Wetenschap - 6 mei 2011

Plant roots get help from bacteria

Arno van 't Hoog

Bacteria protect plant roots against fungus infections, report researchers from Wageningen, Utrecht and the United States in Science.


Foto: .

When plants are affected by soil fungi, the disease can become considerably milder after several years. 'While the disease appears to be still present in the soil, infections occur much less often. This is somewhat similar to the immunity system in humans, which also develops only after being exposed to a pathogen', says the Wageningen phytopathologist Dr. Jos Raaijmakers. This resistance can be transplanted into non-resistant soils by mixing small amounts of resistant soil into them; this resistance disappears upon heating. All these point to the presence of a microbial factor.
Sugar beet
To find out which micro-organisms are involved, the phytopathologists used DNA chips to research into different soil samples with varying degrees of resistance collected from sugar beet farms. This so-called PhyloChip can distinguish more than 60,000 types of bacteria. Raaijmakers: 'We have seen that our rhizosphere samples alone have 33,000 different types, but the differences are in the quantities. Resistant soils have relatively larger quantities of certain bacteria groups, including proteobacteria, actinobacteria and furmicutes.'
In the laboratory, various types of proteobacteria from resistant soils were isolated and characterized. It appeared that one of these groups is able to protect sugar beet against fungus infections. Genetic research into the bacteria further showed that they discharge chlorinated lipopeptides, a class of small proteins with strong anti-microbial effects. This would account for the protective effect against fungus infections.
All in all, the research lasted six years. Raaijmakers: 'A lot of investigative work was involved. Many metagenomics-based studies often just show which micro-organisms are present in certain environments. We have moved towards a complete functional analysis, from population to gene and bio-active substances.'
Cry for help
It is still not clear what role the plant itself plays in the selection of these bacterial groups. 'We think that the plant discharges signal substances which enrich certain groups of bacteria. It looks like the 'cry for help' signal substances which plants secrete when they are being bitten by insects. But we haven't found any hard evidences yet for this. In any case, the research work has clearly shown that the plant is much more than a plant. Plants recruit micro-organisms and together, a "super organism" is formed, like gut flora in humans.'