Comparative Studies on Inducible Antimicrobial Defense Reactions in Soybean Cultivars

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Central Research Institute for Food Crops
Plants react to microbial attack with a number of defence mechanisms which include a rapid formation of reactive oxygen species (ROS), cross-linking of cell wall proteins and strengthening of the plant cell wall, induction of the phenylpropanoid pathway, synthesis of phenolic compounds, accumulation of antimicrobial compounds named phytoatexins, synthesis of fungal wall degrading enzymes {glucanases, chitinases) and a rapid localized cell death. These defence responses can also be Induced by application of various biotic and abiotic substances (elicitors) to cell suspension cultures. Most analyses of defence reactions were performed on a few cuttivars and elicitors. There is little knowledge about the reactivity of different cultivars against various stimuli. Therefore, suspension cultured soybean (Glycine max [L] Merr.) cells of four cultivars (Wills, Lumut, Kalmit, Dokb RC) were treated either with crude cell wall extracts of the fungal pathogens Phytophlhora sojae, Rhizoctonia solani, Athelia mlfsii (Pmg-, Riso-, Aro-elicitor, respectively) or with two isolates of the bacterial pathogen Pseudomonas syrmgae pv. glycinea (Psg01'02). Cells of all four cultivars showed the same range of defence reactions but time courses and intensities differed significantly between the cultivars. Furthermore, the reactivity of the cultivars depended on the tested elicitor. A common response to el citation is the rapid release of ROS called oxidative burst. Comparing the amount of H^O; induced by the fungal elicitors three cuttivars produced significantly less H^Oj after application of Pmg-elicitor than with Riso-elictor, only Doko RC showed equal values for both elicitors. These differences in the expression of an oxidative burst corresponded with a rapid and transient alkalinization of the ceil culture medium after elicitation. Those elicitors that caused a strong oxidative burst were also capable of Inducing a greater alkalinization. Nevertheless there is no causal relationship beetween the two reactions as could be shown by specific inhibitors of the oxidative burst. Activation of the phenylpropanoid metabolism is an important step for the synthesis of various secondary compounds (i.e. [iso]-flavonoids, Itgnin-like substances, soluble phenolics). Phenylaianine ammonia-lyase (PAL), the key enzyme of phenylpropanoid metabolism, was activated after elicitor-treatment. The enzyme activity was stronger after addition of Riso-elicitor than after application of bacterial suspensions. The intensity of PAL-activity correlated with an increase of soluble phenolic compounds in the culture medium but not with an accumulation of the isoflavonoid phytoalexins. These results suggest that soybean cultivars recognise the same stimuli but differ In the intensity of elicltor-induced defence reactions. This is probably due to a difrent recognition of the elicitors at the plasma membrane.
Antimicrobial, Glycine max, fungal pathogen, elicitor