Phenotype pictures of Arabidopsis thaliana in high light and low light conditions

The rate at which plants grow is a major functional trait in plant
ecology. However, little is known about its evolution in natural
populations. Here, we investigate evolutionary and environmental factors
shaping variation in the growth rate of Arabidopsis thaliana. We used
plant diameter as a proxy to monitor plant growth over time in
environments that mimicked latitudinal differences in the intensity of
natural light radiation, across a set of 278 genotypes sampled within four
broad regions, including an outgroup set of genotypes from China. The
observed variation was validated in a field experiment conducted under
natural conditions. All genotypes grew markedly wider rosettes when the
light supply was decreased, demonstrating that environmental plasticity is
a predominant source of variation to adapt plant size to prevailing light
conditions. Yet, we detected significant levels of genetic variation both
in growth rate and growth plasticity. Genome-wide association studies
revealed that only 2 single nucleotide polymorphisms associate with
genetic variation for growth above Bonferroni confidence levels. However,
marginally associated variants were strongly enriched among genes with an
annotated role in growth and stress reactions. Polygenic scores computed
from marginally associated variants confirmed the polygenic basis of
growth variation. For both light regimes, phenotypic divergence between
Europe and the distantly related populations of China is smaller than the
variation observed within Europe indicating that plant growth is
constrained by stabilizing selection. We observed that Spanish genotypes,
however, reach a significantly larger size than Northern European
genotypes. Tests of adaptive divergence and analysis of the individual
burden of deleterious mutations reveal that adaptive processes have played
a more important role in shaping regional differences in rosette growth
than maladaptive evolution.