HAR1 mediates systemic regulation of symbiotic organ development

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Abstract
Symbiotic root nodules are beneficial to leguminous host plants; however, excessive nodulation damages the host because it interferes with the distribution of nutrients in the plant. To keep a steady balance, the nodulation programme is regulated systemically in leguminous hosts1,2. Leguminous mutants that have lost this ability display a hypernodulating phenotype. Through the use of reciprocal and self-grafting studies using Lotus japonicus hypernodulating mutants, har1 (also known as sym78)3, we show that the shoot genotype is responsible for the negative regulation of nodule development. A map-based cloning strategy revealed that HAR1 encodes a protein with a relative molecular mass of 108,000, which contains 21 leucine-rich repeats, a single transmembrane domain and serine/threonine kinase domains. The har1 mutant phenotype was rescued by transfection of the HAR1 gene. In a comparison of Arabidopsis receptor-like kinases, HAR1 showed the highest level of similarity with CLAVATA1 (CLV1)4. CLV1 negatively regulates formation of the shoot and floral meristems through cell–cell communication involving the CLV3 peptide5. Identification of hypernodulation genes thus indicates that genes in leguminous plants bearing a close resemblance to CLV1 regulate nodule development systemically, by means of organ–organ communication.