Internode length in Pisum . Gibberellins and the slender phenotype

Pea plants ( Pisum sativum L.) possessing the slender phenotype (conferred by the gene combination la cry s ) have extremely long, thin internodes and are phenotypically similar to dwarf plants (possessing genes La and/or Cry ) that have been treated with a non‐limiting dose of gibberellin (GA 3 ). In contrast to tall and dwarf plants, slender plants are virtually insensitive to treatment with AMO 1618, PP333 or GA 3 and addition of the “gibberellin‐less” mutant gene na does not alter the phenotype of slender plants. Na slender segregates possessed lower levels of gibberellin‐like substances than comparable dwarf segregates when extracts from shoots were assayed using the lettuce hypocotyl or rice seedling bioassays. In addition, na slenders possessed little or no gibberellin‐like activity even though they possessed a slender phenotype. Thus the gene combination la cry s causes slender plants to respond as if they are saturated with gibberellins for growth. In addition, the gene combinations la cry s and le la cry c (allele cry c is less extreme in effect than cry s ) are shown to be almost completely epistatic to the alleles at the na locus. All these results suggest that gibberellin levels are not important in determining the internode length of slender peas (genotype la cry s ). The possible mechanisms by which this could occur are discussed.