid) has been studied in detail. Rosmarinic acid (alpha-O-caffeoyl-3, 4-dihydroxyphenyllactic acid) has been reported to occur in a number of plants but nothing was known of its biosynthesis or metabolic role. Tracer studies demonstrated that in Mentha the caffeic acid moiety was formed from phenylalanine via cinnamic and para-coumaric acids. In contrast, the structurally similar 3, 4-dihydroxyphenyllactic acid moiety was formed from tyrosine and 3, 4-dihydroxyphenylalanine. There was no evidence of the participation of a para-coumaroyl ester intermediate. Time-course studies and use of labelled rosmarinic acid showed that endogenous rosmarinic acid was turning over slowly. The caffeoyl moiety, however, does not appear to be contributing to the formation of insoluble polymers, as has been suggested for chlorogenic acid in other plants. III. Bacteria and fungi readily degrade aromatic compounds to carbon dioxide. Despite the large quantities of aromatic compounds formed in plants, little attention has been paid to the ability of plant tissues to degrade aromatic rings. No reported studies have used completely sterile plants and techniques. This has left open the possibility that the microflora associated with the plant might be carrying out the observed reactions. The ability of sterile plant tissue cultures to degrade aromatic ring-¹⁴C compounds to carbon dioxide was studied. It was established that a number of tissues (Ruta, Triticum, Phaseolus, Melilotus) have the ability to cleave the aromatic ring of phenylalanine. Melilotus tissue could also degrade cinnamic acid-ring-¹⁴C suggesting that a dihydroxy phenolic acid may be the ring-cleavage substrate. Neither Ruta nor Melilotus tissues were able to degrade benzoic acid or salicylic acid-ring-¹⁴C. Tryptophan benzene ring-¹⁴C was shown to be degraded to carbon dioxide by both Ruta and Melilotus. In summary, the ability of plants to cleave the benzene ring of aromatic compounds when free of micro-organisms was thus established.

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