Eternal Rice: a case study of sustainable management of plant resistance

Important insights into ways to reach durability of plant disease resistance can be gained by studying existing sustainable systems, like the traditional rice agrosystem of the Yuan Yang Terraces (YYT) in China. In YYT, more than 190 traditional rice varieties have been grown for centuries without noticeable erosion of resistance to the blast fungus Magnaporthe oryzae. The main objectives of the multi-disciplinary project "Riz Eternel" project was to test the hypothesis that the interaction between diversity and spatio-temporal arrangement of varieties prevents the development of large epidemics of M. oryzae. Genome analysis of hundreds of plants showed that the traditional rice varieties are populations, highly diversified, with resistance genes under diversifying selection. Socio-economic surveys allowed the identification of social rules underlying seed exchange within villages, producing additional levels of spatial and temporal diversity. As a consequence, there is no genetic co-structure between plants and M. oryzae populations, suggesting that M. oryzae is maladapted in YYT. Combining socio-economic approaches and molecular genetics, we also present recent evidence that shrinking of rice cultivated diversity in YYT destabilizes the co-evolutionary equilibrium and favors epidemics.