RESPONSE OF ACTIN MICROFILAMENTS DURING PHYTOCHROME-CONTROLLED GROWTHOF MAIZE SEEDLINGS

In seedlings of maize (Zea mays L. cv. Percival), growth is controlled by the plant photoreceptor phytochrome. Whereas coleoptile growth is promoted by continuous far-red light, a dramatic block of mesocotyl el ongation is observed. The response of the coleoptile is based entirely upon light-induced stimulation of cell elongation, whereas the respon se of the mesocotyl involves light-induced inhibition of cell elongati on. The light response of actin microfilaments was followed over time in the epidermis by staining with fluorescence-labelled phalloidin. Ln contrast to the underlying tissue, epidermal cells are characterized by dense longitudinal bundles of microfilaments. These bundles become loosened during phases of rapid elongation (between 2-3 days in irradi ated coleoptiles, between 5-6 days in dark-grown coleoptiles). The con densed bundles re-form when growth gradually ceases. The response of a ctin to light is fast. If etiolated mesocotyls are transferred to far- red light, condensation of microfilaments can be clearly seen 1 h afte r the onset of stimulation together with an almost complete block of m esocotyl elongation. The observations are discussed in relation to a p ossible role of actin microfilaments in the signal-dependent control o f cell elongation.