Two separate photoreceptors control hypocotyl growth in green seedlings

Abstract

PHOTOCONTROL of elongation growth has been extensively studied in dark-grown (etiolated) seedlings for some time. However, only relatively recently has detailed attention been given to green (de-etiolated) plants. One reason for this is that the photoreceptor, phytochrome, can easily be detected spectrophotometrically in non-green tissues but not in tissue containing chlorophyll. There is agreement that the photoreceptor operates in its low-energy mode (low irradiances or fluence rates, with red/far-red reversibility) in green seedlings as well as in dark-grown, non-green seedlings. In its high-energy mode (high irradiances and time dependency), exemplified by the action of prolonged irradiation with far-red, phytochrome might act only in etiolated seedlings^1–3. Photoregulation of stem extension in etiolated and de-etiolated seedlings is also achieved by blue light (400–500 nm), but it is still not clear if a photoreceptor other than phytochrome is responsible for this effect. Kinetic studies on etiolated seedlings suggest the operation of a separate blue-responsive photosystem². Notwithstanding this, it has recently been suggested that in green seedlings blue-light control of hypocotyl extension is due solely to absorption by phytochrome⁴. This claim, together with the evidence that changes in phytochrome photoequilibria can exert profound effects upon stem extension, may lead one to believe that this is the major photoreceptor controlling elongation growth in green plants. We report here, however, work that strongly indicates that two separate pigment systems control hypocotyl extension in green seedlings—phytochrome and a separate blue-lightreceptor. Both systems produce the same response—inhibition—but we have found it possible to separate their effects by examining the times at which the response commences.