Gibberellin-deficient dwarfs in potato vary in exogenous GA3 response when the ga1 allele is in different genetic backgrounds

Gibberellins (GAs) are involved in internode elongation and other important processes such as seed germination, flowering, maturation, tuberization, and tuber dormancy. The discovery of GA-deficient mutants enabled further study of the role of these hormones in many plant processes. GA-deficient mutants lack the ability to produce adequate amounts of gibberellin for normal growth, resulting in a rosette type growth and short internodes. The ga(1) mutant allele was introduced into various genetic backgrounds including different Solanum species and ploidies. Diploid GA-deficient genotypes were obtained by crossing haploid Solanum tuberosum ssp. andigena with Solanum chacoense. The progeny was then bulked and intermated to produce F, individuals. Tetraploid GA-deficient genotypes were obtained by crossing S. tuberosum ssp. andigena with Solan um sucrense and with Solanum gourlayi. The two resulting progenies were then bulked and intermated. Diploid and tetraploid GA-deficient genotypes were grown on MS media containing different levels of gibberellin (GA(3)). Plant height and visual observations were made as a way to assess the response of these genotypes to GA(3). Concentration of 0.1 mu M GA(3) and lower failed to restore normal plant height in both diploid and tetraploid genotypes. Normal plant height was restored in most of the GA-deficient genotypes when concentrations between 0.8 and 1.2 mu M GA(3) were used. We found some important differences between these genotypes: (1) the level of GA(3) to restore normal plant height varies among the GA-deficient genotypes, some needed more GA(3), than others to grow normally; (2) the time to respond to the presence of GA(3) in the media differs between the GA-deficient genotypes, (3) tetraploid genotypes exhibited normal growth and internode length in response to GA(3), while diploid genotypes tended to show a rosette-type growth at the apical. end. These results suggest that ga(1) mutants can be affected by a series of modifier genes and/or iso-alleles. The importance of variable response to GA among dwarf individuals is two fold: (1) experiments measuring GA response should choose and clonally multiply one genotype to ensure uniform optimal response to GA application; and (2) variation between ga(1) mutant phenotypes could be used to characterize GA-response modifier genes.