Phytochrome-induced transmissible signal elicits germination response in turions of Spirodela polyrhiza

The role of cell competence, including the spatiotemporal aspect of phytoch rome-induced long-distance signal transmission, was investigated in turions of Spirodela polyrhiza (L.) Schleiden, Irradiation of the dorsal surface o f the turions triggered a significant germination response, while identical treatment of the ventral surface was less effective. Red-light (R) microbe am irradiation of a subregion (ca. 1 mu m(2)) of the dorsal surface could i nduce the germination response. Therefore, photoactivation of phytochrome i n a single cell or few cells is sufficient to trigger the photomorphogeneti c response, The ultimate response occurs at the proximal end of the turion by way of growth and emergence of the frond primordia about 1.3 mm away fro m the microbeam-irradiated distal cell(s), This photoinduction was reversib le by a pulse of far-red light (FR) given less than 24 h after R microbeam irradiation. Microsurgical separation of distal (irradiated) and proximal ( primordium bearing) halves of the turions following microbeam irradiation f urther revealed that the light-induced transmissible signal can be intercep ted and that it required more than 48 h to traverse one half distance of th e turions, Based on the kinetics of the signal transmission, the possible i nvolvement of light scattering, light piping, or transfer of electrophysiol ogical signals can be excluded, Taken together, the results indicate that a transmissible signal is generated by the irradiated cell(s) and propagated across to the non-irradiated cells, leading to induction of the photomorph ogenetic response.