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 phy A 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.