Short root mutant of Lotus japonicus with a dramatically altered symbioticphenotype

Legume plants carefully control the extent of nodulation in response to rhi zobial infection. To examine the mechanism underlying this process we condu cted a detailed analysis of the Lotus japonicus hypernodulating mutants, ha r1-1, 2 and 3 that define a new locus, HYPERNODULATION ABERRANT ROOT FORMAT ION (Har1), involved in root and symbiotic development. Mutations in the Ha r1 locus alter root architecture by inhibiting root elongation, diminishing root diameter and stimulating lateral root initiation. At the cellular lev el these developmental alterations are associated with changes in the posit ion and duration of root cell growth and result in a premature differentiat ion of har1-1 mutant root. No significant differences between har1-1 mutant and wild-type plants were detected with respect to root growth responses t o 1-aminocyclopropane1-carboxylic acid, the immediate precursor of ethylene , and auxin; however, cytokinin in the presence of AVG (aminoetoxyvinylglyc ine) was found to stimulate root elongation of the har1-1 mutant but not th e wild-type. After inoculation with Mesorhizobium loti, the har1 mutant lin es display an unusual hypernodulation (HNR) response, characterized by unre stricted nodulation (hypernodulation), and a concomitant drastic inhibition of root and shoot growth. These observations implicate a role for the Har1 locus in both symbiotic and non-symbiotic development of L. japonicus, and suggest that regulatory processes controlling nodule organogenesis and nod ule number are integrated in an overall mechanism governing root growth and development.