Going underground: Benefits of phototropism in Arabidopsis depend on the soil environment [abstract]
Abstract
Phototropins are blue-light photoreceptors that control shoot and root phototropism in
Arabidopsis thaliana. We investigated whether the soil environment influences the benefit of
phototropism. We hypothesized that root phototropism should improve survival and growth
under dry soil conditions, but not under wet conditions. We used a greenhouse experiment to
test whether phototropism under dim light benefits plants in drought conditions. Seed of two
non-phototropic (phot 1, nph3) mutants and wildtype A. thaliana (Columbia ecotype) were
planted into a range of soil moisture regimes created by altering the sand content in the soil
and varying the watering schedule. The soil water content varied from 26%-65% (w/w) mainly
due to the sand content. Seedling size (rosette diameter) was measured three weeks after
germination. We found several significant results. First, seedling emerged earlier in wet soil
(11.5+/- 4.3 days after planting) then dry soil (15.3 +/- 5.6 d). Second, wild type plants were
significantly larger than the mutant genotypes, but the magnitude of the size advantage
depended on the soil environment. In sandy soil the wild type plants were 20% larger then the
non-phototropic mutant plants (P<0.0001), and in the clay soil, the wild type plants were only
12% larger then the non-phototropic mutant plants(P<0.01). These results agree with previous
findings that the growth advantage associated with phototropism increases in dry conditions.
These results led us to wonder what was happening below the soil surface. We asked do nonphototropic
plants differ from wild type in their rooting profiles, due to impaired root
phototropism of mutant genotypes. We tested this idea in another greenhouse experiment in
which the same genotypes were grown under dry and wet soils resembling the moisture
extremes of the first experiment. Seeds were planted along the sides of clear plastic window
boxes to enable us to track the roots. All sides except the top of the boxes were wrapped in
foil to prevent light from entering. One month later, the boxes were unwrapped and the roots
traced to determine the length and distance that each had traveled through the soil.
Measurements are currently in progress.