Abstract

Lectins are generally defined as non-immune proteins or glycoproteins having at least one non-catalytic domain to bind reversibly to a specific sugar molecule, and extensively detected in diverse organisms including plants, animals and microorganisms. But their possible roles and functions in planta have not been elucidated yet. The novel lectin that we found binds glucose/mannose residues in rice seedlings (Teraoka, 1990), namely mannose-binding rice lectin (MRL). The lectin has potential to agglutinate spores of Magnaporthe grisea and also intact cells of Xanthomonas campestris pv. oryzae. The expression of MRL is induced by salt- or water-stress, treatment with abscisic acid, jasmonate and plant activators such as probenzol, and inoculation with M. grisea (Hirano, 2000). So we focus on the hypothesis that MRL may function as plant antibodies in a defensive reaction in rice plants. The transformants that express MRL constitutively up-regulated by introducing the MRL4.85 gene with a strong promoter or down-regulated by RNAi system, were established to prove the hypothesis. In the up-regulated transformants, the growth of the infective hyphae of M. grisea were strongly suppressed and finally the lesions decreased in the transformants. Although the down-regulated transformants had little effects in enhancing the growth of infective hyphae, probably the MRL functions in the disease resistance or in the pathogen recognition system.