Gibberellin Signal Transduction in Stem Elongation & Leaf Growth

Abstract

The effect of gibberellin (GA) on promoting stem growth was first discovered in 1930s by studies of the Bakanae(foolish seedling) disease in rice (57). Gibberella fujikuroi, a pathogenic fungus, produces gibberellic acid (GA3) that causes the infected rice plants to grow so tall that they fall over. Later studies of dwarf mutants and analysis of their GA contents revealed that bioactive GAs are endogenous hormones that regulate the natural developmental processes including stem growth in plants. An increase in both cell elongation and cell division occurs during stem growth. GA induces transcription of genes involved in these processes. For example, expression of some of the genes encoding xyloglucan endotransglycosylases (XETs1) and expansins are upregulated by GA in elongating internodes in rice and in Arabidopsis (7, 62, 66). XET is thought to increase the plasticity of the cell wall because this enzyme is involved in xyloglucan reorganization through cleaving and re-ligating xyloglucan polymers in the cell wall. Expansins are also extracellular proteins that cause plant cell wall loosening, probably by disrupting the polysaccharide adhesion. Transcripts of the genes encoding for cyclin-dependent protein kinases are also elevated in intercalary meristem in rice after GA treatment (12).