Elicitor induction of S-adenosyl-L-methionine: Anthranilic acid N-methyltransferase activity in cell suspension and organ cultures of Ruta graveolens L.

Abstract

Suspension-cultured cells of Ruta graveolens respond with rapid accumulation of acridone epoxides when treated with autoclaved culture homogenate of Rhodotorula rubra. Here the effect of elicitation on S-adenosyl-L-methionine:anthranilic acid N-methyltransferase (NMT), the first specific step in acridone alkaloid biosynthesis, was investigated. In dark-grown and in light-grown cultures, NMT activity remained at low levels (0.1–1 μkat kg-1 protein) over the entire culturing period of 35 days. Elicitation caused a rapid transient increase of NMT activity up to levels of 9–15 μkat kg-1 for light-grown cultures and 16–22 μkat kg-1 for dark-grown ones. Correlation of elicitor dose and induced NMT activity showed saturation rather than an optimum. Culture age and elicitor concentration strongly affected the return of NMT activity to the constitutive level. Addition of inhibitors of transcription or translation suppressed induction of both NMT activity and acridone epoxide accumulation by 50% (actinomycin) and 90–100% (cycloheximide), respectively. Of all the different suspension and organ cultures tested, roots, the site of high constitutive acridone epoxide accumulation, possessed the highest constitutive NMT activities. Different suspension cultures lines, shoot teratomas, hairy root cultures, and roots and shoots of hydroponically and sterile grown plants all showed increased NMT activity upon elicitation. Roots, however, showed the weakest response and, in contrast to all the other systems, induction was not followed by an increase in acridone epoxide content. Tissue-specific acridone epoxides may function as constitutive metabolites or as phytoalexins.