Commonalities and complexities in rubber biosynthesis

Abstract Rubber biosynthesis in plants underpins the production of this strategically vital polymer. A fundamental understanding of the regulation of rate and polymer quality is essential to the development of alternate rubber-producing crops and new rubber materials with novel properties. Alternate rubber crops are needed to meet the projected shortfalls in global rubber production caused by the burgeoning economies of China and India and disease-induced production losses. Rubber, cis-1,4-polyisoprene, is synthesized from an allylic pyrophosphate (PP), which serves both as the initiator and chain transfer agent, and the nonallylic isopentenyl PP from which the polymer is made. The reaction requires a divalent cation activator, magnesium in vivo. The concentrations of both substrates and activator, relative to their binding affinity to the species-specific rubber transferase (RT-ase), are key to the production of high molecular weight rubber. Rubber biosynthesis may be largely regulated by the functioning of the laticifer in latex-producing species. In contrast, the RT-ase of Parthenium argentatum (guayule), a species that makes rubber in bark parenchyma cells, exerts significant control over final molecular weight. The structural elucidation of a functional RT-ase complex and how the different components of such complexes regulate rate, catalytic activity, and molecular weight is still a challenge to researchers.