PIGLIUCCI, MASSIMO1* and JOHANNA SCHMITT2. 1Department of Botany, University of Tennessee, Knoxville, TN 37996-1100; 2Dept. Ecology & Evolutionary Biology, Brown University, Providence, RI 02917. - Phenotypic plasticity to foliar and neutral shade in gibberellin mutants of Arabidopsis thaliana.
To examine the role of gibberellins in plasticity to foliage shade, we
characterized the reaction norms of gibberellin-insensitive and
deficient mutants of Arabidopsis thaliana to variation in
photosynthetically active radiation (PAR) and in the ratio of red :
far red light (R:FR). We asked: (1) Do mutations in the gibberellin
(GA) signaling system alter the phenotypic plasticity of A. thaliana
to foliage shade? (2) Do GA-deficient mutants at distinct loci differ
in their effects on reaction norms when compared to a GA-insensitive
mutant? (3) If a mutation in GA signaling affects plasticity to
foliage shade, does it affect resource-mediated plasticity to reduced
PAR, or phytochrome-mediated plasticity to the R:FR cue? Mutations at
GA signaling loci altered the reaction norms of A. thaliana, but
mostly in their height, not in the degree of plasticity. There were
clear quantitative differences in reaction norms among GA loci, but
the gibberellin-insensitive mutant was not phenotypically
distinguishable from the gibberellin-deficient ones. The only effect
of mutations on the shape of reaction norm was detected for fruits
production, implying the existence of other traits affected by GA that
we did not measure and which mediate the role of these hormones in
response to light quantity but not quality. Certain GA mutants
dramatically increased reproductive fitness relative to the wild type
under the favorable conditions and unlimited growing season
encountered in the greenhouse. While this fitness advantage might not
occur under the stricter selective regime imposed by field conditions,
it does demonstrate that mutations at major regulatory loci can
dramatically and positively affect fitness, depending on the
environmental circumstances.
Key words: fitness, GA, irradiance, phenotypic plasticity, phytochromes, R:FR