1.9.4S
PHYTOPHTHORA INFESTANS-STIMULATED BIOSYNTHESIS OF HYDROXYCINNAMIC ACID AMIDES IN SOLANUM TUBEROSUM AND CLONING OF A CDNA-ENCODING TYRAMINE HYDROXYCINNAMOYLTRANSFERASE (THT)

D STRACK, A SCHMIDT, D SCHEEL and S ROSAHL

lnstitut fur Pfianzenbiochemie (IPB), Weinberg 3, D-06120 Halle (Saale), Germany

Background and objectives
Previous work from our laboratories described changes in the accumulation of phenylpropanoids in Phytophthora infestans (Pi)-elicitor-treated cells and Pi-infected leaves of potato [1]. One of the enzymes that catalyses the formation of hydroxycinnamoyltyramines (4coumaroyl- and feruloyltyramine in our potato cell cultures), hydroxycinnamoyl-CoA:tyramine hydroxycinnamoyltransferase (THT; EC 2.3.1.110), has been purified and characterised [2, 3]. This enzyme has extensively been studied with regard to biotic and abiotic elicitor- and stress-stimulated activity increases in tobacco and other plants [4]. It is well known that the enzymic products, tyramine amides, are frequently incorporated into the cell walls.

We report here elicitor-induced changes in the accumulation pattern of hydroxycinnamic acid tyramine amides in suspension cultures of potato (Solanum tuberosum), increases in the activities of the biosynthetic enzymes, and cloning of a CDNA encoding the THT.

Results and conclusions
Treatment of suspension-cultured potato cells with an elicitor from Pi induced the biosynthesis of various soluble and cell wall-bound as well as secreted phenylpropanoids including hydroxycinnamic acid amides. Induced metabolite accumulation was preceded by rapid and transient increases in activities of phenylalanine ammonia-lyase (EC 1.11.1.7) and tyrosine decarboxylase (EC 4.1.1.25). THT increased later and remained at high levels [5]. The enzyme catalysed the formation of 4-coumaroyl- and feruloyltyramine along with some other amides, such as octopamine and normetadrenaline conjugates. Some of these phenolic defense compounds appear to be involved in cell wall reinforcement and may furthermore directly affect fungal growth in the apoplastic space.

A CDNA library constructed from RNA of elicited potato cells was screened with a THTspecific PCR fragment as a probe and several THT-specific cDNAs were isolated. One clone was used for expression of THT in E. coli. Northern analyses showed that in potato cell cultures, elicitor-induced THT transcript accumulation correlated well with the development of enzyme activity. Genomic Southern analyses indicated that in potato THT is encoded by a multigene family.

Future studies will have to show whether THT-transgenic potato plants, expressing increased THT activity, show increased amounts of cell wall-bound hydroxycinnamic acid amides and thus increased resistance against Pi.

References
et al., 1996. Phytochemistry 42, 389-396.
2. Hohifeld H, Schormann W, Scheel D, Strack D, 1995. Plant Physiology 107, 545-552.
3. Hohifeld H, Scheel D, Strack D, 1996. Planta 199, 166-168.
4. Negrel J, Javelle F, 1997. European Journal of Biochemistry 247, 1127-1135.
5. Schmidt A, Scheel D, Strack D, 1998. Planta (in press).