Unraveling the paradoxes of plant hormone signaling integration

Abstract

Plant hormones play a major role in plant growth and development. They affect similar processes but, paradoxically, their signaling pathways act nonredundantly. Hormone signals are integrated at the gene-network level rather than by cross-talk during signal transduction. In contrast to hormone-hormone integration, recent data suggest that light and plant hormone pathways share common signaling components, which allows photoreceptors to influence the growth program. We propose a role for the plant hormone auxin as an integrator of the activities of multiple plant hormones to control plant growth in response to the environment.

Figure 1

Phytohormone structures and functional interactions. Lines with arrowheads, upregulation of hormone biosynthetic genes or downregulation of genes involved in hormone inactivation; blocked arrows, downregulation of genes involved in hormone biosynthesis or upregulation of genes involved in inactivation of a hormone; diamond arrowheads, changes in gene expression with ambiguous outcome. Figure is adapted from previous work.

Figure 2

Tissue-specific action of hormones in the root. (a) Schematic representation of the root apex. (b) Gibberellin acts in the endodermis to control meristem size and cell elongation in the root, whereas cytokinin acts at the transition zone (gray oval), also to control meristem size. Auxin accumulates in stem cells and the columella root cap. Auxin distribution is ensured by specific carriers that transport auxin through the root (green arrow).

Figure 3

Signal integration during the shade-avoidance response. (a) Comparison of a tomato plant grown under sunlight (left) with a high R:FR ratio and a tomato plant grown under vegetative shade (right) with a low R:FR ratio. (b) Hormone and light pathway interaction in response to white light (high R:FR, left) and shade (low R:FR, right).