PHOTORESPONSES OF ARABIDOPSIS SEEDLINGS EXPRESSING AN INTRODUCED OAT PHYA CDNA - PERSISTENCE OF ETIOLATED PLANT TYPE RESPONSES IN LIGHT-GROWN PLANTS

The photocontrol of hypocotyl elongation has been studied in etiolated and light-grown wild type (WT) Arabidopsis thaliana (L. Heynh) seedlings, and in two homozygous isogenic lines that have been transformed with the oat phyA gene coding sequence under the control of the cauliflower mosaic virus (CaMV) 35S promoter. For etiolated seedlings the inhibition of hypocotyl elongation by continuous broad band far-red light (FR) is saturated at much lower photon fluence rates in the transgenic seedlings compared with WT seedlings. Furthermore, whereas de-etiolation of WT seedlings leads to loss of responsiveness of the hypocotyls to prolonged FR, de-etiolated transgenic seedlings continue to show a pronounced FR-mediated inhibition of elongation. This may reflect the persistence of a FR-high irradiance response (HIR) mediated by the introduced oat phytochrome A. Although the hypocotyls of light-grown transgenic seedlings display a qualitatively-normal end-of-day FR growth promotion, such seedlings display an aberrant shade-avoidance response to reduced red:far-red ratio (R:FR). These results are discussed in relation to the proposal that the constitutive expression of phytochrome A leads to the persistence of photoresponse modes normally restricted to etiolated plants.