Avena phytochrome a overexpressed in transgenic tobacco seedlings differentially affects red/far-red reversible and very-low-fluence responses (cotyledon unfolding) during de-etiolation

Etiolated seedlings of tobacco (Nicotiana tabacum L.) were exposed to single light pulses predicted to establish different proportions of phytochrome in its far-red absorbing form (Pfr/P). The angle between the cotyledons was compared in wild-type and transgenic seedling overexpressing Avena phytochrome A over the range of both very low-fluence responses (VLFR) and low-fluence responses (LFR). The unfolding of the cotyledons increased linearly for 24 h after the light pulse. At this time the Pfr/P-response curve showed two linear segments. The segment below a calculated Pfr/P = 3% (i.e. VLFR) was steeper than the segment above 3% (i.e. LFR). In the VLFR range the slope was almost threefold higher in transgenic than wild-type seedlings. However, in the LFR range the difference was less than 50%. From these data we propose that Avena phytochrome A makes a higher contribution to VLFR than LFR in etiolated tobacco seedlings.Abbreviations FR far-red light - LFR low-fluence response - Pfr/P proportion of phytochrome (P) in its FR-absorbing form (Pfr) - R red light - VLFR very low-fluence response Financial support was provided by the University of Buenos Aires and Fundación Antorchas (Argentina) to J.J.C., CONICET (Argentina) to R.A.S. and the U.S. Department of Energy (DE-FG02-88ER13968) to R.D.V.