Sulfur assimilation in developing lupin cotyledons could contribute significantly to the accumulation of organic sulfur reserves in the seed

It is currently assumed that the assimilation of sulfur into reduced forms occurs predominantly in the leaves of plants. However, developing seeds hav e a strong requirement for sulfur amino acids for storage protein synthesis . We have assessed the capacity of developing seeds of narrow-leaf lupin (L upinus angustifolius) for sulfur assimilation. Cotyledons of developing lup in seeds were able to transfer the sulfur atom from S-35-labeled sulfate in to seed proteins in vitro, demonstrating the ability of the developing coty ledons to perform all the steps of sulfur reduction and sulfur amino acid b iosynthesis. Oxidized sulfur constituted approximately 30% of the sulfur in mature seeds of lupins grown in the field and almost all of the sulfur det ected in phloem exuded from developing pods. The activities of three enzyme s of the sulfur amino acid biosynthetic pathway were found in developing co tyledons in quantities theoretically sufficient to account for all of the s ulfur amino acids that accumulate in the protein of mature lupin seeds. We conclude that sulfur assimilation by developing cotyledons is likely to be an important source of sulfur amino acids for the synthesis of storage prot eins during lupin seed maturation.