Gravistimulation leads to asymmetry of both auxin and gibberellin levels in barley pulvini

The auxin indole-3-acetic acid (IAA) is known to promote the biosynthesis of active gibberellins (GAs) in barley (Hordeum vulgare). We therefore investigated the possibility that this interaction might contribute to the gravitropic response of barley leaf sheath pulvini. Barley plants at the inflorescence stage were gravistimulated for varying times, and the pulvini were then separated into upper and lower halves for quantification of IAA and GAs by GC-MS. Consistent with the Cholodny-Went theory, the lower portion contained more IAA than did the upper portion. This difference was detected as early as 2.5 h after the start of gravistimulation, and bending was also observed at this stage. At later time points tested (6 h and 24 h), but not at 2.5 h or 3 h, the higher auxin content of the lower half was associated with a higher level of GA1, the main bioactive GA in barley. Consistent with that result, the expression of Hv3ox2, which encodes a key enzyme for the conversion of GA20 to GA1, was higher in the lower side than in the upper, after 6 h. It is suggested that in gravistimulated leaf sheath pulvini, auxin accumulates in the lower side, leading to a higher level of GA 1, which contributes to the bending response. Further evidence that GAs play a role in the gravitropic response was obtained from GA-related mutants, including the elongated sln1c mutant, in which GA signalling is constitutive. Pulvinar bending in the sln1c mutant was greater than in the wild-type. This result indicates that in the lower side of the gravistimulated pulvinus, the relatively high level of bioactive GA facilitates, but does not mediate, the bending response. © Physiologia Plantarum 2007.