A cell wall-oriented genomic approach reveals a new and unexpected complexity of the softening in peaches

During ripening, fleshy fruits undergo textural changes that lead to loss of tissue firmness and consequent softening. It is a common idea that this process is the consequence of cell wall dismantling carried out by different and orderly expressed enzymes. For this purpose, by using a single enzyme family approach many enzymes and related genes have been characterized in different fruits. In this work, the softening of the climacteric peach fruits (Prunus persica (L.) Batsch.) has been studied by using a genomic approach, and the results obtained are novel and partly unexpected. The genes analysed encode proteins involved in the main metabolic aspects of a primary cell wall: degradation, synthesis, structure. In addition, some genes encoding cell-wall-related proteins with an unknown function have been studied. The gene expression profiles show that the softening actually begins well before the climacteric rise and continues thereafter. Genes whose expression starts before the climacteric rise are mostly down-regulated by ethylene, while genes with a ripening-specific expression are mostly upregulated by the hormone. A few other genes are apparently insensitive to ethylene. Besides the expected parietal degradation, the softening that results from this study also comprises some repairing of the cell wall performed by enzymes involved in the synthesis of parietal polysaccharides and, especially, by proteins with structural functions. The newly synthesized polysaccharides and the structural proteins would thus help to hold together the fruit cell wall while not preventing the softening.