-
24/12/2010
-
René Geurts en Ton Bisseling
Providing the world population with food in 2050 will prove a daunting task in which it will be crucially important to remove limiting factors in food production. One of these limiting factors is nitrogen. It is essential that we respond to the massive challenge presented by the fixation of atmospheric nitrogen in relation to the fertilisation and cultivation of crops. Recent insights into symbiosis in leguminous and non-leguminous plants and technological breakthroughs in genomics and plant breeding make it incumbent on us to invest in research that will harness new potential for sustainable food production.
The symbiosis between legumes and rhizobial bacteria is a key biological process in the reduction of nitrogen compounds. This unlimited nitrogen fixation is used directly by the leguminous host plant and indirectly by all organisms, including humans. The transposition of this symbiotic capacity to important (non-leguminous) crops is widely acknowledged as a major agricultural challenge for 2050 (Gewin, Nature 466, 552, 2010).
This rhizobium-root-nodule symbiosis evolved independently in a non-legume, the parasponia from New Guinea. Parasponia acquired this symbiotic ability fairly soon compared with legumes and can therefore help us to find ways of transposing it to non-legumes.
The scientific community has been intrigued ever since parasponia emerged in 1973 as the first – and hitherto only – non-legume that is capable of nitrogen-fixing root-nodule symbiosis with rhizobium. This ‘bridging species’ will shed light on how such a unique process was able to evolve. A deeper understanding of the potential of this plant will inevitably lead to the utilisation of its properties. In the past ten years vital knowledge has been acquired of the molecular mechanisms that lie at the heart of the rhizobium-leguminous symbiosis. Research on, for example, the model leguminous medicago and the lotus can now be used to ascertain how parasponia enlists existing botanical processes to create a root-nodule symbiosis.
The formation of leguminous root-nodules is prompted by specific molecules, called nod factors, which are produced by rhizobium. The model systems of leguminous plants have made it possible to clone and characterise the leguminous genes that are needed for the perception and signalling of nod factors. Interestingly, nod factors are also essential in the parasponia-rhizobium symbiosis, which strongly suggests that, although nod factors have a unique structure, they are recognised by receptors that occur across the plant kingdom. We unravelled this paradox in our studies on parasponia. Our findings (published in Science, 23 December 2010) showed that nod factors in parasponia are recognised by the plant receptor that also recognises signal molecules of mycorrhiza fungi – which can form a symbiosis with most of the higher taxonomic ranks.
Parasponia gives us an opportunity to observe an evolutionary process ‘shortly’ after it has emerged. As the symbiotic code stems from the long-standing evolutionary mycorrhiza symbiosis, it will be present in most of the higher ranks, including important agricultural crops. Meticulous research on parasponia will show us how to transpose an evolutionary process to these crops.
The ultimate aim is to give these crops the capacity to satisfy their own nitrogen needs in an environmentally-friendly and energy-saving way. It will take courage to invest in research with such a long-term pay-off, but future generations will thank us for it.
Rene Geurts Ton Bisseling