Differences in the recognition of glucan elicitor signals between rice andsoybean: beta-glucan fragments from the rice blast disease fungus Pyricularia oryzae that elicit phytoalexin biosynthesis in suspension-cultured ricecells

Partial acid/enzymatic hydrolysis of the beta-(1-->3, 1-->6)-glucan from th e cell walls of the rice blast disease fungus Pyricularia oryzae (Magnaport he grisea) released elicitor-active fragments that induced phytoalexin bios ynthesis in suspension-cultured rice cells. From the digestion of the gluca n by an endo-beta-(1-->3)-glucanase, one highly elicitor-active glucopentao se was purified as a reduced compound, tetraglucosyl glucitol. The structur e of this tetraglucosyl glucitol as well as two other related tetraglucosyl glucitols was elucidated as follows: (1) Glc beta(1-->3)Glc beta(1-->3)(Gl c beta(1-->6)) Glc beta(1-->3)Gtucitol (most active fragment); (2) Glc beta (1-->3)(Glc beta(1-->3)Glc beta(1-->3))Glc beta(1-->3)Glucitol; and (3) Glc beta(1-->6) Glc beta(1-->3)Glc beta(1-->3)Glc beta(1-->3)Glucitol. However , a synthetic hexa-beta-glucoside, known as a minimal structural element fo r the phytoalexin elicitor for soybean cotyledon cells, did not induce phyt oalexin biosynthesis in the rice cells. Conversely, the beta-glucan fragmen t from P. oryzae did not induce phytoalexin biosynthesis in the soybean cot yledon cells, indicating differences in the recognition of glucooligosaccha ride elicitor signals in these two plants. Because rice cells have been sho wn to recognize chitin fragments larger than pentamers as potent elicitors, these results also indicate that the rice cells can recognize at least two types of oligosaccharides from fungal cell walls as signal molecules to in itiate defense response.