Phytochrome, photosynthesis and flowering of Arabidopsis thaliana:: photophysiological studies using mutants and transgenic lines

A number of phytochrome mutants have been examined for involvement in high irradiance (HIR) or red/far-red (R/FR) end-of-day (EOD) photoresponses during flowering of the long-day (LD) plant, Arabidopsis thaliana (L.) Heynh. A large component of phytochrome A (phyA) response is shown to involve an indirect effect via photosynthesis. When grown autotrophically in soil at a low irradiance (80 mu mol m(-2) s(-1)), the phyA-211 mutant flowered extremely late compared with wild type and its leaf area was halved, both effects being reversed by increase in photosynthetic irradiance. Supplying sucrose via agar led to very early flowering with little indication of an additional direct phyA HIR. For light-stable phytochrome apoprotein mutants (phyB, phyD) or chromophore mutants (hy1, hy2), flowering was early and R/FR photoreversible EOD response was erased. Conversely, flowering was delayed in a transgenic line overexpressing the PHYB apoprotein. The FR EOD promotion of flowering via phyB was retained in darkness, brief night interruptions mimicking LD response. This novel finding emphasizes the importance of phyB-like phytochromes, with phyA acting indirectly. Whether phyB influences time measurement remains uncertain as we found no rhythmicity in this response to night interruptions. Overall, the role(s) of phytochromes in the regulation of flowering of Arabidopsis include EOD phyB-type response, a minor phyA photoperiodic response, and a large indirect phyA effect involving photosynthesis.