The low temperature induced rate of 3-hydroxy-3-methylglutaryl-CoA reductase in Parthenium argentatum Gray limits the theoretical rate of rubber formation

Abstract

The accumulative low temperature of 5–7 in the fall and winter of the Chihuahuan Desert induces rubber biosynthesis in Parthenium argentatum Gray (guayule) and the enzyme activities of 3-hydroxy-3-methylglutaryl-CoA reductase and rubber transferase. The conclusion that the induced rates of 3-hydroxy-3-methylglutaryl-CoA reductase and rubber transferase account, in part, for the induced rate of rubber biosynthesis is incomplete. The objective of this Short Communication was to determine if the induced rate of 3-hydroxy-3-methylglutaryl-CoA reductase supported the rate of rubber biosynthesis as determined by the rate isopentenyl pyrophosphate polymerization to rubber polymers catalyzed by rubber transferase. The rates of these 2 enzymes were measured in the bark of lower stems of guayule from July to December in plants transplanted to the field in the Chihuahuan Desert in May. At the peak of the low temperature induced rubber formation the induced rate of 3-hydroxy-3-methylglutaryl-CoA reductase of 29.9nmolMVAh−1gfrwt−1 was significantly lower than the induced rate of rubber transferase of 357.5nmolIPPh−1gfrwt−1. The rate of 3-hydroxy-3-methylglutaryl-CoA reductase was 8.3% of the theoretical rate of rubber formation based on the rate of rubber transferase at saturating concentrations of isopentenyl pyrophosphate Mg2+ and dimethylallyl pyrophosphate initiator in vivo. A new interpretation of the induced developmental rate curves of 3-hydroxy-3-methylglutaryl-CoA reductase and rubber transferase is that the low temperature induced rate of 3-hydroxy-3-methylglutaryl-CoA reductase limits rubber biosynthesis in guayule. The new calculations support the conclusion that isopentenyl pyrophosphate from an alternate source, other than the mevalonic acid pathway may additionally supply isopentenyl pyrophosphate to rubber transerase for polymerization into natural rubber.