THE GIGAS MUTANT IN PEA IS DEFICIENT IN THE FLORAL STIMULUS

Identification of a gene acting in the floral stimulus pathway should provide a basis for determining the identity of this elusive substance . Our tests indicate the Gi (gigas) gene in pea (Pisum sativum L.) act s in this manner. The gigas mutant was selected by Dr M. Vassileva fol lowing gamma radiation of the late flowering, quantitative long day cu ltivar Virtus. The gigas trait showed single gene recessive inheritanc e and the mutant allele was symbolised gi consistent with our prelimin ary report. Gigas plants were later flowering than the initial line in all conditions tested and they showed an enhanced response to photope riod and vernalisation. Unvernalised gigas plants did not flower under a 24-h photoperiod comprising 8 h of daylight and 16 h of weak (3 mu mol m(-2) s(-1)) incandescent light and they took on a phenotype simil ar to the veg1 (vegetative) mutant in pea. However, genetic tests show ed the two mutants were not allelic. Three or four weeks vernalisation at 4 degrees C resulted in 100% flowering of gigas plants under the 2 4-h photoperiod. Applied gibberellin A(3) inhibited flowering in gigas plants given partial cold induction. Grafting studies showed the prom otive effect of vernalisation occurred in the shoot. Grafting studies were also used to examine the physiological basis of delayed flowering in the gigas mutant. These studies indicated that gigas plants produc ed normal levels of flower inhibitor and they responded in a normal ma nner to the floral stimulus. Reciprocal grafts were made between the g igas mutant and the wild-type initial line. Under the 24-h photoperiod , either a wild-type root-stock with cotyledons or a wild-type shoot i nduced flowering in a gigas graft partner. However, under a 9-h photop eriod, flowering was only induced if the wild-type partner possessed b oth roots and a shoot. We conclude that gigas plants are deficient in the floral stimulus or a precursor which can be supplied across a graf t union by a wild-type donor. Of the 12 major flowering genes known in pea, Gi is the first found to act on the synthesis pathway for the fl oral stimulus.