Oligosaccharide Elicitor Signaling in Plant-Pathogen Interactions

Elicitors are substances that can induce defense responses when applied to plant tissues or cultured-plant cells (oligosaccharides, glycoproteins, peptides and lipids). The well- studied oligosaccharide elicitors include oligoglucan, oligochitin, oligochitosan and oligogalacturonic acid.

When a plant that has developed a resistance mechanism is challenged by a pathogen, rapid and highly localized cell death (hypersensitive cell death) occurs at the attempted infection sites and a variety of biochemical defense responses occur in the surrounding cells. These include the production of reactive oxygen species, structural changes in the cell wall, accumulation of defense-related proteins and phytoalexin biosynthesis. Elicitors of these defense responses are known to be present in the culture filtrate of pathogens.

The culture filtrate of Phytophthora megasperma sojae, a fungal pathogen of soybean, has the ability to stimulate the accumulation of a soybean phytoalexin in soybean tissues. The elicitor turned out to be a component of the mycerial cell wall of the pathogen. The minimum structure required for the eliciting activity was a branched 1-3,1-6 hepta beta-glucoside. The half-maximum concentration necessary for the elicitation was as low as nM. This low concentration suggests that the hepta beta-glucoside acts as an infection signal and soybean has a mechanism to perceive and transduce this specific signal. The elicitor is effective not only in soybean but also in several other leguminous plants. Studies on the receptor of the elicitor are under way.

Chitin is another major component of the cell walls of various fungi. Fragments of chitin also stimulate defense responses in many plants. The chitin oligosaccharide (DP=7 and 8) induces phytoalexin biosynthesis in suspension-cultured rice cells and stimulates various early cellular responses such as membrane depolarization, a transient increase in ion efflux, alkalinization of the medium, generation of reactive oxygen species and protein phosphorylation as well as the expression of several unique early responsive genes. These early response may play important roles in the signal transduction pathway. The presence of a receptor for the elicitor is also suggested.

Chitosan oligosaccharides elicit accumulation of lignin, callose, phytoalexin, or protease inhibitors in various plant tissues and induces early cellular responses. The elicitor is effective in rather high concentration ranges and the mechanism of the elicitation is not clear.

The elicitors of the fungal cell wall are released from the wall by cell wall-degrading enzymes (chitinases, glucanases) secreted by the plants upon infection. On the other hand, oligogalacturonic acids are digest products of the plant cell wall, the result of microbial pectic enzymes secreted by pathogens. Oligogalacturonide (DP=10 to 12) induces phytoalexin in soybean tissues and stimulates lignin and protease inhibitor in several other plants. Plants are capable of responding not only to fragments of the pathogen but also to their own cell wall produced by the enzyme of the pathogen. Oligogalacturonides are effective on the concentration from microM to milliM and there is a synergistic effect when hepta beta-glucoside and oligogalacturonides are both applied to soybean tissues.

Yeast is not a plant pathogen, but when several peptides derived from yeast invertase were applied to cultured tomato cells, they induced ethylene. The peptide elicitor (gp8c) consists of 10 mannosyl residues attached to the Asn of a pentapeptide. The interesting characteristic of this elicitor is that the oligosaccharide moiety itself does not have eliciting activity but inhibited the eliciting activity of the intact gp8C. The oligosaccharides seem to act as suppressors to the elicitor.

Higher plants have developed complex mechanism of perception and transduction of pathogen signals against pathogen attack. It has been clearly proved that oligosaccharide elicitors have important functions in the plant-pathogen interactions.

Yuki Ito (Department of Biotechnology, National Institute of Agrobiological Resources)
References (1) A.G.Darvill and P.Albersheim : Phytoalexin and their elicitors---a defense against microbial infection in plants. Annu.Rev.Plant Physiol.,35, 243-275,1984,
(2) F.Cote and M.G.Hahn : Oligosaccharins:structures and signal transduction. Plant Mol. Biol., 26, 1375-1411,1994
(3) J.Ebel and A.Mithofer : Early events in the elicitation of plant defense. Planta, 206, 335-348,1998
Sep. 15, 1999

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