Abundance of the Major Chloroplast Polypeptides during Development and Ripening of Tomato Fruits

Abstract

During maturation and ripening of tomato (Lycopersicon esculentum, cv Tamar) fruits, there are differential changes in the steady state levels of chloroplast proteins. Western blot analysis indicated that with the exception of the core polypeptide of photosystem I (PSI) (subunit I) the whole complex disappears during the transition of chloroplast to chromoplast. The amounts of the core polypeptide of photosystem II (PSII) (43 kilodaltons) and the light harvesting chlorophyll protein complex increase during maturation and decrease thereafter. In contrast, the 33 kilodalton subunit of PSII is found at the highest levels from the early recorded stages and decreases gradually until late stages of ripening. The level of cytochrome f decreases slowly during the maturation and ripening process, whereas the Rieske protein of the same complex disappears at a faster rate. There are also differential changes in the subunits of the chloroplast coupling factor·ATPase complex; α and β subunits increase during maturation, whereas the level of the γ subunit is already maximal at the earliest recorded stage of development and depleted thereafter. The two subunits of the ribulose-1,5 bisphosphate carboxylase increase in abundance during chloroplast maturation and gradually disappear after the transition from chloroplast to chromoplast. However, there are substantial differences in the rates of increase and disappearance of the large and small subunits of this enzyme. This imbalance is attributed to different regulation of nuclear and chloroplast gene expression. In addition, the steady state levels of chloroplastic superoxide dismutase and phosphoenolpyruvate carboxylase have been followed. Both enzymes reach their maxima at the final stages of ripening. This increase coincides with the climacteric rise of CO₂ release.