Induction of Phytoalexin Formation in Suspension-cultured Rice Cells by N-Acetyl-chitooligosaccharides

Abstract

Induction of phytoalexin formation in suspension-cultured rice cells by a series of N-acetylchitooligosaccharides and chitooligosaccharides was studied. N-acetylchitooligosaccharides larger than hexaose induced the formation of momilactones A and B as well as oryzalexins A, B, and D at very low concentrations like 10^{− 9}–10^{− 6} M (N-acetylchitoheptaose). GlcNAc oligomers smaller than trimers had almost no activity and a series of deacetylated chitooligosaccharides were also inactive. Strict requirement for the size and structure of GlcNAc oligomers as well as the sensitivity to them strongly indicates the presence of recognition systems specific for these compounds in rice cells. The level of momilactone A produced reached 100–500 μg/g of cultured cells, which appeared to be enough to prevent the growth of pathogenic fungi such as Pyricularia oryzae, thus indicating the importance of this phenomenon in the defense systems of rice plants. Suspension-cultured cells obtained only from a suitable period of cultivation, mainly those from lag phase, could respond to the elicitor and produce phytoalexins.