A proteomic analysis shows the stimulation of light reactions and inhibition of the Calvin cycle in the skin chloroplasts of ripe red grape berries

Abstract

The role of photosynthesis in fruits still challenges scientists. This is especially true in the case of mature grape berries of red varieties lined by an anthocyanin-enriched exocarp (skin) almost impermeable to gases. Although chlorophylls are degraded and replaced by carotenoids in several fruits, available evidence suggests that they may persist in red grapes at maturity. In the present study, chloroplasts were isolated from the skin of red grape berries (cv. Vinhão) to measure chlorophyll levels and the organelle proteome. The results showed that chloroplasts (and chlorophylls) are maintained in ripe berries masked by anthocyanin accumulation and that the proteome of chloroplasts from green and mature berries is distinct. Several proteins of the light reactions significantly accumulated in chloroplasts at the mature stage including those of light-harvesting complexes of photosystems I (PSI) and II (PSII), redox chain, and ATP synthase, while chloroplasts at the green stage accumulated more proteins involved in the Calvin cycle and the biosynthesis of amino acids, including precursors of secondary metabolism. Taken together, results suggest that although chloroplasts are more involved in biosynthetic reactions in green berries, at the mature stage, they may provide ATP for cell maintenance and metabolism or even O2 to feed the respiratory demand of inner tissues.

Biosynthesis of Chlorophyll and Other Isoprenoids in the Plastid of Red Grape Berry Skins

“…In our experimental conditions, the two most accumulated metabolites in the plastids of berry skins at E-L 34 were flavoxanthin and zeaxanthin, at 0.9-and 0.5-fold, respectively (Figure 2C, Supplementary Figure 3A). Chlorophylls a and b, and related degradation products, showed no significant differences between mature (E-L 38) and green (E-L 34) stages in purified plastids from berry skins (Figure 2C), supporting previous observations (Table 1 and Figure 1; Teixeira et al 2022) 5 that there is no loss of chloroplasts during the transition in color of the fruit after veraison.…”

“…This biosynthetic activity may serve to preserve the photosynthetic machinery of grape skin chloroplasts during development and ripening (Table 1, Figure 1, Teixeira et al 2022). 5 Because Calvin cycle proteins were found to be impoverished in skin plastids at the mature stage (Teixeira et al 2022), 5 cytosolic precursors like isopentenyl pyrophosphate (IPP) may feed the synthesis of some isoprenoids, including phytoene or phytofluene, that accumulate at the mature stage, as shown in the present study.…”

“…Plastids from grape berry skins and leaves at E-L 34 and E-L 38 were isolated as previously described. 5 with minor modifications. Chlorophyll quantification was performed according to Lichtenthaler and Wellburn.…”

“…"Vinhaõ" revealed different proteins of the light reactions significantly accumulated in chloroplasts at the mature stage, suggesting that energy-yielding reactions predominate at this stage, while chloroplasts at the green stage accumulate proteins involved in the Calvin cycle. 5 In tomato fruit, photosynthesis may rely exclusively on the CO 2 released from internal respiration, 9 due to the absence of stomata. 4 Much like in tomato, the net uptake of CO 2 by grape berries is likely very low because, depending on the cultivar, young fruits 10−12 have few stomata, and these become waxfilled during maturation.…”

“…Photosynthesis in fruits like coffee, pea, soybean, avocado, orange, apple, tomato, cucumber, or grape berry is still puzzling as fruits are typically sink tissues. It has been proposed that the green tissues of fruits may contribute to carbon accumulation, providing O 2 for respiration in the inner tissues, or even energy for metabolism and cell maintenance. − Indeed, early studies showed that tomato fruit photosynthesis contributes to 10–15% of sugars accumulated in the fruit. , More recently, proteomic analysis demonstrated that all components of the Calvin cycle, photorespiratory cycle, and the major light-harvesting proteins accumulate in tomato fruit at both mature green and breaker stages . Moreover, a recent study on grape berries cv.…”