[Intellectual Contribution]

Identification of natural products that inhibit bacterial wilt, an important disease of crops

Shigemi Seo, Michie Kobayashi, Ichiro Mitsuhara, Yuko Ohashi
Plant-Microbe Interactions Research Unit
[Abstract]
We biochemically isolated two bacterial wilt disease-inhibiting compounds from tobacco and identified them as labdane-type diterpenes, namely, sclareol and cis-abienol. Exogenous application of sclareol and cis-abienol to the roots of tobacco, tomato and Arabidopsis infected with bacterial wilt inhibited the wilt disease without exhibiting any antibacterial activity. Our results suggested that sclareol and cis-abienol seem to have a different mode of action than the chemicals known to be capable of activating host resistance to pathogens.
[Keywords]
bacterial wilt disease, plant activator, natural compounds, eco-friendly agriculture

[Background]

Plant activators are compounds capable of inducing disease resistance without exhibiting antimicrobial activity. In recent years, much attention has focused on their potential benefits in reducing the harmful effects of chemical pesticides on the environment. However, most of plant activators that have been identified so far are effective against only a limited number of diseases such as rice blast. Wilt disease caused by the bacterial pathogen Ralstonia solanacearum is one of the most devastating plant diseases. We attempted to isolate a new type of plant activator using tobacco-bacterial wilt disease as a model system.
[Results and Discussion]
  1. We developed a bacterial wilt disease resistance assay system with young tobacco plants. Using this system, we isolated two bacterial wilt disease-inhibiting compounds from tobacco which were identified as labdane-type diterpenes, namely, sclareol and cis-abienol (Fig. 1).
  2. These compounds were exogenously applied to roots of tobacco, tomato and Arabidopsis plants and inhibited wilt disease without exhibiting any antibacterial activity (Fig. 2).
  3. Structure-activity correlation assay revealed an important role of the OH group at C8 of these diterpenes in their biological activities.
  4. Genetic analysis suggested that inhibition of bacterial wilt disease by sclareol occurs independently of SA and JA biosynthetic and signaling pathways, but is partially dependent on ET and ABA signaling pathways. Sclareol and cis-abienol seem to have a different mode of action than the chemicals known to be capable of activating host resistance to pathogens such as probenazole and BTH.
  5. Sclareol also exhibited an inhibitory activity toward infection of root-knot nematodes (RKN), which cause yield losses in many crops (Fig. 3).
[Future prospects]
  1. The identification of sclareol and cis-abienol as bacterial wilt disease-inhibiting compounds using the assay system with young tobacco plants demonstrates the effectiveness of this system for screening disease-inhibiting compounds such as plant activators.
  2. The result obtained from structure-activity correlation test will be useful in the synthesis of more potent inhibitors.

Fig.1. Chemical structures of sclareol-related compounds.


Fig.2. Effect of exogenously applied sclareol on bacterial wilt disease in tobacco and tomato. Roots of tobacco (left panel) or tomato (right panel) plants were treated with 100 mM sclareol or 0.1% methanol (control) for 48 h and inoculated with R. solanacearum.


Fig.3. Effect of exogenously applied sclareol on infection of RKN in tomato. Roots of tomato plants were treated with 100 mM sclareol or 0.1% methanol (control) for 48 h and inoculated with RKN, and the number of invaded RKN was measured.

 

[Reference]

  1. Seo S, Gomi K, Kaku H, Abe H, Seto H, Nakatsu S, Neya M, Kobayashi M, Nakaho K, Ichinose Y, Mitsuhara I, Ohashi Y (2012) Identification of natural diterpenes that inhibit bacterial wilt disease in tobacco, tomato and Arabidopsis Plant and Cell Physiology 53(8):1432-1444
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