It has long been recognized that some soils are disease-suppressive soils – that is they have the ability to suppress the occurrence of soil-borne plant diseases caused by soil microorganisms. The mechanisms by which this disease-suppression is brought about have been the subject of study.
The soil layer surrounding plant roots (the rhizospere) is a complex community in which a diversity of microorganisms (archaea, bacteria, and fungi) and plants interact with one another. If plants are parasitized by fungal or bacterial pathogens, it is thought that soil from the rhizosphere may protect plants from future attacks. This disease-suppression is thought to be the result of a mutualism between the plants that provide nutrients for microorganisms while the microorganisms protect the plant from parasitism by other soil organisms.
In an attempt to better understand disease-suppressive soils, researchers investigated whether microorganisms were responsible for the protective effects of these soils. Disease-suppressive soil was obtained from an agricultural field in the Netherlands in which sugar beet crops had previously been attacked by a fungal pathogen, Rhizoctonia solani . Other soil was collected from the margin of the field. The researchers predicted that soil from the margin would not offer protection against pathogens.
The researchers then planted and raised sugar beets in greenhouses, using 5 different soil treatments. Each soil treatment was applied to 4 pots, and each pot contained 8 plants. The pots were inoculated with Rhizoctonia solani and placed in greenhouses kept at 20°C with a 16-hour light: 8-hour dark cycle. After 20 days, researchers determined the percentage of infected sugar beet seedlings for each soil treatment. The results of the experiment are shown in the table.
| Deciphering the Rhizosphere for Disease-Suppressive Bacteria | |
|---|---|
| Soil Treatment | Percentage of Seedlings with Fungal Disease |
| A. Disease-suppressive soil | 3.0 |
| B. Soil from margin of field | 62 |
| C. Soil from margin of field +10% disease-suppressive soil | 39 |
| D. Disease-suppressive soil heated to 50° C for 1 hour | 31 |
| E. Disease-suppressive soil heated to 80° C for 1 hour | 70 |
Data from R. Mendes, et al. Deciphering the rhizosphere for disease-suppressive bacteria, Science 332 : 2097-1100 (2011).
Be prepared to answer the following questions in class: