The what, when & how of seedling growth regulation.

Abstract

Photomorphogenesis has two stages. First, seedlings detect light and open their embryonic leaves (cotyledons). Second, seedlings optimize their light environment by regulated elongation of the embryonic stem (hypocotyl). Several hormones, including auxin, brassinosteroids (BR) and gibberellins (GA), orchestrate the growth of the seedlings stem. The work presented in this dissertation used time-lapse imaging coupled with molecular biology to investigate the dynamics of these hormones across photomorphogenesis. These studies revealed distinct growth dynamics during each stage, in addition to changes in growth hormone sensitivity across these stages. For example, the interaction between the brassinosteroid and gibberellin pathways is quite different in early versus late promotion of seedling growth. In late stages of seedling development, the hormone auxin and the <italic>PHYTOCHROME INTERACTING FACTOR (PIF)</italic> family of transcription factors mediate an endogenous carbon-sensing mechanism regulating `light-foraging' hypocotyl growth. Dynamic analysis of growth, protein abundance and gene expression support a `regulatory relay' model of the seedling growth network where the importance of each hub changes over time.