This review covers recent advances in gibberellin
(GA) signaling. GA signaling is now understood to
hinge on DELLA proteins. DELLAs negatively regulate
GA response by activating the promoters of several genes
including Xerico, which upregulates the abscisic acid pathway
which is antagonistic to GA. DELLAs also promote
transcription of the GA receptor, GIBBERELLIN INSENSITIVE
DWARF 1 (GID1) and indirectly regulate GA
biosynthesis genes enhancing GA responsiveness and
feedback control. A structural analysis of GID1 provides a
model for understanding GA signaling. GA binds within a
pocket of GID1, changes GID1 conformation and increases
the aYnity of GID1 for DELLA proteins. GA/GID1/
DELLA has increased aYnity for an F-Box protein and
DELLAs are subsequently degraded via the proteasome.
Therefore, GA induces growth through degradation of the
DELLAs. The binding of DELLA proteins to three of the
PHYTOCHROME INTERACTING FACTOR (PIF) proteins
integrates light and GA signaling pathways. This
binding prevents PIFs 3, 4, and 5 from functioning as positive
transcriptional regulators of growth in the dark. Since
PIFs are degraded in light, these PIFs can only function in
the combined absence of light and presence of GA. New
analyses suggest that GA signaling evolved at the same
time or just after the plant vascular system and before
plants acquired the capacity for seed reproduction. An analysis
of sequences cloned from Physcomitrella suggests that
GID1 and DELLAs were the Wrst to evolve but did not
initially interact. The more recently diverging spike moss
Selaginella has all the genes required for GA biosynthesisand signaling, but the role of GA response in Selaginella
physiology remains a mystery.