The Reversible Inhibition by Red and Far-Red Light of Auxin-Induced Lateral Root Initiation in Isolated Pea Roots

lated roots by decapitation can be inhibited by a single short exposure to white light and red light inhibits lateral root formation much more effectively than blue or green light (10). With the development of knowledge about phytochrome, a photomorphogenetically active red/far-red pigment system (3, 7), it became of interest to know whether the phytochrome system acts in the light inhibition of lateral root formation. To investigate such a possibili.ty, the present w.ork was carried out using an assay based on IAA-induced root initiation in isolated pea root segments grown aseptically in nutrient culture. The results obtained show that the inhibition of lateral root formation is induced by low-intensity red light, and can be reversed by subsequent treatment with far-red light; this reversibility can be repeated numerous times by successive red and far-red irradiation. Since this assay is based on the initiation of cell division in the root pericycle, the results suggest the interesting possibility that the phytochrome system may be involved in some way in auxin-induced mitotic activity in these roots.