Auxin coordinates shoot and root development during shade avoidance response

Plants have evolved sophisticated mechanisms to sense the presence of other plants growing nearby and adjust their growth rate accordingly. The early perception of neighbor proximity depends on the detection of light quality changes. Within a vegetation community, the ratio of red (R) to far-red (FR) light is lowered by the absorption of R light by photosynthetic pigments. This light quality change is perceived through phytochrome (phyB, phyD and phyE in Arabidopsis) as a signal of the proximity of neighbors, and induces a suite of developmental responses (termed the shade avoidance response). In Arabidopsis shade avoidance is regulated by a balance of positive (PIF) and negative (HFR1/SICS1) regulators of gene expression which ensures a fast reshaping of the plant body towards an environment optimal for growth while at the same time avoiding an exaggerated reaction to low R/FR. Persistency of a low R/FR signal enhances the activity of phyA and, in turn, of HY5, a master regulator of seedling de-etiolation. Several hormones, such as gibberellins and brassinosteroids have been implicated in shade-induced elongation. However, a compelling amount of evidence indicates that low R/FR-induced changes in auxin homeostasis and auxin transport are central in the shade avoidance response. This chapter describes the recent advances in understanding how auxin coordinates plant growth in a low R/FR light environment.