Elicitor induced secondary metabolism in Ruta graveolens L

Abstract

In vitro cultures of Ruta graveolens L. respond with rapid accumulation of acridone epoxides, furoquinolines and furanocoumarins, when challenged with autoclaved homogenate of the yeast Rhodotorula rubra. A transient increase of several enzymes of the respective biosynthetic pathways was measured but we still look for the key regulatory enzymes. We investigated whether the branch point enzymes of the shikimic acid pathway anthranilate synthase (AS) and chorismate mutase (CM) possibly play such a role. The two enzymes compete for chorismate. AS forms anthranilate, the precursor amino acid of acridone and furoquinoline alkaloids. CM channels chorismate into phenylalanine, tyrosine and phenylpropanoid biosynthesis. Elicitation resulted in a transient increase of the activity of both enzymes. Relative induction rates were 2--4 fold for AS and about 1.5 fold for CM. Constitutive CM activity, however, is about 1000 fold higher than AS activity. As in other plants 2 isoforms of CM are expected to be present in R. graveolens. A differential determination of the activity of the isoforms via the tryptophan activation rate proved to be ambiguous. Some evidence for the specific induction of a plastidic form of CM was obtained by inhibition of translation. The time courses of CM induction show CM not to be a key enzyme in elicitor induction of furanocoumarin accumulation. In comparison to other enzyme activities induction of anthranilate synthase activity corresponds closest to inducible acridone epoxide accumulation indicating a key role in its regulation. Induction of AS and CM was inhibited by actinomycin D and chloramphenicol while cycloheximid inhibited AS induction only.