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

Abstract

The activities of 3-hydroxy-3-methylglutarylcoenzyme 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 midmaturation 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 D 5 -end-product sterols. The implications of the results in terms of enhancing seed sterol levels by genetic modification are also discussed.

Spatial and temporal regulation of sterol biosynthesis in Nicotiana benthamiana

“…), corroborating earlier conclusions that multiple control points for sterol biosynthesis operate in plants (Vögeli and Chappell , Chappell et al , Newman and Chappell ). Overall, the data herein and findings from other studies (Harker et al , Schrick et al ) provide support for co‐regulation of enzymes for sterol biosynthesis during plant development.…”

“…While there have been many prior studies of profiling sterol composition at discrete stages of plant development, such as developing (Davis and Poneleit , Harker et al , Schrick et al ) or germinating (Baisted ) seeds or in specific tissues, such as tubers (Bergenstråhle et al ) or leaves (Grunwald , Schrick et al ), we sought initially to establish a whole‐plant perspective and thus to determine the profile of dominant sterols in tissues representing a cross‐section of development (Fig. B).…”

“…For instance, while induction and suppression of this enzyme activity have been associated with changes in the flux of carbon through the central mevalonate pathway, other enzymes exhibit similar magnitude changes (Vögeli and Chappell ). Also, during rape ( Brassica napus ) and tobacco ( Nicotiana tabacum ) seed development, increased HMGR and STEROL METHYLTRANSFERASE 1 (SMT1) activities coincide with periods of high rates of sterol synthesis (Harker et al ). Moreover, over‐ and knockdown/out‐expression studies of HMGR and other sterol biosynthetic genes also argue that HMGR is not an exclusive control point for sterol biosynthesis in plants (Chappell , Holmberg et al ).…”

Enhanced seed phytosterol accumulation through expression of a modified HMG‐CoA reductase

“…There is evidence of co-ordinated regulation of genes encoding HMG-CoA reductase and other enzymes in the phytosterol biosynthesis pathway in the developing seeds of tobacco and oilseed rape (Brassica napus), expression peaking at midmaturation (Harker et al, 2003a). The expression of an introduced HMG-CoA reductase gene would have to coincide with this.…”

Modules of co-regulated metabolites in turmeric (Curcuma longa) rhizome suggest the existence of biosynthetic modules in plant specialized metabolism

“…When it is considered that only around 4–8% of all plants have been investigated in any detail for the metabolites that they produce (422 000 plant species estimated, 35 000 species tested for anti-cancer activity by NCI, 15 254 registered in the KNApSAck database), having such a tool in hand could lead to great strides in our understanding, not only of what compounds plants produce but also of how such compounds are produced and how their production is regulated. It has been known for quite some time, for example, that the activity of enzymes such as HMG-CoA reductase (HMGR) and phenylalanine ammonia lyase (PAL) influence the rates of production of a large number and a wide variety of downstream compounds (Camm and Towers, 1973; Stermer et al , 1994; Fukasawa-Akada et al , 1996; Britton et al , 1998; Weisshaar and Jenkins, 1998; Harker et al , 2003; Winkel, 2004). It could be argued that these ‘key’ enzymes regulate large metabolite modules that represent entire biosynthetic pathways.…”