Proteomic approach to blossom‐end rot in tomato fruits ( Lycopersicon esculentum M.): Antioxidant enzymes and the pentose phosphate pathway

Blossom‐end rot (BER) is a physiopathy that affects tomato fruits causing disorganisation, cell breakage and darkening of the tissues. In this study we describe a tomato fruit protein extraction protocol that includes polyvinyl polypyrrolidone, ascorbic acid and protease inhibitors to promote depletion of phenolics and to avoid protein degradation. The temperature‐induced phase separation of plant extracts with nonionic detergent Triton X‐114 favours the solubilisation of partially‐hydrophobic species in the low‐detergent upper phase, making them suitable for further analysis using two‐dimensional gel electrophoresis. The analysis of two‐dimensional images revealed differences in number and expression levels of several proteins from the control and BER‐affected tomato fruits. Although the appearance of BER in tomato is primarily attributed to a lack of calcium supply to fruits, very little is known about the molecular and biochemical mechanisms involved. The identification of differential proteins from affected fruits with matrix‐assisted laser desorption/ionisation‐time of flight and peptide mass fingerprinting analysis revealed the induction of proteins participating in antioxidant processes (ascorbate‐glutathione cycle) and the pentose phosphate pathway. We suggest that these two biochemical pathways, acting as reactive oxygen species scavengers in BER‐affected fruits, restrain the spread of the blackening to the whole fruit.