title: Root exudates drive interspecific facilitation by enhancing nodulation and N2 fixation.
abstract: Plant diversity in experimental systems often enhances ecosystem productivity, but the mechanisms causing this overyielding are only partly understood. Intercropping faba beans (Vicia faba L.) and maize (Zea mays L.) result in overyielding and also, enhanced nodulation by faba beans. By using permeable and impermeable root barriers in a 2-y field experiment, we show that root-root interactions between faba bean and maize significantly increase both nodulation and symbiotic N2 fixation in intercropped faba bean. Furthermore, root exudates from maize promote faba bean nodulation, whereas root exudates from wheat and barley do not. Thus, a decline of soil nitrate concentrations caused by intercropped cereals is not the sole mechanism for maize promoting faba bean nodulation. Intercropped maize also caused a twofold increase in exudation of flavonoids (signaling compounds for rhizobia) in the systems. Roots of faba bean treated with maize root exudates exhibited an immediate 11-fold increase in the expression of chalcone-flavanone isomerase (involved in flavonoid synthesis) gene together with a significantly increased expression of genes mediating nodulation and auxin response. After 35 d, faba beans treated with maize root exudate continued to show up-regulation of key nodulation genes, such as early nodulin 93 (ENOD93), and promoted nitrogen fixation. Our results reveal a mechanism for how intercropped maize promotes nitrogen fixation of faba bean, where maize root exudates promote flavonoid synthesis in faba bean, increase nodulation, and stimulate nitrogen fixation after enhanced gene expression. These results indicate facilitative root-root interactions and provide a mechanism for a positive relationship between species diversity and ecosystem productivity.
authors.affiliation: Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing Key Laboratory of Biodiversity and Organic Farming, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, People's Republic of China;
authors.forename: Bai
authors.initials: B
authors.lastname: Li
authors.affiliation: Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing Key Laboratory of Biodiversity and Organic Farming, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, People's Republic of China;
authors.forename: Yu-Ying
authors.initials: YY
authors.lastname: Li
authors.affiliation: Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing Key Laboratory of Biodiversity and Organic Farming, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, People's Republic of China;
authors.forename: Hua-Mao
authors.initials: HM
authors.lastname: Wu
authors.affiliation: Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing Key Laboratory of Biodiversity and Organic Farming, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, People's Republic of China;
authors.forename: Fang-Fang
authors.initials: FF
authors.lastname: Zhang
authors.affiliation: Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing Key Laboratory of Biodiversity and Organic Farming, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, People's Republic of China;
authors.forename: Chun-Jie
authors.initials: CJ
authors.lastname: Li
authors.affiliation: Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing Key Laboratory of Biodiversity and Organic Farming, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, People's Republic of China;
authors.forename: Xue-Xian
authors.initials: XX
authors.lastname: Li
authors.affiliation: School of Plant Biology and Institute of Agriculture, University of Western Australia, Crawley, Perth, WA 6009, Australia.
authors.forename: Hans
authors.initials: H
authors.lastname: Lambers
authors.affiliation: Key Laboratory of Plant-Soil Interactions, Ministry of Education, Beijing Key Laboratory of Biodiversity and Organic Farming, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, People's Republic of China; lilong@cau.edu.cn.
authors.forename: Long
authors.initials: L
authors.lastname: Li
mesh_terms: D000383:Agriculture / Q000379:methods; D015870:Gene Expression; D019833:Genistein / Q000378:metabolism; D007529:Isoflavones / Q000378:metabolism; D009586:Nitrogen Fixation; D010940:Plant Proteins / Q000235:genetics; D018517:Plant Roots / Q000737:chemistry / Q000378:metabolism*; D031307:Vicia faba / Q000378:metabolism*; D003313:Zea mays / Q000378:metabolism*
chemical_list: D007529:Isoflavones; D010940:Plant Proteins; C004742:daidzein; D019833:Genistein
keywords: flavanoids; gene expression; intercropping; root–root interactions; signals