1.4.8
DODECAN-1-OL, A NEW CUCUMBER CUTIN MONOMER, ELICITS DEFENCE REACTIONS IN CUCUMBER HYPOCOTYLS EXHIBITING SYSTEMIC ACQUIRED RESISTANCE

M FAUTH, J BECKER and H KAUSS

Universität Kaiserslautern, FB Biologie, Erwin-Schrödinger-Strasse, D-67663 Kaiserslautern, Germany

Background and objectives
The cuticle is a constitutive barrier protecting plants against environmental stress, including pathogens. Even though it is known that fungal pathogens can recognize the cuticle by its wax constituents and cutin monomers, little is known about the role of the cuticle for recognition of attacking pathogens by the plant. It was recently demonstrated that hypocotyls of etiolated cucumber (Cucumis sativus) seedlings that acquired resistance by treating the roots with 2,6-isonicotinic acid (DCIA, INA) can express the class-III chitinase only when the pathogen Colletotrichum lagenarium adheres and germinates on the epidermal surface [1]. It has been speculated that the growing fungal spores generate - before penetration - a lipophilic signal compound, resulting in recognition of the pathogen by the epidermal cells. As the apoplastic chitinase accumulates at the site of an attempted penetration, it could contribute to defence. It was also shown (Fauth M et al., unpublished data) that the alkaline hydrolysate of cucumber cutin contains few classical cutin monomers (hydroxy fatty acids), but is rich in dodecan-1-ol (DDO). Both types of cutin monomers have now been used as elicitors of hydrogen peroxide and of chitinase expression in cucumber hypocotyls which were gently abraded at their surface to allow permeation of elicitors.

Results and conclusions
Freshly abraded segments, even from resistant hypocotyls, are not constitutively competent for hydrogen peroxide elicitation by a polymeric elicitor from fungal walls [2]. This observation was confirmed for another six plant species, and shows that plant tissues are clearly distinct from suspension cultures. As studied in detail for cucumber, hydrogen peroxide elicitation competence develops only after abrasion, and it is this process which is enhanced by resistance inducers such as DCIA and a benzothiadiazol (BTH). The requirements of both competence induction and its enhancement in resistant cucumber hypocotyls were also shown for elicitation by hydroxy fatty acids. In this case, hydrogen peroxide production is inhibited by diphenyleneiodonium (DPI) and the protein kinase inhibitor K-252a, indicating the involvement of the NAD(P)H-oxidase complex, a mechanism suggested for many suspension cultures. In contrast, DDO elicits hydrogen peroxide also in freshly abraded hypocotyls. This constitutive hydrogen peroxide elicitor mechanism is not sensitive to the above inhibitors and, therefore, based on other enzymes. As KCN and sodium azide are also not inhibitory, peroxidases appear not to be involved.

DDO also elicits chitinase expression in the resistant abraded cucumber hypocotyls. In addition, DDO provides protection against Colletotrichum lagenarium when sprayed as an aqueous suspension on leaves several days prior to infection. Taken together, these results suggest that monomers generated at the site of fungal attack could play a role as signals in the cucumber/pathogen interaction and in disease resistance.

References
1. Kästner B, Tenhaken R, Kauss H, 1998. Plant Journal 13, 101-108.
2. Fauth M, Merten A, Hahn M et al., 1996. Plant Physiology 11, 347-354.