THE CHARACTERIZATION OF GIO, A NEW PEA MUTANT, SHOWS THE ROLE OF INDOLEACETIC-ACID IN THE CONTROL OF FRUIT-DEVELOPMENT BY THE APICAL SHOOT

Fruit-set and fruit growth in pea (Pisum sativum L.) depend on gibbere llins (GAs). The authors have isolated a new pea mutant, gio, which ap peared spontaneously within the population of the cultivar Alaska, cha racterized by unpollinated ovaries much less sensitive to applied GAs. The mutant also has elongated peduncles, and is taller than the wild- type (WT) because the upper plant internodes are longer. Contrary to W T, the gio ovaries respond very little to benzylaminopurine (BAP) and 2,4-dichlorophenoxyacetic acid, but become fully sensitive to GA(3) wh en this hormone is applied together with BAP. The gio phenotype is det ermined by a mutation at a single mendelian locus. The mutation is rec esive, shows incomplete penetrance, and its expression depends on envi ronmental culture conditions. The sensitivity of the ovaries to GA(3) can be recovered by removing the apical shoot (plant decapitation) and by blocking the transport of indoleacetic acid (IAA) from the apical shoot with 2,3,5-triiodobenzoic acid. The content of IAA in methanolic extracts and phloematic exudates of the apical shoot of gio is about double that in the WT. The rate of transport of [H-3]IAA applied to th e apex of the mutant is also twice that in the WT. This indicates that the insensitivity of the gio ovaries to GAs is due to the inhibitory effect of the higher basipetal IAA transport from the shoot. The inter action between the fruit and the apical shoot mediated by IAA probably also involves cytokinins transported from the basal part of the plant .