Abstract

Fruit of tomato (Solanum lycopersicum), like those from many species, have been characterized to undergo a shift from partially photosynthetic to truly heterotrophic metabolism. While there is plentiful evidence for functional photosynthesis in young tomato fruit, the rates of carbon assimilation rarely exceed those of carbon dioxide release, raising the question of its role in this tissue. Here, we describe the generation and characterization of lines exhibiting a fruit-specific reduction in the expression of glutamate 1-semialdehyde aminotransferase (GSA). Despite the fact that these plants contained less GSA protein and lowered chlorophyll levels and photosynthetic activity, they were characterized by few other differences. Indeed, they displayed almost no differences in fruit size, weight, or ripening capacity and furthermore displayed few alterations in other primary or intermediary metabolites. Although GSA antisense lines were characterized by significant alterations in the expression of genes associated with photosynthesis, as well as with cell wall and amino acid metabolism, these changes were not manifested at the phenotypic level. One striking feature of the antisense plants was their seed phenotype: the transformants displayed a reduced seed set and altered morphology and metabolism at early stages of fruit development, although these differences did not affect the final seed number or fecundity. Taken together, these results suggest that fruit photosynthesis is, at least under ambient conditions, not necessary for fruit energy metabolism or development but is essential for properly timed seed development and therefore may confer an advantage under conditions of stress.

Highlights

  • Fruit of tomato (Solanum lycopersicum), like those from many species, have been characterized to undergo a shift from partially photosynthetic to truly heterotrophic metabolism

  • Given the major purpose of this work, which was to study the role of fruit photosynthesis and the lack of a suitable enhancer line population analogous to those used in Arabidopsis (Arabidopsis thaliana; Janacek et al, 2009), our initial aim was to find a suitable promoter to confer loss of function only in the tissue of choice in tomato

  • We chose glutamate 1-semialdehyde aminotransferase (GSA) because it has previously been reported to play an essential role in chlorophyll synthesis (Kannangara and Gough, 1978; Hofgen et al, 1994; Ilag et al, 1994; Chen et al, 2003) and leaf chlorophyll content itself has a major impact on the rate of photosynthesis of higher plants (Hofgen et al, 1994; Yaronskaya et al, 2003)

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Summary

Introduction

Fruit of tomato (Solanum lycopersicum), like those from many species, have been characterized to undergo a shift from partially photosynthetic to truly heterotrophic metabolism. Despite the fact that these plants contained less GSA protein and lowered chlorophyll levels and photosynthetic activity, they were characterized by few other differences They displayed almost no differences in fruit size, weight, or ripening capacity and displayed few alterations in other primary or intermediary metabolites. One striking feature of the antisense plants was their seed phenotype: the transformants displayed a reduced seed set and altered morphology and metabolism at early stages of fruit development, these differences did not affect the final seed number or fecundity. We generated transgenic tomato plants exhibiting decreased expression of glutamate 1-semialdehyde aminotransferase (GSA), which has previously been documented to contribute to the control of chlorophyll biosynthesis (Hofgen et al, 1994), under the control of the TFM5 promoter, which confers early fruit specificity. These results are discussed with respect to the proposed roles of photosynthesis during fruit metabolism, ripening, and development, in particular with respect to carbon provision for seed set in tomato

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