Characterisation of mevalonic acid (MVA) pathway genes in Periploca sepium, a promising alternative source of natural rubber

Natural rubber is an indispensable raw material for modern industries. Currently, its commercial supply depends almost exclusively on Hevea brasiliensis , whose cultivation is restricted to tropical climates. This geographic limitation threatens sustainable supply security and highlights the need to identify alternative sources. Periploca sepium is a perennial shrub with a broad distribution spanning tropical to temperate regions. Its laticifers synthesise and accumulate the natural rubber component cis -polyisoprene, a polymer that is structurally and physiochemically similar to the rubber from H. brasiliensis . This study focuses on the upstream mevalonate (MVA) pathway, which generates the primary source of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), the key precursors of natural rubber biosynthesis in this species. Using an integrated approach combining transcriptome sequencing, yeast functional complementation, and weighted gene co-expression network analysis (WGCNA), we identified PsAACT3 (PX726067), PsHMGS1 (PX726068), and PsHMGR2 (PX726069) as three putative functional genes in the MVA pathway. Phenotypic analysis of transgenic overexpressing and RNAi-treated plants demonstrated distinct roles for these genes. PsAACT3 primarily influences the molecular weight of cis -polyisoprene, whereas PsHMGS1 affects both IPP/DMAPP accumulation and molecular weight without altering the cis -polyisoprene content. In contrast, PsHMGR2 appears to be a rate-limiting enzyme in the MVA pathway and plays a central role in rubber biosynthesis, as its expression modulates IPP/DMAPP accumulation, elevates cis -polyisoprene content, and concurrently shifts the molecular weight profile. Overall, these findings establish a robust theoretical foundation for elucidating the natural rubber biosynthesis network in P. sepium and advancing its development as an alternative natural rubber source.