Signal Transduction and Gene Expression Induced by Oligosaccharides in Plants

Some kinds of oligosaccharides are recognized by plant cells and induce a set of defense reactions such as membrane depolarization, ion fluxes, production of reactive oxygen species (ROS) and activation of defense-related genes.

The earliest event so far reported is the membrane depolarization observed in the root cells of alfalfa treated with nod factor produced by Shinorhizobium meliloti, which nodulates on alfalfa roots, and in suspension-cultured rice cells treated with N-acetylchitooligosaccharides. This was observed within seconds after treatment with oligosaccharides. The mechanisms and role of membrane depolarization in oligosaccharide signaling are still unclear.

In many experimental systems, including suspension-cultured tomato cells treated with N-acetylchitooligosaccharides (DP=4, 5) and soybean cells treated with heptaglucoside, occurrence of the influx of Ca2+ and H+, and efflux of K+ and Cl- has been reported. Among these, Ca2+ influx resulting in the elevation of cytoplasmic [Ca2+] is considered to play a crucial role in the signal transduction, because removal of Ca2+ from the medium or addition of Ca2+ channel blocker completely cancels the following response such as production of reactive oxygen species or gene expression. In tobacco cells, cytoplasmic acidification was observed in a transient manner when treated with purified polygalacturonic acids, whereas it was in observed to be when treated with the cell wall fraction of a pathogenic fungi, Phytopthora glycinea, which did not induce change in cytoplasmic pH in suspension-cultured soybean cells. Treatment of alfalfa root cells with nod factor induced cytoplasmic alkalinization, indicating the occurrence of H+-efflux.

N-acetylchitooligosaccharides are potent elicitors against suspension-cultured rice cells, inducing all sets of defense reactions described above. The production of ROS and gene activation require extracellular Ca2+, indicating the occurrence of Ca2+ influx at an early stage of signal recognition. It has been shown that, in rice cells, the cytoplasmic acidification due to the H+ influx is the trigger for the induction of some specific genes. Addition of calyculin A, an inhibitor of protein phosphatases, also activated the same genes and induced the cytoplasmic acidification, whereas k-252a, an inhibitor of protein kinases, inhibited both cytoplasmic acidification and gene activation. Based on these results, the hypothetical role of cytoplasmic acidification as a signal to the set of genes was proposed.

Oligosaccharides with elicitor activities induce the production of ROS. ROS are considered to be a signal to the activation of genes involved in the detoxification of ROS and the production of antimicrobial substances, phytoalexins, as well as the material for the crosslinking of the cell wall protein and biosynthesis of lignins, resulting in the construction of physical barriers against pathogen attack.

Defense reactions such as production of ROS or gene activation are canceled in the presence of protein kinase inhibitors such as k-252a or staurosporin. In contrast, the addition of protein phosphatase inhibitor such as calyculin A or cantheridine induces these responses without oligosaccharides. These observations lead us to speculate that protein phosphorylation/dephosphorylation cycles are essential to signal transduction. In some experimental systems, rapid and transient activation of MAP-kinase activities has been reported.
Figure 1
Eiichi MINAMI (Department of Biotechnology, National Institute of Agrobiological Resources)
References (1) RA, Dixon, MJ, Harrison, CJ, Lamb : Early events in the activation of plant defense responses. Annu. Rev. of Phytopathol. 32, 479-501, 1994
(2) CA, Ryan, EE, Farmer : Oligosaccharide signals in plants: A current assessment. Annu. Rev. Plant Physiol. Mol. Bio. 42, 651-674, 1992
(3) J, Ebel, A, Mithofer : Early events in the elicitation of plant defense. Planta, l206, 335-348, 1998
Jun. 15, 2000

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