Gibberellin-Mediated Synergism of Xylogenesis in Lettuce Pith Cultures

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Abstract
Major gibberellins (GAs) in lettuce (Lactuca sativa L. cv. Romaine) pith explants have been identified by gas chromatography-mass spectrometery (GC-MS) or GC-selected ion monitoring (GC-SIM) as GA1, 3-epi-GA1, GA8, GA19, and GA20. Treatment of pith explants with indole-3-acetic acid (IAA) (57 micromolar) plus kinetin (0.5 micromolar) induced xylogenesis. In this xylogenic treatment, the concentration of a biologically active, polar GA-like substance(s) increased during the first 2 days of culture, although all of the above GAs decreased (as measured by GC-SIM). In non-xylogenic treatments, where explants were cultured without exogenous hormones, or with IAA or kinetin alone, the concentration of the biologically active, polar GA-like substance(s) decreased during the first two days of culture, as did all of the above GAs (as measured by GC-SIM). Treatment of pith explants with exogenous GA1 alone did not induce xylogenesis, but GA1 at very low concentrations (0.0014 and 0.003 micromolar) synergized xylogenesis in the IAA plus kinetin-treated cultures. These results suggest that changes in the concentration of certain endogenous GAs may be involved in xylogenesis mediated by IAA plus kinetinc in lettuce pith cultures.