Involved in providing solanesyl diphosphate for plastoquinone-9 (PQ-9) formation in plastids (PubMed:12437513, PubMed:23913686, PubMed:26039552). Catalyzes the elongation of the prenyl side chain of PQ-9 in plastids (PubMed:23913686, PubMed:26039552). Contributes to the biosynthesis of plastochromanol-8 (PC-8) in plastids (PubMed:23913686, PubMed:26039552). Does not contribute to the synthesis of tocopherol or ubiquinone (PubMed:23913686). PQ-9 and PC-8 are lipophilic antioxidants that act as protectant against photooxidative stress under high light stress conditions (PubMed:23913686, PubMed:26039552, PubMed:29901834). Prefers geranylgeranyl diphosphate to farnesyl diphosphate as substrate (PubMed:12437513). No activity with geranyl diphosphate or dimethylallyl diphosphate as substrate (PubMed:12437513). (2E,6E,10E)-geranylgeranyl diphosphate + 5 isopentenyl diphosphate = all-trans-nonaprenyl diphosphate + 5 diphosphate (2E,6E)-farnesyl diphosphate + isopentenyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate Binds 2 Mg(2+) ions per subunit. kcat is 29.7 sec(-1) with farnesyl diphosphate as substrate (in the presence of 500 uM of isopentenyl diphosphate). kcat is 21.0 sec(-1) with geranylgeranyl diphosphate as substrate (in the presence of 100 uM of isopentenyl diphosphate). kcat is 14.7 sec(-1) with isopentenyl diphosphate as substrate (in the presence of 20 uM of farnesyl diphosphate). kcat is 15.0 sec(-1) with isopentenyl diphosphate as substrate (in the presence of 4 uM of geranylgeranyl diphosphate). Optimum pH is 8.0. Homodimer (PubMed:12437513). Interacts with FBN5 (PubMed:26432861). Higher expression in leaves than in roots. Induced by high light conditions. Decreased levels of plastoquinone-9 (PQ-9) and plastochromanol-8 (PC-8) in leaves (PubMed:23913686). The double mutants sps1 and sps2 exhibit an albino phenotype and are devoided of both PQ-9 and PC-8 in cotyledons (PubMed:23913686). Plants over-expressing SPS1 exhibit increased resistance to photooxidative stress, decreases in leaf bleaching, lipid peroxidation and PSII photoinhibition under excess light due to their increased capacities for plastoquinone-9 (PQ-9) biosynthesis (PubMed:26039552). Plants over-expressing SPS1 exhibit decreased singlet oxygen production in response to photooxidative stress due to increased levels of PQ-9, which acts as chemical quencher of singlet oxygen (PubMed:29901834). Belongs to the FPP/GGPP synthase family.