GENETIC-ENGINEERING FOR HIGH METHIONINE GRAIN LEGUMES

Methionine (Met) is the primary limiting essential amino acid in grain legumes. The imbalance in amino acid composition restricts their biol ogical value (BV) to 55 to 75% of that of animal protein. So far impro vement of the BV could not be achieved by conventional breeding. There fore, genetic engineering was employed by several laboratories to reso lve the problem. Three strategies have been followed. A) Engineering f or increased free Met levels; B) engineering of endogenous storage pro teins with increased numbers of Met residues; C) transfer of foreign g enes encoding Met-rich proteins, e. g. the Brazil nut 2S albumin (BNA) and its homologue from sunflower, into grain legumes. The latter stra tegy turned out to be most promising. In all cases the gene was put un der the control of a developmentally regulated seed specific promoter and transferred into grain legumes using the bacterial Agrobacterium t umefaciens-system. Integration into and copy numbers in the plant geno me as well as Mendelian inheritance and gene dosage effects were verif ied. After correct precursor processing the mature 2S albumin was intr acellularly deposited in protein bodies which are part of the vacuolar compartment. The foreign protein amounted to 5 to 10% of the total se ed protein in the best transgenic lines of narbon bean (Vicia narbonen sis L., used in the authors' laboratories), lupins (Lupinus angustifol ius L., used in CSIRO, Australia), and soybean (Glycine max (L.) Merr. , used by Pioneer Hi-Bred, Inc., USA). In the narbon bean the increase of Met was directly related to the amount of 2S albumin in the transg enic seeds, but in soybean it remained below the theoretically expecte d value. Nevertheless, trangenic soybean reached 100%, whereas narbon bean and lupins reached approximately 80% of the FAO-standard for nutr itionally balanced food proteins. These results document that the Met problem of grain legumes can be resolved by genetic engineering.