Endogenous gibberellin A1 levels control thermoperiodic stem elongation in Pisum sativum?

Gibberellin (GA) is believed to be involved in thermoperiodic stem elongation. With this in mind, we studied the correlation between gibberellin A(1) (GA(1)) levels and stem elongation affected by alternating day (DT) and night temperature (NT) in 5 genotypes of Pisum sativum differing in their degree of dwarfism. The endogenous GA content in the tissue of two of the genotypes was determined by combined gas chromatography and mass spectrometry. The wild genotype developed 40 to 50% shorter stems and internodes under a low DT and high NT combination (negative difference [DIF] between DT and NT, DT/NT 15.5/21.5 or 14/24 degrees C) than under the opposite regime of high DT and low NT (positive DIF, DT/NT 22.5/16.5 or 24/14 degrees C). The GA biosynthetic mutants Is and ie, and the auxin and brassinosteroid mutant lkb responded in a similar way, but not as strongly as the wild type. The stem length of the GA-insensitive slender mutant (la cry(5)) was reduced by only 8% under negative compared to positive DIF. In the wild type endogenous GA levels decreased by 60% from positive to negative DIF in the upper part of the stem. Further, there was a corresponding decrease in the levels of precursors to GA(1), i.e. GA(53), GA(44), GA(19) and GA(20), while 2 beta-hydroxylated GA(20) and GA(1), GA(29) and GA(8), respectively, were unaffected by DIF. A similar increase in the ratios of GA(29) to GA(20) and GA(8) to GA(1) from positive to negative DIF was seen in the stem tissue of the le mutant as in the wild type. The temperature regimes affected the levels of GA(1) and its precursors in combined leaf and petiole samples and in the shoot tip in a similar manner as in the stem tissue. However, the different temperature regimes did nor affect the ratio of GA(8)/GA(1) in the shoot tip. The results indicate that altered stem elongation of the pea plants in response to diurnal temperature alternations may be mediated by changes in endogenous levels of GA(1). The GA(1) levels may be controlled by an effect of DIF on both biosynthetic and inactivation steps.