Biosynthesis of camalexin from tryptophan pathway intermediates in cell-suspension cultures of arabidopsis

Camalexin (3-thiazol-2'-yl-indole) is the principal phytoalexin that accumu lates in Arabidopsis after infection by fungi or bacteria. Camalexin accumu lation was detectable in Arabidopsis cell-suspension cultures 3 to 5 h afte r inoculation with Cochliobolus carbonum (Race 1), and then increased rapid ly from 7 to 24 h after inoculation. Levels of radioactivity incorporated i nto camalexin during a 1.5-h pulse labeling with [C-14]anthranilate also in creased with time after fungal inoculation. The levels of radioactive incor poration into camalexin increased rapidly between 7 and 18 h after inoculat ion, and then decreased along with camalexin accumulation. Relatively low l evels of radioactivity from [C-14]anthranilate incorporated into camalexin in the noninoculated controls. Autoradiographic analysis of the accumulatio n of chloroform-extractable metabolites labeled with [C-14]anthranilate rev ealed a transient increase in the incorporation of radioactivity into indol e in fungus-inoculated Arabidopsis cell cultures. The time-course measureme nt of radioactive incorporation into camalexin during a 1.5-h pulse labelin g with [C-14]indole was similar to that with [C-14]anthranilate. These data suggest that indole destined for camalexin synthesis is produced by a sepa rate enzymatic reaction that does not involve tryptophan synthase.