Flesh softening in melting flesh, non melting flesh and stony hard peaches : endopolygalacturonase expression and phosphorylation of soluble polypeptides in relation to ethylene production

In peach, fruit flesh texture is an important quality parameter affecting fruit harvest and subsequent shelf-life. Peaches can be classified as Non Melting flesh (NMF, preferred for canning), Melting flesh (MF, appreciated for the fresh market) and Stony Hard (SH, maintaining high flesh firmness also when ripe and with a long shelf life), characterized by different ethylene production. Furthermore, there exist also mutants (M) with altered ripening phenotype (impaired fruit development and altered ethylene production) which represent a very interesting model for the study of ripening. Previous data obtained in our laboratory showed that, although MF produced less ethylene than NMF fruits, their softening was characterized by increased levels of an endoPG polypeptide which was also present, although in very limited amounts, in NMF fruits. The differences in endoPG levels seemed to be due to different transcription of an endoPG gene. The ethylene signalling pathway seems to involve, among others, also protein phosphorylation/dephosphorylation events also. Our previous results indicated that the Ca2+-dependent phosphorylation of a soluble polypeptide (Mw 52 ca kDa) decreased with ripening in NMF and remained constant in MF fruits. In M phenotypes phosphorylation of this polypeptide was detectable only in ethylene-producing fruits, that softened and produced the endoPG polypeptide. The data suggested that phosphorylation of the 52 kDa soluble polypeptide may be involved in the modulation of ethylene signalling and flesh softening. In the present work the study has been widened to the evaluation of endoPG expression to SH phenotype as well as to other NMF, MF and M genotypes. Moreover, the changes in the phosphorylation patterns of soluble polypeptides in relation to softening have been investigated and the results discussed in relation to ethylene production in fruits with the different phenotypes.