Identification of linkages between cell wall polymers provided evidence for the existence of the protein APAP1, composed of an AGP core, with attached RG-I, HG and arabinoxylan ( Tan et al., 2013 Hijazi et al., 2014 a). Among the HRGPs, there are highly glycosylated members found in cells throughout the plant kingdom, acting as covalent cross-linkers in the polysaccharide–proteoglycan network. In addition, as dynamic cell wall components, AGPs are up-regulated by plant growth regulators, and interact with auxin and cytokinin ( Seifert and Roberts, 2007).ĪGPs are classified as a category of the superfamily of hydroxyproline-rich glycoproteins (HRGPs). Moreover, constitutive expression during developmental processes indicates AGPs functions during fruit ripening ( Fragkostefanakis et al., 2012). Studies of AGP gene expression profiles confirmed its involvement in oxygen deficiency adaptation in fruit. Furthermore, one of the structural proteins of the plant extracellular matrix, arabinogalactan protein (AGP), is involved in the response to abiotic factors, i.e. Moreover, it has been established that specific proteins are involved in cell wall synthesis, contributing to the stabilization of the actin cytoskeleton, and are associated with changes in the content of soluble solids ( Shi et al., 2014). Proteins that are differentially expressed in apple fruit during maturation and storage were characterized by Shi et al. reduced firmness reduction and cell wall degradation during post-harvest storage. Proteomic research on dynamic changes occurring during apple fruit ripening and storage indicated that proteins exert an impact on specific metabolic pathways and in this way have an influence on complex physiological changes, e.g. By contrast, an entirely different distribution is shown by rhamnogalacturonan type I (RG-I) epitopes, which are restricted to a region close to the plasma membrane, associated with its role in wall rearrangements during cell growth ( Guillon et al., 2017). Also, methyl-esterified HG epitopes generally occur at cell junctions and in the middle lamella ( Christiaens et al., 2011 Zamil and Geitmann, 2017).

Moreover, the correlation with Ca 2+ cross-linking suggests a role of de-esterified HG in the mechanism of intercellular adhesion ( Ng et al., 2013). Numerous studies describe the presence of de-esterified homogalacturonan (HG) at the corners of intercellular spaces their presence is related to their function in cell-to-cell contact. Among many factors, the distribution of the above constituents, their mutual interactions, cross-linking and multidomain organization determine the mechanical properties of fruit such as firmness ( Chen et al., 2009 Geitmann, 2010 Zhang et al., 2010 Jarvis, 2011). The prevailing structural model of the cell wall consists of a cellulose–hemicellulose network, an amorphous matrix composed of pectins, and small amounts of proteins and phenolic compounds ( Bidhendi and Geitmann, 2016 Guillon et al., 2017).

The cell wall of fruit plays a significant biological function and determines crop quality, which is important for both the grower and the consumer. Arabinogalactan proteins, cell wall, fruit senescence, Malus × domestica, pectin, plasma membrane INTRODUCTION

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