Gibberellin-responsive genes: high level of transcript accumulation in leaf sheath meristematic tissue from Zea mays L.

In order to identify genes that are related to the gibberellin (GA) response in maize (Zea mays L.), mRNA species from wild-type and single-gene dwarf mutants, d5 and D8, were compared by fluorescent differential display. The d5 mutant is unable to produce biologically active GA, but recovers its tall stature on exogenous application of GA. D8 is insensitive to GA, despite the accumulation of a high level of endogenous GA, suggesting it to be a receptor mutant or a mutant in signal transduction pathway(s). After screening 7000 cDNA populations, one clone was isolated, for which transcripts were rare in d5 shoots but accumulated within 1 h after GA3 application. This clone, designated as ZmGR1a, encodes a polypeptide with a relative molecular mass of ca. 13 kDa, which shows significant homology to proline-rich proteins from several plant species. A similar experiment with D8 identified a clone, ZmGR2a, with low transcript levels, but accumulation within 6 h after GA3 treatment of d5 shoots. ZmGR2a encodes a polypeptide with a relative molecular mass of ca. 19 kDa, which shows no significant homology with any known protein. Southern blot analysis indicated that ZmGR1a and ZmGR2a form a small multigene family within the maize genome. In situ hybridization with wild-type seedlings showed transcripts on both to be abundant in leaf sheath meristematic tissue, in which GA enhances cell elongation and cell division.