CONTROL OF SEED STORAGE PROTEIN GENE-EXPRESSION - NEW ASPECTS ON AN OLD PROBLEM

Legume storage proteins are thought to be synthesized embryo-specifica lly during the cell-expansion phase of development and stored and meta bolized not before seed germination. Instead we have shown that Vicia faba legumin and vicilin storage proteins are synthesized early in dev elopment in the suspensor, the endosperm, the globular stage embryo an d even in the thin-walled parenchyma cells of the maternal seed coat. The proteins appear and disappear at precise developmental stages spec ific for each tissue before they again and finally accumulate in cotyl edon-stage embryos. The final expression level of the V. faba legumin B4 gene is influenced by cis-regulatory elements upstream and downstre am of the transcription initiation site acting at both the transcripti onal and posttranscriptional/translational level. Studies in transgeni c tobacco seeds revealed complex interactions between homologous trans gene sequences resulting in both trans-suppression and trans-stimulati on of legumin B4 target transgen expression. Seed protein gene express ion is not only developmentally but also metabolically regulated. In V icia faba cotyledons incubated ex planta for 2 to 4 days osmotic press ure 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 valu es 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'-f lanking, cis-regulatory elements are involved in the response.The comp lexity of the regulatory network is underlined by the fact that in Ara bidopsis thaliana the FUS3 locus specifically regulates storage protei n synthesis but also induces changes in lipid and carbohydrate metabol ism seemingly by partially omitting the dessication phase of developme nt.