Characterization of cis-polyisoprene produced in Periploca sepium, a novel promising alternative source of natural rubber

Natural rubber is an important industrial raw material and is almost exclusively produced from Hevea brasiliensis latex. Because H. brasiliensis is limited its cultivation to specific tropical regions, the insufficient capacity of natural rubber has become increasingly urgent. To develop a novel alternative plant for natural rubber production, we selected a perennial shrub Periploca sepium, which is widely distributed from tropical to temperate regions. P. sepium can produce latex and contains the rubber component polyisoprene with a high-molecular-weight distribution ranging from 10⁴-10⁶. Its main chain structure was identified as cis-1,4-polyisoprene, similar to that of H. brasiliensis. The polyisoprenes were observed to be present mainly in the secondary phloem adjacent to the cambium and pith, and almost entirely overlapped with the distributions of three rubber particle-associated proteins, cis-prenyltransferase (CPT), small rubber particle protein (SRPP) and rubber elongation factor (REF). The three genes were genome edited via CRISPR-Cas9 in P. sepium, and the total contents and high-molecular-mass regions of the cis-polyisoprenes in the transgenic plants with mutations were reduced to different degrees, indicating that the three genes apparently play important roles in natural rubber biosynthesis. This research will promote the development of P. sepium as an alternative source of natural rubber.