Analysis of in vivo and in vitro Globulin Formation during Cotyledon Development of Field Beans (V icia faba L. var. minor)

Abstract

Cotyledons of field beans synthesize and accumulate the storage globulins vicilin and legumin concomitantly with the rapid cell elongation growth of storage tissue during stage 2 of embryogenesis. Biosynthesis of vicilin and legumin starts simultaneously when the cotyledon length reaches 2-3 mm. Vicilin is more actively formed in the beginning whereas legumin synthesis becomes the predominant event of globulin accumulation towards the end of developmental stage 2. Some minor globulin components of unknown nature appear in the middle of the globulin deposition period but do not contribute significantly to the storage protein quantity finally contained in mature seeds. In vitro translation of total cellular as well as of polysomal poly(A)-containing RNA leads to the formation of primary globulin polypeptides with larger Mr than that of in vivo formed globulins from mature seeds. The Mr of the primary in vitro translation products of legumin mRNA are of approximately similar size as the Mr of the disulfide-linked alpha- and beta-chains of in vivo formed mature legumins, thus suggesting that in accordance with other legume 11S globulins the subunits are covalently linked in a legumin propolypeptide. Cell-free translation of similar quantities of total cellular poly(A)-containing RNA from different developmental stages in a standardized wheat germ system reflects changes in the pattern of vicilin and legumin formation which correspond to the in vivo experiments. Polysome translation leads to similar results. The availability of mRNA for the globulin synthesis at cytoplasmic membrane-bound polysomes preferentially determines the pattern of vicilin and legumin formation during cotyledon development.