DIFFERENTIAL REGULATION OF TRICHOME FORMATION ON THE ADAXIAL AND ABAXIAL LEAF SURFACES BY GIBBERELLINS AND PHOTOPERIOD IN ARABIDOPSIS-THALIANA (L) HEYNH

In wild-type (WT) Columbia and Landsberg erecta ecotypes of Arabidopsi s thaliana (L.) Heynh., trichomes are present on the adaxial surfaces of all rosette leaves but are absent from the abaxial surfaces of the first-formed leaves. We have determined that both long-day (LD) photop eriod and gibberellin (CA) stimulate trichome formation. WT plants gro wn in LD conditions produce the first abaxial trichome on earlier leav es than plants grown in short-day (SD) conditions. Photoperiod sensiti vity of abaxial trichome formation on Wi plants develops gradually ove r time, reaching the maximum sensitivity about 24 d after germination. Application of gibberellic acid to WT plants growing in SD conditions accelerates the onset of abaxial trichomes. Conversely, application o f 20 to 80 mg L(-1) paclobutrazol, a CA biosynthesis inhibitor, to wil d-type plants suppresses trichome initiation on the abaxial epidermis. The GA-deficient mutants ga1-5 and ga4-1 and the CA-insensitive mutan t gai-1 exhibit delayed onset of abaxial trichomes when grown in LD co nditions. The null mutant ga1-3 produces completely glabrous leaves wh en grown in SD conditions. Application of gibberellic acid to glabrous ga1-3 plants consistently induces earlier formation of trichomes on t he adaxial epidermis than on the abaxial epidermis, demonstrating a di fference between the adaxial and abaxial surfaces in their response to GA with regard to trichome formation.