Control of Seed Storage Protein Gene Expression: New Aspects on an Old Problem

Legume storage proteins are thought to be synthesized embryo-specifically during the cell-expansion phase of development and stored and metabolized not before seed germination. Instead we have shown that Vicia faba legumin and vicilin storage proteins are synthesized early in development in the suspensor, the endosperm, the globular stage embryo and even in the thin-walled parenchyma cells of the maternal seed coat. The proteins appear and disappear at precise developmental stages specific for each tissue before they again and finally accumulate in cotyledon-stage embryos. The final expression level of the V. faba legumin B4 gene is influenced by cis-regulatory elements upstream and downstream of the transcription initiation site acting at both the transcriptional and posttranscriptional/translationallevel. Studies in transgenic tobacco seeds revealed complex interactions between homologous transgene sequences resulting in both trans-suppression and trans-stimulation of legumin B4 target transgen expression. Seed protein gene expression is not only developmentally but also metabolically regulated. In Vicia faba cotyledons incubated ex planta for 2 to 4 days osmotic pressure and soluble sugar concentrations differentially influence storage protein mRNA levels. Legumin B mRNA levels are dramatically reduced by high hexose concentrations but only little influenced by osmotic values whereas vicilin mRNA levels strongly react to the osmotic value of the medium but much less to sugar concentrations. Particle bombardment experiments with legumin B4 promoter/GUS constructs suggest that 5'-flanking, cis-regulatory elements are involved in the response. The complexity of the regulatory network is underlined by the fact that in Arabidopsis thaliana the FUS3 locus specifically regulates storage protein synthesis but also induces changes in lipid and carbohydrate metabolism seemingly by partially omitting the dessication phase of development.