Ethylene and auxin control the Arabidopsis response to decreased light intensity.

Morphological responses of plants to shading have long been studied as a function of light quality, in particular the ratio of red to far red light that affects phytochrome activity. However, changes in light quantity are also expected to be important for the shading response because plants have to adapt to the reduction in overall energy input. Here, we present data on the involvement of auxin and ethylene in the response to low light intensities. Decreased light intensities coincided with increased ethylene production in Arabidopsis rosettes. This response was rapid because the plants reacted within minutes. In addition, ethylene- and auxin-insensitive mutants are impaired in their reaction to shading, which is reflected by a defect in leaf elevation and an aberrant leaf biomass allocation. On the molecular level, several auxin-inducible genes are up-regulated in wild-type Arabidopsis in response to a reduction in light intensity, including the primary auxin response gene IAA3 and a protein with similarity to AUX22 and the 1-aminocyclopropane-1-carboxylic acid synthase genes ACS6, ACS8, and ACS9 that are involved in ethylene biosynthesis. Taken together, the data show that ethylene and auxin signaling are required for the response to low light intensities.