Tomato geranylgeranyl diphosphate synthase isoform 1 is involved in the stress‐triggered production of diterpenes in leaves and strigolactones in roots

Abstract

Summary Carotenoids are photoprotectant pigments and precursors of hormones such as 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), three genes encode plastid‐targeted GGPP synthases (SlG1 to SlG3) and three genes encode PSY isoforms (PSY1 to PSY3). Here, we investigated the function of SlG1 by generating loss‐of‐function lines and combining their metabolic and physiological phenotyping with gene co‐expression and co‐immunoprecipitation analyses. Leaves and fruits of slg1 lines showed a wild‐type phenotype in terms of carotenoid accumulation, photosynthesis, and development under normal growth conditions. In response to bacterial infection, however, slg1 leaves produced lower levels of defensive GGPP‐derived diterpenoids. In roots, SlG1 was co‐expressed with PSY3 and other genes involved in SL production, and slg1 lines grown under phosphate starvation exuded less SLs. However, slg1 plants did not display the branched shoot phenotype observed in other SL‐defective mutants. At the protein level, SlG1 physically interacted with the root‐specific PSY3 isoform but not with PSY1 and PSY2. Our results confirm specific roles for SlG1 in producing GGPP for defensive diterpenoids in leaves and carotenoid‐derived SLs (in combination with PSY3) in roots.

Overlapping and specialized roles of tomato phytoene synthases in carotenoid and abscisic acid production

“…Besides showing that stress-induced ABA biosynthesis in tomato roots relies on PSY2 but not PSY1, the results suggest that root-localized PSY3 does not substantially contribute to this process. This is in agreement with the conclusion that only monocot-type PSY3 enzymes (that are phylogenetically close to PSY1 and PSY2 isoforms) participate in root ABA synthesis, whereas the unrelated dicot-type PSY3 enzymes (including the tomato PSY3 isoform) are involved in root SL production ( Li et al 2008 ; Welsch et al 2008 ; Stauder et al 2018 ; Ezquerro et al 2023 ). Our results also show that loss of PSY1 or PSY2 in fruit tissues differentially impacts ABA-regulated processes in the pericarp and the seeds.…”

“…In the fruit, GGPP required to produce pericarp carotenoids and ABA during fruit ripening is mainly supplied by the GGPPS isoform SlG3 with a supporting contribution of SlG2 ( Barja et al 2021 ). While SlG2 can interact with both PSY1 and PSY2, no interaction was reported for SlG3 in transient coexpression assays in N. benthamiana leaves ( Barja et al 2021 ; Ezquerro et al 2023 ). It is possible that interaction of SlG3 with particular PSY isoforms requires specific partners only found in tomato pericarp (to interact with PSY1) or seed (to interact with PSY2) tissues.…”

“…Similar to most members of the dicot PSY3 clade, tomato PSY3 expression is highest in roots, where it is induced during arbuscular mycorrhizal (AM) fungi colonization ( Barja et al 2021 ; Stauder et al 2018 ; Walter et al 2015 ). Other lines of evidence concluded that the main role of PSY3 is in roots, supplying phytoene to produce carotenoids and derived SLs and apocarotenoid molecules essential for the establishment of the AM symbiosis ( Baslam et al 2013 ; Ezquerro et al 2023 ; Fester et al 2002 ; Ruiz-Lozano et al 2016 ; Stauder et al 2018 ). This work aimed to genetically test the hypothesis that besides the main role of PSY1 for carotenoid production in flowers and fruit (chromoplasts), PSY2 in green tissues (chloroplasts), and PSY3 in roots (leucoplasts), tomato PSY isoforms might also provide extra phytoene when a sudden requirement of carotenoid production could not be met by the isoform normally operating in a particular tissue.…”

“…Tomato lines defective in PSY1 have been reported as yellow-flesh ( r ) mutants ( Fray and Grierson 1993 ; Kachanovsky et al 2012 ; Kang et al 2014 ; Karniel et al 2022 ), silenced lines ( Bird et al 1991 ; Bramley et al 1992 ; Fantini et al 2013 ; Fraser et al 1999 ) and CRISPR-Cas9–edited lines ( D’Ambrosio et al 2018 ), but lines impaired in PSY2 or PSY3 have not been described yet. Based mostly on sequence homology and gene expression data, it was proposed that tomato PSY3 function might be restricted to the production of SLs and other apocarotenoids in roots whereas PSY1 and PSY2 appear to differentially support carotenogenesis in shoot tissues: PSY1 for pigmentation in chromoplasts and PSY2 for photosynthesis in chloroplasts ( Fraser et al 1999 ; Giorio et al 2008 ; Hirschberg 2001 ; Stauder et al 2018 ; Ezquerro et al 2023 ). However, other sources of evidence suggest that isoform specialization is not complete.…”