No abstract

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 up-regulated 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.

MADS-box genes have been shown to play a role in the formation of fruits, both in Arabidopsis and in tomato. In peach, two C-class MADS-box genes have been isolated. Both of them are expressed during flower and mesocarp development. Here a detailed analysis of a gene that belongs to the PLENA subfamily of MADS-box genes is shown. The expression of this PLENA-like gene (PpPLENA) increases during fruit ripening, and its ectopic expression in tomato plants causes the transformation of sepals into carpel-like structures that become fleshy and ripen like real fruits. Interestingly, the transgenic berries constitutively expressing the PpPLENA gene show an accelerated ripening, as judged by the expression of genes that are important for tomato fruit ripening. It is suggested that PpPLENA might interfere with the endogenous activity of TAGL1, thereby activating the fruit ripening pathway earlier compared with wild-type tomato plants.

In peach (Prunus persica L. Batsch.) the degradation of the pectic compounds of the cell wall is considered to be the principal component responsible for fruit softening. Many genes encoding enzymes acting on the different polymers of the pectic matrix have been shown to be highly expressed during the late phases of softening, with polygalacturonase being the most important. Nevertheless, it is known that softening starts well before the ethylene climacteric rise which occurs concomitant with the maximal expression of the pectolytic enzymes. The cloning and characterization of PpEG4, an endo-beta-1,4-glucanase (EGase) gene preferentially expressed in preclimacteric fruits, are presented here. PpEG4 belongs to the group of EGases containing, at their carboxy-terminus, a peptide similar to the cellulose binding domain of microbial origin. This EGase is also expressed during abscission of both leaves and fruits. The effect of exogenous ethylene treatments on PpEG4 transcription is null in young fruits and negative in preclimacteric ones, while it is positive in abscission zones. Thus, the expression of PpEG4 seems to be more dependent on the type of separation process rather than being influenced by a direct hormone action. The ability of the PpEG4 regulatory sequences to drive transcription in cells undergoing separation events is also maintained in tomato, where about 3 kb of the gene promoter could drive the expression of gusA in preclimacteric fruits and in the fruit abscission zones.