Evidence for a mevalonate shunt in a tracheophyte

Primary leaf tissue from light and dark grown wheat seedlings incubated with [2-¹⁴C]acetate and [2-³H]mevalonolactone (MVA) synthesized doubly labelled sterols and long chain fatty alcohols (LCFA). While [2-³H]MVA was incorporated into LCFA as efficiently as into sterols, [5-³H]MVA was metabolized only to sterols. Mevinolin, a specific inhibitor of HMG-CoA reductase, completely inhibited [2-¹⁴C]acetate incorporation into sterols but it did not completely prevent [2-³H]MVA from being incorporated into LCFA. In the presence and absence of mevinolin, ³H in the purified LCFA was found associated primarily with C₂₂, C₂₄, and C₂₆ (components isolated from the subcellular membranes), while ¹⁴C was present additionally in C₂₈ (the major LCFA isolated from the epicuticular wax). Substantial ¹⁴C and ³H was incorporated into the membrane-bound 24-desalkyl and 24-alkylsterols with no loss of label associated with increasing the side chain length, that is, by alkylation at C₂₄. The results demonstrate for the first time that: (i) the MVA shunt operates in a tracheophyte; (ii) preferential utilization of acetate, formed by the shunt and presumably compartmentalized, may exist in wheat for the synthesis of LCFA having a chain length distribution more suitable for membrane than wax construction; and (iii) the MVA shunt is not a minor vestigial lipid pathway but may, under certain physiological and developmental conditions, represent a pathway for routing isopentenyl pyrophosphate C-atoms away from their inclusion into the sterol pathway. Photosynthesis had no apparent effect on shunt activity.