The regulation of carotenoid formation in tomato fruit

Summary Carotenoid biosynthesis in plants includes a complex series of desaturation/isomerisation reactions, catalyzed by four independent enzymes. In bacteria and fungi one desaturase/isomerase enzyme completes the same series of reactions. In the present study, a bacterial desaturase ( crtI ) from Pantoea ananatis has been overexpressed in the tangerine mutant of tomato ( Solanum lycopersicon ) which accumulates cis ‐carotene isomers in the fruit due to a defective isomerase ( CRTISO ) and the old gold crimson ( og c ) tomato mutant, which is defective in the fruit‐enhanced lycopene β‐cyclase ( CYCB ). Comprehensive molecular and biochemical characterization of the resulting lines expressing crtI has revealed negative feedback mechanisms, acting predominantly at the level of phytoene synthase‐1 ( PSY 1 ), and feed‐forward mechanisms inducing cyclisation. In both cases, altered transcription appears to be the progenitor, with subsequent post‐transcriptional modulation highlighting the complexity of the processes involved in modulating carotenoid homeostasis in plant tissues.