A BIOTECHNOLOGICAL APPROACH TO IMPROVING THE NUTRITIVE-VALUE OF ALFALFA

The postruminal supply of the sulfur-containing amino acids, methionin e and cysteine, has been reported to be a major limitation to wool gro wth in sheep. We aim to improve the protein quality of forage for rumi nants by introducing into alfalfa chimeric genes encoding a ruminally stable, sulfur amino acid-rich protein from sunflower seeds. Four gene constructs were transferred to Australian commercial cultivars of alf alfa using Agrobacterium tumefaciens-mediated transformation and selec tion with phosphinothricin (PPT). Modification of the sunflower seed a lbumin protein-coding region by addition of the coding information for an endoplasmic reticulum (ER) retention signal was found to greatly i ncrease the level to which the sulfur amino acid-rich protein accumula ted in the leaves of transgenic alfalfa plants. The Cauliflower Mosaic Virus (CaMV) 35S promoter and two light-regulated plant gene promoter regions were compared for their ability to direct high-level expressi on of the introduced genes in alfalfa leaves. The highest expression o f sunflower seed albumin was found in transformants bearing a gene inc orporating the promoter from the Arabidopsis thaliana ats1A gene, whic h encodes the ribulose bisphosphate carboxylase small subunit. The hig hest level of sunflower seed albumin found in transgenic alfalfa leave s was estimated to constitute .1% of soluble leaf protein. This level of accumulation of the foreign protein would be predicted to supply an extra 40 mg of sulfur amino acids daily to sheep fed the modified for age. Published studies in which wool growth rates were significantly i ncreased employed supplementation of approximately 1 to 2 g of sulfur amino acids daily.