The enzyme tryptophan decarboxylase (TDC) (EC 4.1.1.28) converts tryptophan into tryptamine, and thereby channels primary metabolites into indole alkaloid biosynthesis. The production of these secondary metabolites in suspension cells of Catharanthus roseus depends on medium composition. Of the possible variables, we investigated the effect of hormones on the expression of the tdc gene in cell cultures. Omission of NAA from the growth medium resulted in accumulation of tdc mRNA. The addition of 1-naphthaleneacetic acid (NAA), indoleacetic acid (IAA) or 2,4-dichlorophenoxyacetic acid (2,4-D) rapidly reduced the enhanced tdc transcript level. Cytokinin was unable to suppress the enhanced transcript level. Hairy roots transformed by Agrobacterium rhizogenes also showed a reduction of the tdc mRNA level after NAA addition. Run-off transcription experiments showed that the down-regulation takes place at the transcriptional level within 15 minutes and independent of de novo protein synthesis. Thus one of the mechanisms which control the activity of terpenoid indole alkaloid biosynthesis in C. roseus cell cultures is the negative regulation by auxin of the gene involved in the first committed step.

In culture, Phytophthora fungi - except P. nicotianae - secrete proteins, called elicitins, which cause necrosis on the leaf of the non-host tobacco (Nicotiana tabacum L.) at a distance from the inoculation site, and are responsible for the incompatible reaction. Cryptogein and capsicein are elicitins secreted by P. cryptogea and P. capsici, respectively, and form part of a novel family of 10-kDa holoproteins. On tobacco, the necrotic activity of cryptogein is approx. 100-fold higher than that of capsicein. Using elicitins radioactively labelled in vivo, we have demonstrated that cryptogein and capsicein (i) move from a wound in the stem towards the leaves where they interact directly, (ii) reach their target without undergoing any molecular alteration, (iii) are carried in, and at the same rate as, the sap flow in the xylem, (iv) do not alter the rate of the xylem flow although they are able to provoke drastic damage to the lamina. Consequently, the remote necrotic activity of elicitins does not require any transportable secondary plant elicitor, so the differences in necrotic properties should be due to structural features involved in the interaction of elicitins with the leaf target cells.