Gibberellin Signaling in Barley Aleurone Grain
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Gibberellin Signaling in Barley Aleurone Grain

GAs (Gibberellins) are members of a large family of Diterpenoid compounds, which are essential for a number of processes, including Gene Expression in Cereal Aleurones, Seed Germination, Elongation, Growth, and Flowering. During the last four decades, Barley Aleurone has been a valuable system for studying GA regulation of gene expression. After germination, GAs are released from the Embryo into the Endosperm, triggering the expression of a number of genes encoding Hydrolytic enzymes in Aleurone cells. Many of these Hydrolytic enzymes, which include Alpha-Amylase, Proteases, and Cell Wall–degrading enzymes, are secreted and are responsible for digestion of the stored reserves in the starchy endosperm. The Signal transduction events leading from the Receptor to the coordination of the Complex events that make up and regulate the secretory activity of these cells are still poorly understood. A range of downstream-signaling components and events has been implicated in GA signaling in Barley Aleurone. These include the G-Alpha subunit of a Heterotrimeric G-Protein, a transient elevation in cGMP (Cyclic Guanine Monophosphate), Ca2+ (Calcium ion) dependent and Ca2+ independent events in the Cytoplasm, reversible protein phosphorylation, and several promoter cis-elements and transcription factors, including GAMyb (Gibberellin Myb) (Ref.1 & 2).

GA is perceived on the surface of Barley Aleurone cells by an unidentified outward-facing Plasmamembrane–associated GA Receptor. Binding activates, directly or indirectly, Second messengers and G-proteins. This interaction stimulates a Signal Transduction Cascade that involves the phosphorylation or dephosphorylation of proteins on Serine, Threonine, or Tyrosine. Eventually, the Signal reaches the nuclear DELLA Protein, Sln1. The DELLA proteins are highly conserved negative regulators of GA signaling in Barley. These DELLA proteins are named after a conserved Amino Acid motif near their N-termini. The DELLA proteins form a subfamily within a family of putative transcriptional regulators known as GRAS. Sln1 acts as a repressor of GA responses, inhibiting the transcription of the gene encoding the GAMyb activator of the Alpha-Amylase response. The GA signal alters Sln1, resulting in Proteasome-dependent Sln1 destabilization. The inactivation of Sln1 repressor allows the expression of GAMyb genes, as well as other genes, to proceed through transcription, processing and translation. The lag time between Sln1 disappearance and the expression of GAMyb remains poorly understood. The newly synthesized GAMyb proteins then enter the nucleus and binds to the promotor genes for Alpha-Amylase and other hydrolyting enzymes. Spy (Spindly) protein negatively regulates GA responses in Aleurone. Two of the HSI (Hordeum Spy-Interacting) Proteins, HSImyb and HSINac inhibited the GA up-regulation of Alpha-Amylase expression in Aleurone.  Recent evidence suggests that the GA regulation of GAMyb involves both transcriptional and post-transcriptional control. A GAMyb binding protein, Kgm, has been identified as a repressor of transcriptional activation of an Alpha-Amylase promoter by GAMyb. Kgm, a Mitogen-Activated Protein–like Kinase, is expressed in Aleurone cells in the absence of GA. HRT is a repressor of Alpha-Amylase gene expression. This nucleus-localized Zinc-finger protein binds to a 21-bp sequence containing the TAACAAA element, but at present there is no evidence for GA control of its repressor function (Ref.3 & 4).

Along with GAMyb, other early GA responses in Aleurone cells include increase in cytosolic Calcium, Calm(Calmodulin), and ER (Endoplasmic Reticulum)-localized Ca2+-ATPase. GA, in presence of Calcium, increased the level of Calm in Barley Aleurone layers by twofold. Calm binds to Ca2+ ions, and the resulting Ca2+–Calm Complex is capable of activating specific enzymes, such as Ca2+–Calm-dependent Protein Kinases. GA stimulates the secretion of Alpha-Amylase and other Hydrolases via a Ca2+-dependent pathway, whereas GA appears to stimulate expression of the Alpha-Amylase gene via a Calcium-independent pathway. cGMP is a candidate for a Calcium-independent signaling intermediate involved in GA-induced gene expression. cGMP play an intermediary role between Sln1 and early response genes. Increase in cGMP in response to GA correlate closely with increase in GAMyb protein in Barley Aleurone cells.  An inhibitor of Guanylyl Cyclase, the enzyme that synthesizes cGMP from GTP, blocks GA-induced Alpha-Amylase production (Ref.4 & 5). Various GA response complexes have been identified in the promoters of Alpha-Amylase genes. TAACAAA-like sequence motifs present in these GA response complexes play a key role in mediating the GA activation of transcription. The addition of the TAACAAA element to a minimal 35S promoter conferred GA responsiveness, indicating that the element can act as a GA response element. Alpha-Amylase and other Hydrolases are synthesized on the RER (Rough Endoplasmic Reticulum). Then they are secreted via the Golgi. The Secretory Pathway requires GA stimulation via Calcium-Calm dependent Signal Transduction Pathway. In conclusion, GA Signal Transduction seems to involve Calcium ions as well as cGMP, but the detailed Signaling pathways have not been worked out. Alpha-Amylase secretion is regulated by a Calcium-dependent pathway, whereas Alpha-Amylase gene expression is regulated by a Calcium-independent pathway (Ref.6).