Integration of carbohydrate and nitrogen metabolism during legume seed development: Implications for storage product synthesis

Abstract

Summary This paper reviews recent data from our laboratory on aspects of carbon and nitrogen metabolism in developing legume embryos. The specific accumulation patterns of the major storage products starch and protein are different. Starch deposition is correlated with cell size and is arranged as a gradient increasing from the outer to the inner cotyledon side. Storage proteins start to accumulate in the outer epidermal and subepidermal cells and further accumulation does not follow the cell size gradient. Genes associated with storage activities and related to assimilate transport are expressed in a specific temporal pattern. In the embryo a H+-ATPase and a sucrose transporter are coordinately expressed together with sucrose synthase during early cotyledon stage. Expression of an amino acid transporter is highest at the mid to late cotyledon stage when legumin B transcripts start to accumulate. In order to identify branch points between C and N metabolism we changed the sugar and nitrogen state using in vitro culture of storage phase-cotyledons. Hexose sugars as well as nitrogen starvation induced transcripts of sucrose phosphate synthase and of the small subunit of ADP-glucose pyrophosphorylase, whereas a sucrose medium including nitrogen increased PEP-carboxylase and legumin B-mRNA. Hexose sugars as well as nitrogen starvation elevated sucrose and decreased amino acid levels in the cotyledons indicating that sugars and nitrogen are involved in the control of carbohydrate partitioning into starch and amino acids. In an in vivo approach, the sugar state of Vicia narbonensis embryos was changed by expressing apoplastically a legumin B4 promoter-yeast invertase construct. Embryos containing invertase have lower sucrose and higher hexose levels. Starch and protein content per seed are reduced. An increased fresh weight to dry weight ratio with severely wrinkled seeds indicates altered osmotic conditions due to a higher concentration of reducing sugars.