Research process on peach fruit flesh texture

Abstract: The fruit of peach (Prunus persica L. Batsch) is popular with domestic and foreign consumers,based on thin skin, delicious taste and abundant nutrition. However, the fruit firmness declined rapidlyduring peach ripening and post-harvest storage, resulting in the decrease of the fruit quality and storabili⁃ty, which is one of the major bottleneck constraints of peach industry development. To understand the re⁃search survey of peach fruit texture, and provide a reference to further study the mechanism of fruit ripen⁃ing and softening of peach fruits, this paper summarized the classification and genetic mapping of peachfruit texture, molecular mechanism of peach fruit texture formation. Flesh texture of peach was classifiedinto melting (MF) and non-melting flesh (NMF), most of the table peaches belong to melting flesh. Themelting-type fruit soften rapidly during ripening, non-melting type peaches showed slow softening rateand no notable decrease in flesh firmness, even when overripe. This texture has been also referred to asrubbery. Besides MF and NMF, several other flesh types including stony hard (SH), and slow ripening(SR) were found in current peach cultivars and breeding materials. Peach melting flesh and flesh adhesionto stone (endocarp) are simply inherited and controlled by the F-M locus on linkage group (LG) 4. A num⁃ber of studies have been conducted to identify potential candidate genes for melting flesh and stone adhe⁃sion in peach. Initially, biochemical studies revealed that an endopolygalacturonase (endoPG) is highly ex⁃pressed in ripe MF peaches, but extremely low in NMF peaches. Thus, the endoPG gene is deemed to be acandidate for the M locus in peach. Two genes encoding endopolygalacturonase (endoPG) in the F-M lo⁃cus, designated PpendoPGF and PpendoPGM, are associated with the melting flesh and stone adhesiontraits. PpendoPGM controls melting flesh while PpendoPGF has pleiotropic effects on both melting fleshand stone adhesion. However, recombination between the M and the endoPG gene was observed in threeprogeny derived from a cross between NMF and MF cultivars. The F-M locus has three allelic copy num⁃ber variants of endoPG, H1 (PpendoPGF and PpendoPGM), H2 (PpendoPGM), and H3 (null). The H2 haplo⁃type may represent the ancestral one while the H1 and H3 haplotypes are two variants due to duplicationand deletion of PpendoPGM, respectively. Accessions with H1H1, H1H2, or H1H3 genotypes show the free⁃stone or semi-freestone and melting flesh phenotype, while both H2H2 and H2H3 accessions have the cling⁃stone and melting flesh phenotype. The H3H3 accessions have the clingstone and non-melting flesh pheno⁃type. The genetic separation of the flesh texture characteristics in F1 and F2 offspring from the cross ofnon-melting and stony hard cultivars indicated that the stony hard trait was controlled by a single gene,and inherited independently of melting flesh/non-melting flesh traits. The fruit of melting and non-melt⁃ing cultivars showed increased ethylene production and softer flesh, although their variation degrees dif⁃fered. Conversely, the stony hard texture was characterized by the absence of both ethylene productionand postharvest softening in mature fruit, while the firmness of the stony hard peach decreased effectivelywith continuous ethylene treatment. The fruit of melting-flesh peach cultivars produce high levels of ethyl⁃ene caused by high expression of PpACS1 (an isogene of 1-aminocyclopropane-1-carboxylic acid syn⁃thase), resulting in rapid fruit softening at the late-ripening stage. In contrast, the fruit of stony hard peachcultivars do not soften and produce little ethylene due to low expression of PpACS1. Recently studyshowed that suppression of PpACS1 expression at the late-ripening stage of stony hard peach may resultfrom a low level of IAA and that a high concentration of IAA is required to generate a large amount of sys⁃tem 2 ethylene in peaches. It is suggested that a YUCCA flavin mono- oxygenase gene (PpYUC11,ppa008176m), a key gene in auxin biosynthesis, displayed an identical differential expression profile tothe profiles of IAA accumulation and PpACS1 transcription: the mRNA transcripts increased at the lateripening stage in melting flesh peaches but were below the limit of detection in mature fruits of stony hardpeaches. In addition, the strong association between intron TC microsatellite genotypes of PpYUC11 andthe flesh texture (normal or stony hard) is described in 43 peach varieties, indicating that this locus maybe responsible for the stony hard phenotype in peach. These findings support the hypothesis thatPpYUC11 may play an essential role in auxin biosynthesis during peach fruit ripening and is a candidategene for the control of the stony hard phenotype in peach. The paper pointed out that the current researchof fruit ripening and softening of peach were concentrated in the changes of cell wall material compositionand structure, enzyme activity changes of cell wall degradation related enzyme, and the gene cloning andfunction analysis of cell wall degradation enzyme, and proposed the research directions. Transcriptionalregulation mechanism of hormone regulation on ripening and softening of peach fruit, the combination ofgenomics, transcriptomics, proteomics, and metabolomics studies will be the development direction ofpeach fruit texture research.