Manipulation of Phytoene Levels in Tomato Fruit: Effects on Isoprenoids, Plastids, and Intermediary Metabolism

Paul D. Fraser ¹, Eugenia M.A. Enfissi ¹, John M. Halket ², Mark R. Truesdale ¹, Dongmei Yu ¹, Christopher Gerrish ¹, and Peter M. Bramley ¹^*

¹ School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 OEX, United Kingdom
² Specialist Bioanalytical Services Laboratory, Royal Holloway, University of London, Egham, Surrey, TW20 OEX, United Kingdom

^* To whom correspondence should be addressed. E-mail: p.bramley{at}rhul.ac.uk.

In tomato (Solanum lycopersicum), phytoene synthase-1 (PSY-1)is the key biosynthetic enzyme responsible for the synthesisof fruit carotenoids. To further our understanding of carotenoidformation in tomato fruit, we characterized the effect of constitutiveexpression of an additional tomato Psy-1 gene product. A quantitativedata set defining levels of carotenoid/isoprenoid gene expression,enzyme activities, and metabolites was generated from fruitthat showed the greatest perturbation in carotenoid content.Transcriptional upregulation, resulting in increased enzymeactivities and metabolites, occurred only in the case of Psy-1,Psy-2, and lycopene cyclase B. For reactions involving 1-deoxy-D-xylulose5-phosphatesynthase, geranylgeranyl diphosphate synthase, phytoene desaturase, ${zeta}$ -carotene desaturase, carotene isomerase, and lycopene ${beta}$ -cyclase,there were no correlations between gene expression, enzyme activities,and metabolites. Perturbations in carotenoid composition wereassociated with changes in plastid type and with chromoplast-likestructures arising prematurely during fruit development. Thelevels of >120 known metabolites were determined. Comparisonwith the wild type illustrated that key metabolites (sucrose,glucose/fructose, and Glu) and sectors of intermediary metabolism(e.g., trichloroacetic acid cycle intermediates and fatty acids)in the Psy-1 transgenic mature green fruit resembled changesin metabolism associated with fruit ripening. General fruitdevelopmental and ripening properties, such as ethylene productionand fruit firmness, were unaffected. Therefore, it appears thatthe changes to pigmentation, plastid type, and metabolism associatedwith Psy-1 overexpression are not connected with the ripeningprocess.