High-Irradiance Response Signaling Is More Ancient Than Phytochrome A

AbstractThe control of seed germination by red and far-red light is one of the earliest documented phytochrome-mediated processes Phytochrome is now known to be a small family of photoreceptors whose apoproteins are encoded by different genes Phytochrome B (phyB) is present in dry seeds and affects germination of dark imbibed seeds but other phytochromes could also be involved Phytochrome A (phyA) appears after several hours of imbibition and mediates very-low-fluence responses PhyB and other phytochromes different from phyA mediate the classical low-fluence responses The phytochrome involved in high-irradiance responses of seed germination (inhibition of germination under continuous far-red) has not been unequivocally established, although phyA is the most likely candidate Phytochrome can affect embryo growth capacity and/or the constraint imposed by the tissues surrounding the embryo At least in some species, gibberellins participate in the signalling process In the field, phyA has been implicated in the perception of light during soil cultivations, and phyB would be involved in the perception of red/far-red ratios associated with the presence of gaps in the canopy This review describes recent advances in phytochrome research, particularly those derived from the analysis of germination in specific mutants, and their connection with traditional observations on phytochrome control of seed germination

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

Functional Plant Biology ◽

10.1071/pp99123 ◽

2001 ◽

Vol 28 (5) ◽

pp. 401 ◽

Cited By ~ 4

Author(s):

David J. Bagnall ◽

Rod W. King

Keyword(s):

Arabidopsis Thaliana ◽

Photoperiodic Response ◽

High Irradiance ◽

Phytochrome A ◽

Wild Type ◽

Long Day ◽

Transgenic Lines ◽

A Minor ◽

Type Response ◽

Plant Arabidopsis Thaliana

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 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.

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