Tomato geranylgeranyl diphosphate synthase isoform 1 specifically interacts with phytoene synthase isoform 3 to produce strigolactones in tomato roots

Carotenoids are photoprotectant pigments and precursors of the hormones abscisic acid (ABA) and strigolactones (SL). Carotenoids are produced in plastids from geranylgeranyl diphosphate (GGPP), which is diverted to the carotenoid pathway by phytoene synthase (PSY). In tomato (Solanum lycopersicum), 3 genes encode plastid-targeted GGPP synthases (SlG1 to 3) and 3 genes encode PSY isoforms (PSY1 to 3). Here we investigated the function of SlG1 by generating loss-of-function lines and combining their metabolic and physiological phenotypes with gene co-expression and co-immunoprecipitation analyses. Leaves and fruits of slg1 lines showed a wild-type phenotype in terms of isoprenoid accumulation, photosynthesis and development. Consistently, SlG1 is co-expressed with PSY3 and other genes involved in the production of carotenoids and SL (but not ABA) only in roots. SlG1 was also found to physically interact with the root-specific PSY3 isoform (and not with PSY1 and PSY2). Root SL (but not ABA) levels were reduced in slg1 lines. Our results confirm a specific role of SlG1 in SL production in combination with PSY3. This role appears to be restricted to roots as slg1 plants do not exhibit the shoot phenotype displayed by other SL-deficient mutants.