Enhanced seed phytosterol accumulation through expression of a modified HMG-CoA reductase.
The regulation of phytosterol biosynthesis in seeds is of interest to biotechnologists because of the efficacy of dietary phytosterols in reducing blood cholesterol in humans. Mevalonate synthesis via 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase) is a key step in phytosterol biosynthesis. HMG-CoA reductase is inactivated by phosphorylation by SNF1-related protein kinase 1 (SnRK1). With the aim of increasing seed phytosterol levels, transgenic tobacco plants were produced expressing a full-length Arabidopsis (Arabidopsis thaliana) HMG-CoA reductase gene (HMG1) coding sequence, a modified HMG1 sequence encoding a protein lacking the target serine residue for phosphorylation by SnRK1, or a chimaeric sequence encoding the N-terminal domain of the Arabidopsis HMG1 enzyme fused with the catalytic domain of yeast HMG-CoA reductase, which lacks an SnRK1 target site. All three transgenes (35S-AtHMG1, 35S-AtHMG1m and 35S-AtScHMG1) were under the control of a cauliflower mosaic virus 35S RNA promoter. Levels of seed phytosterols were up to 2.44-fold higher in plants transformed with the 35S-AtHMG1m gene than in the wild-type, and were significantly higher than in plants expressing 35S-AtHMG1 or 35S-AtScHMG1. In contrast, levels of phytosterols in leaves of plants transformed with the 35S-AtHMG1m gene were unchanged, suggesting that regulation of HMG-CoA reductase by SnRK1 is an important factor in seeds but not in leaves. A total of 11 independent transgenic lines expressing 35S-AtHMG1m or 35S-AtScHMG1 also showed an altered flower phenotype, comprising a compact floret, prolonged flowering, short, pale petals, a protruding style, short stamens, late anther development, little or no pollen production, premature flower abscission and poor seed set. Because of this phenotype, the modified HMG-CoA reductase gene would have to be expressed seed specifically if it were to be engineered into a crop plant for biotechnological purposes.
Hey
Sandra J
SJ
Crop Performance and Improvement, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK.
Powers
Stephen J
SJ
Beale
Michael H
MH
Hawkins
Nathaniel D
ND
Ward
Jane L
JL
Halford
Nigel G
NG
D017360:Arabidopsis / Q000235:genetics; D017796:Caulimovirus / Q000235:genetics; D035264:Flowers / Q000033:anatomy & histology / Q000235:genetics / Q000378:metabolism; D005814:Genes, Viral; D006903:Hydroxymethylglutaryl CoA Reductases / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D010766:Phosphorylation; D010840:Phytosterols / Q000096:biosynthesis*; D018515:Plant Leaves / Q000235:genetics / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D030821:Plants, Genetically Modified / Q000033:anatomy & histology / Q000201:enzymology*; D011401:Promoter Regions, Genetic; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012639:Seeds / Q000235:genetics / Q000378:metabolism; D014026:Nicotiana / Q000235:genetics*; D019076:Transgenes
D010840:Phytosterols; D010940:Plant Proteins; D011993:Recombinant Fusion Proteins; D006903:Hydroxymethylglutaryl CoA Reductases; C071570:SNF1-related protein kinases; D017346:Protein Serine-Threonine Kinases