EXPRESSION PATTERNS OF GASA GENES IN ARABIDOPSIS-THALIANA - THE GASA4GENE IS UP-REGULATED BY GIBBERELLINS IN MERISTEMATIC REGIONS

The GASA gene family previously identified in Arabidopsis belongs to a wide-spread class of genes found in mono- and dicotyledonous plants, all structurally related to the original GA-regulated GAST1 gene from tomato, They encode small peptides (97 to 112 residues) of unknown fun ction sharing a 60 amino acid conserved C-terminal domain comprising t welve conserved cysteine residues which define a pattern not related t o other known cysteine-rich motifs. Northern blot hybridization analys is revealed sequential expression of three genes during flowering, sil ique development and seed germination. GASA4 transcripts were detected in flower buds. GASA1 transcripts markedly accumulated in siliques, a bout five days after pollination, and correlated with the peak of GA b iosynthesis at this stage of silique development. GASA3 transcripts ac cumulated at the end of the maturation stage of the silique, and trans cripts were still present in dry seeds but degraded rapidly during imb ibition. In addition, the GASA4 gene was again actively transcribed af ter germination and this expression was shown to be dependent on the p resence of GAs in GA-deficient mutants. Immunoblot analysis confirmed the presence of the GASA4 gene product in flower buds, seedlings and r oots. We focused on the GASA4 gene and characterized its expression. T he upstream region (-890 to +128) was fused to the GUS reporter gene. GASA4/GUS expression was detected in transgenic Arabidopsis primarily in all meristematic regions, including vegetative, inflorescence and f loral meristems, as well as primary and lateral root tips. In a GA-def icient background (ga1-3), GUS activity in the vegetative meristem was detected only in the presence of supplied GA. In root and flower meri stems, basal GUS activity was slightly enhanced by exogenous GA. Inter estingly, GA strongly inhibit GUS activity in expanding cotyledons and leaves in ga1-3 mutants supplied with exogenous GAs, as well as in th e wild type. The GA-dependent meristem-specific expression pattern sug gests that the GASA4 protein plays a role in dividing cells rather tha n in elongating cells.