This paper (together with the paper in the same issue of Nature by Endre et al.) represents a milestone for the field of symbiosis research. For the first time a gene has been isolated that is necessary for the two types of root symbioses that plants undergo—mycorhiza with fungi and nitrogen-fixing symbiosis with bacteria. The component we isolated is a membrane-spanning receptor-like kinase (RLK) and therefore a prime candidate for the transduction of an extracellular signal inside the cell. This opens a whole field, since we can now ask questions like:

1. Are similar molecules present in important crop plants that do not form nitrogen fixing root nodules?
2. How do plants differentiate between bacterial and fungal symbionts to mount an appropriate response?

This paper shows that the combination of classical map-based cloning technology with an ongoing genome sequencing project allows genes to be cloned via their map position in a relatively short time. This has been achieved via collaboration between my group at the Sainsbury Laboratory and the laboratory of Dr. Satoshi Tabata at the Kazusa DNA Research Institute, where the total genome of Lotus japonicus is currently sequenced.

Almost all land plants live together for mutual benefit with microorganisms, a process called symbiosis. The plants trade carbon (sugars) for phosphate with fungi. Some plants, the legumes (comprising e.g. beans, peas, and alfalfa) can in addition form root nodule symbiosis with bacteria that can fix nitrogen from the air. This is an agriculturally very important interaction, since growth of land plants is often limited by the amounts of nitrogen in the soil. Farmers have to add costly nitrogen fertilizer or employ a crop rotation with legumes and other crops. The gene described in this paper was isolated by mutating wild-type plants and looking for those that can't fix nitrogen any more. Then these mutants are crossed with wild-type plants and by analysis of the progeny it can be determined where on the DNA the gene of interest resides. It has been shown in this and earlier publications that some plant genes are the same for the interaction with fungi and bacteria. This is very encouraging with the view to transferring the potential for symbiotic nitrogen fixation to other crops, since the components for mycorhiza (fungal symbiosis) are already present in most land plants, and thus maybe only a few crucial switches have to be transferred.

I was initially intrigued by the vast gap between the enormous impact of plant-microbe symbioses on world nutrition and sustainable agriculture and our almost complete lack of knowledge about the processes involved in this interaction. When initial mutagenesis showed that both fungal and bacterial symbioses share components, I was increasingly fascinated not only by the evolutionary implications, but also since this use of common mechanisms makes a potential transfer of the symbiotic potential to non-nitrogen fixing plants potentially feasible. We are aware that this is an ambitious and distant goal, but we feel that we are on the way of making it closer and more feasible.

Dr. Martin Parniske
The Sainsbury Laboratory
John Innes Centre
Norwich, UK