Linear beta-1,3 glucans are elicitors of defense responses in tobacco

Laminarin, a linear beta -1,3 glucan (mean degree of polymerization of 33) was extracted and purified from the brown alga Laminaria digitata. Its elic itor activity on tobacco (Nicotiana tabacum) was compared to that of oligog alacturonides with a mean degree of polymerization of 10. The two oligosacc harides were perceived by suspension-cultured cells as distinct chemical st imuli but triggered a similar and broad spectrum of defense responses. A do se of 200 mug mL(-1) laminarin or oligogalacturonides induced within a few minutes a 1.9-pH-units alkalinization of the extracellular medium and a tra nsient release of H2O2. After a few hours, a strong stimulation of Phe ammo nia-lyase, caffeic acid O-methyltransferase, and lipoxygenase activities oc curred, as well as accumulation of salicylic acid. Neither of the two oligo saccharides induced tissue damage or cell death nor did they induce accumul ation of the typical tobacco phytoalexin capsidiol, in contrast with the ef fects of the proteinaceous elicitor beta -megaspermin. Structure activity s tudies with laminarin, laminarin oligomers, high molecular weight beta -1,3 -1,6 glucans from fungal cell walls, and the beta -1,6-1,3 heptaglucan show ed that the elicitor effects observed in tobacco with P-glucans are specifi c to linear beta -1,3 linkages, with laminaripentaose being the smallest el icitor-active structure. In accordance with its strong stimulating effect o n defense responses in tobacco cells, infiltration of 200 mug mL(-1) lamina rin in tobacco leaves triggered accumulation within 48 h of the four famili es of antimicrobial pathogenesis-related proteins investigated. Challenge o f the laminarin-infiltrated leaves 5 d after treatment with the soft rot pa thogen Erwinia carotovora subsp. carotovora resulted in a strong reduction of the infection when compared with water-treated leaves.