Identification of a new expansin gene closely associated with peach fruit softening

Expansins are proteins that have been shown to contribute to fruit softening in tomato. However, expansins that have been correlated with loss of fruit firmness have not yet been identified in peach (Prunus persica (L.) Batsch). Along with the previously isolated PpExp1, two new expansin cDNAs, termed PpExp2 and PpExp3, were isolated from ripe peach fruit, and their mRNA expression patterns were characterized during fruit development and in other tissues, including the flower bud, leaf, and stem. All three expansins were detected in the fruit and not in the other tissues, but each showed differential patterns of mRNA accumulation during fruit development. The PpExp2 mRNA was constitutively expressed throughout fruit development but was abundant in Stage III, when the fruit expands exponentially and then matures. The PpExp1 and PpExp3 mRNAs were up-regulated at the onset of ripening, but PpExp1 was induced at an earlier stage. In order to identify the expansins whose expression correlates with the loss of peach fruit firmness, the mRNA expression levels of the three expansins were compared in the ripe fruit of the 'Akatsuki' and 'Manami' cultivars during postharvest storage. During storage, the ripe fruit of 'Akatsuki' rapidly softened as the level of ethylene increased significantly, while 'Manami' fruit remained firm and exhibited very low levels of ethylene production. The PpExp1 and PpExp2 mRNAs were constitutively detectable during the 8-day storage of both cultivars, whereas PpExp3 mRNA was detectable in 'Akatsuki' but hardly detectable in 'Manami,' suggesting that PpExp3 expression may be related to the changes in fruit firmness. To address the detailed role of PpExp3 in the loss of fruit firmness, the fruit of 'Manami' was treated by ethylene to artificially induce softening. The PpExp3 mRNA accumulation in the ethylene-treated 'Manami' was detectable and similar to that observed in 'Akatsuki.' These results show that, while several expansins show a general increase in expression levels during the later stages of fruit development, some isoforms show a greater association with softening than others. In this regard, PpExp3 is more likely to play a role in peach fruit softening than PpExp1 or PpExp2. (C) 2002 Elsevier Science B.V. All rights reserved.