Differences in Endogenous Levels of Gibberellin-Like Substances in Nodules of Phaseolus lunatus L. Plants Inoculated with Two Rhizobium Strains

Abstract

Lima bean plants (Phaseolus lunatus L.) inoculated with Rhizobium sp. strain 127E14, which lacks constitutive nitrate reductase activity, were significantly taller after 4 weeks of age than plants inoculated with strain 127E15, which contains constitutive nitrate reductase activity. Plants inoculated with either strain responded to application of 5 micrograms gibberellic acid per plant with rapid internode elongation; plants inoculated with strain 127E15 became less responsive to gibberellic acid from 3 to 5 weeks of age, while plants inoculated with strain 127E14 did not. The height of plants inoculated with strain 127E14 was reduced by 20% with application of gibberellin biosynthesis inhibitors to the roots, while height of plants inoculated with strain 127E15 was unaffected.Plants inoculated with strain 127E14 developed nodules containing 4 to 50 times greater quantities of extractable gibberellin-like substances than the nodules of plants inoculated with strain 127E15. Plants inoculated with strain 127E14 had nodules containing larger quantities of gibberellin-like substances in both the acidic and basic ethyl acetate fractions, and these differences were observed before the onset of rapid growth responsible for the difference in plant height.The internode elongation in plants infected with strain 127E14, its reversal by gibberellin biosynthesis inhibitors, and the accumulation of large quantities of gibberellin-like substances in nodules infected with strain 127E14 support the hypothesis that root nodules may contribute gibberellins to the host.