Co-ordinate regulation of sterol biosynthesis enzyme activity during accumulation of sterols in developing rape and tobacco seed

The activities of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, sterol methyl transferase 1 and sterol acyltransferase, key enzymes involved in phytosterol biosynthesis were shown to be co-ordinately regulated during oilseed rape (Brassica napus L.) and tobacco (Nicotiana tabacum L.) seed development. In both plants, enzyme activities were low during the initial stages of seed development, increasing towards mid-maturation where they remained stable for a time, before declining rapidly as the oilseeds reached maturity. During seed development, the level of total sterols increased 12-fold in tobacco and 9-fold in rape, primarily due to an increase in steryl ester production. In both seed tissues, stages of maximum enzyme activity coincided with periods of high rates of sterol production, indicating developmental regulation of the enzymes to be responsible for the increases in the sterol content observed during seed development. Consistent with previous studies the data presented suggest that sterol biosynthesis is regulated by two key steps, although there may be others. The first is the regulation of carbon flux into the isoprenoid pathway to cycloartenol. The second is the flux from cycloartenol to Δ5-end-product sterols. The implications of the results in terms of enhancing seed sterol levels by genetic modification are also discussed.