Differentiation of legume cotyledons as related to metabolic gradients and assimilate transport into seeds

Abstract

Legume seed development is closely related to metabolism and nutrient transport. To analyse this relationship, a combination of biochemical, histological and transgenic approaches was used. Sugars within tissue sections have been quantitatively measured by metabolic imaging. During cotyledon differentiation glucose gradients emerge related to a particular cell type, with higher concentrations in non-differentiated premature regions. Sucrose in creases at the beginning of maturation in a layer underneath the outer epidermis expressing a sucrose transporter. Sucrose distribution is initially controlled by uptake activity and the permeability within the parenchyma and, later on, also by differences in growth and starch accumulation. Increased sucrose levels are accompanied by increased levels of sucrose synthase and ADP-Glc pyrophosphorylase mRNAs, but carbon flux into starch is initially still low. Rates increase at a stage when hexose concentrations become low, allowing increased flux through the sucrose synthase pathway. Transfer cell formation represents a regional specification of the cotyledonary epidermis for embryo nutrition characterized by increased transport-active cell surfaces and up-regulated expression of transport-related genes. The E2748 pea seed mutation blocks epidermal differentiation into transfer cells and leads to the loss of epidermal cell identity. Embryos with impaired epidermis cannot tolerate elevated levels of sucrose and respond with disorganized growth. The E2748 gene product is required for transfer cell formation in developing cotyledons with no other function during plant growth. Seed coat permeability provides a hypoxic environment for embryo development. However, at maturity, seed energy supply is not limited indicating fundamental developmental and metabolic adaptations. Results from transgenic seeds show that altered expression of single genes induces complex and unexpected changes. In AGP-antisense seeds the block in starch synthesis leads to pleiotropic effects of water and nitrogen content and induces temporal changes in seed development.