Although a number of plant hormones have been shown to regulate legume nodule formation, it seems likely that further, as yet unidentified, pathways and regulatory circuits exist. We hypothesized that genes encoding components of these pathways and circuits would be required for aspects of normal plant development as well as for nodule formation. A collection of developmental mutants of M. truncatula does not yet exist, so we first screened for developmental mutants.� We then screened the progeny of these plants for altered nodule formation with Sinorhizobium meliloti. We used a fast-neutron bombarded M2 population generously provided by S. Long (Stanford University). We screened 8,600 M2 individuals corresponding to 349 M1 plants.
giraffe mutants do not have a normal developmental response to light.
eve mutants have long roots and form excess nodules.
Out of this screen we identified 10 independent mutants with defects in both plant and nodule development and have focused on 2: eve and giraffe. eve is an ethylene-sensitive supernodulator with a long root and short root hairs that appears to represent a previously undescribed gene. giraffe is also a supernodulator, but with extensive photomorphogenic defects, indicative of a defect in light signaling. Only one other light-signaling gene has been shown to regulate nodule formation, the Lotus japonicus gene ASTRAY (LjHY5) (Nishimura et al., 2002; Nishimura et al., 2002). giraffe mutants do not exhibit the astray agravitropic lateral roots and have an additional phenotype: yellowish leaves. We are currently testing whether the phenotypes are linked. If so, then GIRAFFE would provide a previously undescribed link between light signaling and nodulation.