Camalexin (3-thiazol-2'yl-indole), an indolic secondary metabolite, is a major phytoalexin in Arabidopsis. Camalexin accumulation caused by infection with plant pathogens and by abiotic elicitors inhibits the growth of pathogenic fungi and bacteria.

The indole ring of camalexin is proposed to be derived from indole-3-glycerol phosphate, an intermediate in tryptophan biosynthesis. The accumulation of this phytoalexin is accompanied by the induction of mRNAs and proteins of all of the tryptophan biosynthetic enzymes. Salicylic acid is required, but is not sufficient for this co-ordinated induction. Surprisingly, however, camalexin induction was only modestly affected in the trp5, trp4, trp1 and trp4:trp1 double mutants (Zhao and Last, 1996). This suggests either that the indole ring of camalexin may be derived from biosynthetic pathways other than tryptophan, or that there may be other isoenzymes important for the production of this secondary metabolite (cf. Ruta graveolens, where fungal elicitors induce a tryptophan-insensitive ASa gene (Bohlmann et al, 1996)). The latter explanation seems unlikely because trp1 is defective in what appears to be the only gene encoding PAT in Arabidopsis (Zhao and Last, 1996). The tryptophan pathway enzymes and camalexin accumulation are induced in response to amino acid starvation in Arabidopsis (Zhao et al, 1998).

Camalexin incorporates label from anthranilate but not from tryptophan in Arabidopsis, suggesting that the camalexin biosynthetic pathway originates from an intermediate of the Trp pathway between anthranilate and indole (Zook and Hammerschmidt, 1997). Zook (1998) has postulated that indole destined for camalexin synthesis is produced by a separate enzymatic reaction that does not involve tryptophan synthase.

Cysteine serves as the precursor of the thiazole ring of camalexin (Zook and Hammerschmidt, 1997). Condensation of indole-3-carboxaldehyde with cysteine is proposed as the first step in this pathway (Zook and Hammerschmidt, 1997).

Bohlmann J, Lins T, Martin W, Eilert U 1996 Anthranilate synthase from Ruta graveolens: duplicated ASa genes encode tryptophan-sensitive and tryptophan-insensitive isoenzymes specific to amino acid and alkaloid biosynthesis. Plant Physiol. 111: 507-514.

Zhao J, Last RL 1996 Coordinate regulation of the tryptophan biosynthetic pathway and indolic phytoalexin accumulation in Arabidopsis. Plant Cell 8: 2235-2244.

Zhao J, Williams CC, Last RL 1998 Induction of Arabidopsis tryptophan pathway enzymes and camalexin by amino acid starvation, oxidative stress, and an abiotic elicitor. Plant Cell 10: 359-370.

Zook M 1998 Biosynthesis of camalexin from tryptophan pathway intermediates in cell-suspension cultures of Arabidopsis. Plant Physiol. 118: 1389-1393.

Zook M, Hammerschmidt R 1997 Origin of the thiazole ring of camalexin, a phytoalexin from Arabidopsis thaliana. Plant Physiol. 113: 463-468.