Enzymic properties and capacities of developing tomato (Lycopersicon esculentum L.) fruit plastids

Abstract

To characterize the developmental stage of tomato fruits, chlorophyll content, photosynthetic O₂ evolution and CO₂ fixation of pericarp slices were determined. During the first developmental stages a higher expression level of the triose phosphate translocator was detected. Transport measurements revealed that both the hexose phosphate and the triose phosphate translocator are very likely to be active at this time. Plastidic and cytosolic fructose-1,6-bisphosphatase are active in green fruit pericarp, whereas in red pericarp only the cytosolic form is present. Tomato fruit chloroplasts are able to synthesize starch from GIc6P. Starch synthesis is strongly dependent on the addition of 3PGA and ATP and on plastid illumination. Fruit chloroplasts exhibit very low CO₂ fixation rates and so the capacities of green pericarp slices were investigated. In relation to chlorophyll content, pericarp slices show the same capacity of starch synthesis as spinach or potato leaves. To investigate the presence of further reactions consuming the products of photosynthetic electron transport, the GOGAT activity was measured. In the light, glutamane/2-oxoglutarate-dependent formation of glutamate occurred with a high activity. In the presence of Glc6P only 18% of the light activity was obtained. Since the Glc6P-dependent activity is rather low, the release of ¹⁴CO₂ from labelled [1-¹⁴C]-Glc6P was also measured. In the dark, the formation of glutamate and oxidation of Glc6P are very tightly coupled to each other in fruit chloroplasts.