Chemical Nature- Gibberellic acid (GA3 ) is the most widely available GA. GA is synthesized from mevalonic acid. GA biosynthesis inhibitor: paelobutrazol (PAC) or uniconazole (UNI). Sites of Biosynthesis: In young tissues of the shoot and developing seeds. Hypothesized that synthesis also occurs in roots. Transport: Hypothesized that GA is transported in the xylem and phloem. Effect: Hyperelongation of shoots by stimulating both cell division and cell elongation, producing tall, as opposed to dwarf plants; induction of seed germination; stimulation of flowering on long day plants and biennials; regulation of production of seed enzymes in cereals. -Constitutive active GA mutants responses have a slender and paler-green phenotype. Mutants with GA signaling impaired resemble the GA biosynthesis mutants i.e. dark green dwarf with compound leaves and some with reduced fertility and this phenotype is not rescued by GA treatment. -Recently GID1 is thought to be the GA receptor. GID1 acts at or is upstream of the DELLA gene, as seen with double mutants in rice. Also GID1 is soluble and primarily located in the nucleus (its proposed site of action). Thought also that there is a membrane bound receptor that interacts with GA.
Positive Regulators of GA Signalling
LOF mutations identified several positive regulators of GA signaling but may function differently in various species: 1) The dwarf1 (di) mutant show a semi-dwarf phenotype but cannot be rescued by GA treatment. Studies done in cereal show that G proteins play a role in GA signaling, this is supported by the finding that DI encodes the α-subunit of the G protein. But an alternative GA signaling pathway must exist because the di null mutant is not as dwarf as a severe GA biosynthetic mutant. Furthermore G proteins may play a role in GA signaling but may function differently in various species.
2) PICKLE (PKL) protein is a positive regulator of GA signaling. It encodes a CHD3 chromatin remodeling factor. PKL mediates GA induced root differentiation during germination.
3) GID2 and SLY1 encode homologous F-box proteins subunits of the SCF complexes (class of ubiquitin E3 ligases). GID2 and SLY1 appear to modulate the GA responses by controlling the stability of a class of negative regulators SLR/RGA of GA signaling.
Negative Regulators of GA Signalling LOF and GOF Mutations Find Several Negative Regulators
1) SPY inhibits an early step in GA signaling and also it regulates additional cellular pathways. Active SPY functions as a homotrimer. It is detected in both cytoplasm and nucleus in plant cells and hence targets proteins that could be present in both cellular compartments. -SPY enhances cytokinin responses and inhibits GA signaling through distinct mechanisms. Mutants of spy are resistant to exogenous application of cytokinin. GA also represesses the effects of cytokinin therefore implicating that there is a crosstalk between the two normal response pathways.
2) DELLA proteins (RGA, GAI, RGL1, RGL2, RGL3) belong to the plant specific GRAS protein family. They function likely as transcriptional regulators or they may act as coactivators or corepressors. DELLA proteins function as repressors is conserved between dicots and monocots but multiple functionally overlapping genes are required for fine tuning GA regulation in dicots. -RGA and GAI have partially redundant functions in maintaining the repressive state of the GA signaling pathway. -DELLA domain is important in the inactivation of the DELLA proteins by the GA signal. DELLA motif is essential for DELLA protein degradation in response to the GA signal and deletion of this motif confers a GA unresponsive dwarfing phenotype but the DELLA domain alone is insufficient for GA-induced degradation.
Model of GA Signalling Pathways in Arabidopsis
The DELLA proteins are transcriptional regulators that directly or indirectly inhibit GA-activated genes. SPY is hypothesized to be a negative regulator by activating RGA and GAI. The GA signal derepresses its signaling pathway by promoting RGA protein degradation through the SCFSLY1 proteosome pathway. The concentrations of bioactive GAs determine the degree of activation of the GA response pathway by controlling the extent of RGA/GAI/RGLs inactivation through SLY1. RGA and GAI are the major repressors that modulate GA-induced leaf and stem growth, and also transition from vegetative to reproductive phase. -biochemical studies need to be performed to demonstrate that RGA/GAI/RGLs are the direct substrates of SPY. Also future biochemical and genetic studies are needed to incorporate the G protein, SHI and PKL into the Arabidopsis model. -cereal aleurone cells are used to analyze GA signal transduction and gene expression. There has been discovery of GA response complexes containing a GA response element (GARE) seen in the promoters of hydrolase genes. -in aleurone cells the action of GA inducing hydrolytic enzymes via activation of calcium/calmodulin signaling, can be blocked by ABA. -the activity of the GA response pathway is tightly linked to the activation in GA biosynthesis and catabolism by a feedback mechanism. GA oxidases control the final steps in the biosynthesis of bioactive GAs and also the inactivation of GAs.
