Dual effect of phytochrome A on hypocotyl growth under continuous red light

Authors

  • M. A. MAZZELLA,

    1. Departamento de Ecología, Facultad de Agronomía, Universidad de Buenos Aires, Av. San Martin 4453, 1417-Buenos Aires Argentina
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  • T. M. ALCONADA MAGLIANO,

    1. Departamento de Ecología, Facultad de Agronomía, Universidad de Buenos Aires, Av. San Martin 4453, 1417-Buenos Aires Argentina
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  • J. J. CASAL

    Corresponding author
    1. Departamento de Ecología, Facultad de Agronomía, Universidad de Buenos Aires, Av. San Martin 4453, 1417-Buenos Aires Argentina
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Dr Jorge J. Casal, Departamento de Ecología, Facultad de Agronomía, Universidad de Buenos Aires, Av. San Martin 4453, 1417-Buenos Aires, Argentina.

ABSTRACT

The role of phytochrome A in the control of hypocotyl growth under continuous red light (Rc) was investigated using phyA and phyB mutants of Arabidopsis thaliana, which lack phytochrome A (phyA) or phytochrome B (phyB), respectively, and transgenic seedlings of Nicotiana tabacum overexpressing Avena phyA, compared to the corresponding wild type (WT). In WT seedlings of A. thaliana, hypocotyl growth inhibition showed a biphasic response to the fluence rate of Rc, with a brake at 10−2μmol m−2 s−1. At equal total fluence rate, hourly pulses of red light caused slightly more inhibition than Rc. The response to very low fluences of continuous or pulsed red light was absent in the phyA and phyA phyB mutants and present in the phyB mutant. The second part of the response was steeper in the phyA mutant than in the WT but was absent in the phyB mutant. In WT tobacco the response to Rc was biphasic. Overexpression of Avena phyA enhanced the response only at very low fluence rates of Rc (< 10−2μmol m−2 s−1). In both species, the effect of hourly pulses of far-red light was similar to the maximum inhibition observed in the first phase of the response to Rc. Using reciprocity failure (i.e. higher inhibition under continuous than pulsed light) as the operational criterion, a ‘true’ high-irradiance reaction occurred under continuous far-red light but not under Rc or red plus far-red light mixtures. Native and overexpressed phyA are proposed to mediate very low fluence responses under Rc. In WT A. thaliana, this effect is counteracted by a negative action of phyA on phyB-mediated low-fluence responses.

Abbreviations
FR

far-red light

FRc

continuous far-red light

FRp

pulses of far-red light

HIR

high-irradiance response(s)

LFR

low-fluence response(s)

Pfr

far-red light absorbing form of phytochrome

phyA

phytochrome A

phyB

phytochrome B

Pr

red light absorbing form of phytochrome

R

red light

Rc

continuous red light

Rp

pulses of red light

VLFR

very-low-fluence response(s)

Ancillary