PHASE-CHANGE AND THE REGULATION OF TRICHOME DISTRIBUTION IN ARABIDOPSIS-THALIANA

Higher plants pass through several phases of shoot growth during which they may produce morphologically distinct vegetative structures. In A rabidopsis thaliana this phenomenon is apparent in the distribution of trichomes on the leaf surface. Leaves produced early in rosette devel opment lack trichomes on their abaxial (lower) surface, leaves produce d later have trichomes on both surfaces, and leaves in the inflorescen ce (bracts) may have few or no trichomes on their adaxial (upper) surf ace. Here we describe some of the factors that regulate this distribut ion pattern. We found that the timing of abaxial trichome production a nd the extent to which bracts lack adaxial trichomes varies in differe nt ecotypes. The production of abaxial trichomes appears to be regulat ed by the age, rather than the size of the plant, This conclusion is b ased on the observation that mutations that affect either the rate (al tered meristem programming1) or onset (paused) of leaf initiation resp ectively increase or decrease the number of leaves that lack abaxial t richomes, but have only a minor effect on the time at which the first leaf with abaxial trichomes is produced. The production of abaxial tri chomes is coordinated with the reproductive development of the shoot a s this trait is delayed by photoperiodic conditions and some mutations that delay flowering. The loss of adaxial trichomes is likely to be a consequence of floral induction, and is accelerated by terminal flowe r1-10, a mutation that accelerates inflorescence development. We demon strate that gibberellins promote trichome production in Arabidopsis an d present evidence indicating that abaxial trichome production is regu lated by both the Level of a trichome inducer and the competence of th e abaxial epidermis to respond to this inducer.