Inhibiting expression of a tomato ripening-associated membrane protein increases organic acids and reduces sugar levels of fruit

Tomato (Lycopersicon esculentum Mill.) ripening-associated membrane protein (TRAMP) is a channel protein of the membrane intrinsic protein (MIP) class encoded by the cDNA clone pNY507 [R.G. Fray et al. (1994) Plant Mol Biol 2 4. 539-543]. It has been suggested that these proteins encode water channel s or aquaporins. TRAMP mRNA accumulated in all tomato tissues tested and wa s elevated in fruit during post-anthesis development and again during ripen ing. Transgenic plants that constitutively expressed a TRAMP antisense RNA sequence were generated with a 94% reduction of endogenous TRAMP mRNA in fr uit. They showed no obvious phenotype that could be associated with gross p erturbation of water relations, but ripening fruit of these plants showed m arked alterations in the normal pattern of accumulation of both organic aci ds and sugars. At the onset and during ripening, levels of the organic acid s L-malate and citrate were significantly elevated while levels of D[+]-glu cose and D[+]-fructose were reduced. Additional transgenic lines were gener ated with reduced TRAMP mRNA, and the phenotype of increased acids and redu ced sugars during fruit maturation and ripening was shown to be reproducibl e and stably inherited. Fruit of plants that over-expressed TRAMP mRNA show ed no significant alteration in the sugars or acids investigated. These res ults suggest a role for TRAMP in the movement of solutes between cell compa rtments.