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Genetic Suppressors of the Lotus japonicus har1-1 Hypernodulation Phenotype

¹Agriculture and Agri-Food Canada, Southern Crop Protection and Food Research Centre, 1391 Sandford Street, London, Ontario N5V 4T3, Canada; ²Graduate Program, Department of Biology, University of Western Ontario, Biological Geological Science Building, 1151 Richmond Street North, London, Ontario N6A 5B7, Canada; ³University of Munich (LMU) Department of Biology I Section Genetics, Maria-Ward-Street 1a D-80638 Munich, Germany; ⁴Department of Integrative Biology, Axelrod Building, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada; ⁵John Innes Centre, Norwich Research Park, Colney Lane, Norwich, Norfolk, United Kingdom; ⁶Laboratory of Gene Expression, Department of Molecular Biology, University of Aarhus, Gustv Wieds Vej 10, 8000 Aarhus C, Denmark

Lotus japonicus har1 mutants respond to inoculation with Mesorhizobium loti by forming an excessive number of nodules due to genetic lesions in the HAR1 autoregulatory receptor kinase gene. In order to expand the repertoire of mutants available for the genetic dissection of the root nodule symbiosis (RNS), a screen for suppressors of the L. japonicus har1-1 hypernodulation phenotype was performed. Of 150,000 M2 plants analyzed, 61 stable L. japonicus double-mutant lines were isolated. In the context of the har1-1 mutation, 26 mutant lines were unable to form RNS, whereas the remaining 35 mutant lines carried more subtle symbiotic phenotypes, either forming white ineffective nodules or showing reduced nodulation capacity. When challenged with Glomus intraradices, 18 of the 61 suppressor lines were unable to establish a symbiosis with this arbuscular mycorrhiza fungus. Using a combined approach of genetic mapping, targeting induced local lesions in genomics, and sequencing, all non-nodulating mutant lines were characterized and shown to represent new alleles of at least nine independent symbiotic loci. The class of mutants with reduced nodulation capacity was of particular interest because some of them may specify novel plant functions that regulate nodule development in L. japonicus. To facilitate mapping of the latter class of mutants, an introgression line, in which the har1-1 allele was introduced into a polymorphic background of L. japonicus ecotype MG20, was constructed.

Additional keywords: root architecture .