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Wanted: bacteria that allow plants to flourish

Plants love favourable microbes such as bacteria and fungi: they grow better and become healthier. Jos Raaijmakers, Professor of Microbial Ecology, is in search of the right microbes to be used in agriculture. Inaugural lecture 13 November.

Favourable soil bacteria ignored

With regard to the domestication and breeding of crops, breeders have until now ignored the relation that crops have with favourable soil bacteria and other micro-organisms. It is therefore unclear whether those micro-organisms still play a role in agriculture, where the growth and health of crops have become dependent on fertilizers and pesticides. Raaijmakers' project Back2Roots, which is financed by Technology Institute STW, aims to re-identify these soil microbes. Together with his team, he travels to regions where the origin of crops can be found so as to recover some of these favourable micro-organisms.

Searching for microbes that are as yet unknown

'Many micro-organisms live on and inside the roots of plants, which encourages the growth and health of these plants,' Raaijmakers explains. 'Classic examples are Rhizobium bacteria and mycorrhiza fungi, that produce nutrients for the plant. But there are many other microbes to be found in the soil that are as yet unknown. Plants attract favourable soil microbes by secreting certain substances. Crop breeders have so far not selected on the basis of this particular feature; as a result, the 'microbiome', which is found in and surrounding the roots of plants, is different and may also be depleted.

Wild relatives

The researchers will travel to the Andes in search of the wild relatives of the tomato and the bean, and to the Middle East to study desert plants that are resistant to extreme drought. They will examine which micro-organisms live on and in the roots of these wild relatives and investigate the role they play in the growth and health of these plants. 'We hope to find a set of favourable microbes and we want to check whether we can increase the growth and health of these plants by stimulating these microbes or by adding them to the soil and to the plant seeds.’

Ecological context essential

Raaijmakers' approach to this subject is relatively new in the field of microbiology. Until recently microbiologists have bred and examined the micro-organisms mainly on the basis of type and strain. During the last few years the interaction between micro-organisms and their relation to plants have increasingly come into focus, partly due to the development of new technologies used for examining microbial communities. Raaijmakers: 'It has proven to be essential to study the micro-organisms in their ecological context, because certain traits only manifest themselves in the presence of other types.'

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Jos Raaijmakers is head of Microbial Ecology at the Dutch Institute of Ecology (NIOO-KNAW) and is affiliated to the Institute of Biology Leiden for one day a week. 'In Leiden I have found a very good addition to the expertise of the NIOO,' he says. 'The IBL has a high level of research in molecular biotechnology, plant chemistry, mycology and evolutionary studies.' 

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