Evol Ecol Res 6: 243-259 (2004)     Full PDF if your library subscribes.

Phenotypic plasticity in response to foliar and neutral shade in gibberellin mutants of Arabidopsis thaliana

Massimo Pigliucci1* and Johanna Schmitt2

1Departments of Botany and of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996-1100 and  2Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02917, USA

Author to whom all correspondence should be addressed.
e-mail: pigliucci@utk.edu


To examine the role of gibberellins in regulating plasticity to foliage shade, we characterized the reaction norms of gibberellin-insensitive and -deficient mutants of Arabidopsis thaliana to variation in photosynthetically active radiation and in the ratio of red : far red light (R : FR). We asked: (1) Do mutations affecting the gibberellin signalling system alter the phenotypic plasticity of A. thaliana to foliage shade? (2) Do gibberellin-deficient and gibberellin-insensitive mutants have different reaction norms? (3) If a mutation in gibberellin signalling alters reaction norms to foliage shade, does it affect resource-mediated plasticity to reduced photosynthetically active radiation, or phytochrome-mediated plasticity to the R : FR cue? Mutations at gibberellin signalling loci altered the reaction norms of A. thaliana, but had greater impact on trait means than on plasticity. There were clear quantitative differences in reaction norms among gibberellin loci, but the gibberellin-insensitive mutant gai-1 was not phenotypically distinguishable from the gibberellin-deficient ga1-5, ga1-6, ga4-1 and ga5-1. The major effect of mutations on the shape of reaction norms was detected for fruit production: some of the gibberellin mutants that we examined dramatically increased reproductive fitness relative to the wild type under the favourable 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 significantly positively affect fitness, depending on the environmental circumstances.

Keywords: gibberellin, irradiance, mutants, phenotypic plasticity, red : far red light ratio, shade avoidance.

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