Free amino acid and storage protein composition of soybean fruit explants and isolated cotyledons cultured with and without methionine

The in vitro culture of immature soybean cotyledons (in direct contact with the medium) and immature fruit explants (stem dipping into the medium) on a defined medium containing glutamine and sulphate as sole sources of N and S for 7 d led to rates of growth and reserve protein accumulation close to, or greater than, those occurring in situ. Supplementation of the medium with 8.4 mM methionine had little effect on growth and protein accumulation. of the cotyledons in the explant system, but did result in significant increases in the isolated cotyledon system. Methionine suppressed the synthesis of the 7S beta-subunit in both systems. The free amino pool of the cotyledons increased more than three-fold when methionine was present in the explant medium. In the isolated cotyledon system, the basal medium alone caused a large increase (over 30-fold) in the free amino acid fraction, but methionine resulted in an even greater increase (over 50-fold). In both systems the expansion involved a very large increase in the methionine pool, but many other amino acids also showed large increases. Specific effects of methionine on individual amino acids were more clear in the explant system, where its presence resulted in marked increases in serine, alanine and asparagine. The data show that an abnormal situation arises on feeding with methionine, a fact to be considered before attributing effects on growth and protein synthesis directly to methionine. (C) 1997 Annals of Botany Company.