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Catalina Pinto, Gabino Reginato, Karen Mesa, Paulina Shinya, Mariana Díaz, and Rodrigo Infante

). Interesting approaches for determining softening in peach during postharvest have been proposed by Tijskens et al. (2007) and Lurie et al. (2013) . On the contrary, other studies have assessed the flesh firmness instead of the softening rate, presenting

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Hiroshi Iwanami, Shigeki Moriya, Nobuhiro Kotoda, and Kazuyuki Abe

Softening of fruit after harvest is a serious problem in the apple industry, and the mechanisms of softening have been thoroughly investigated. In microscopic studies, as apple fruit softened, the middle lamella degraded, and cells separated when

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James P. Mattheis

can prevent scald, softening of treated fruit is delayed and may not predictably occur after fruit are removed from cold storage ( Argenta et al., 2003 ; Bai et al., 2006 ; Baritelle et al., 2001 ; Calvo, 2003 ; Chen and Spotts, 2006 ; Xie et al

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Alan B. Bennett

Fruit softening is integral to the ripening process. It is an important component of fruit quality, but also initiates deterioration and is a limiting determinant of shelf-life. Intensive research has attempted to elucidate the biochemical and genetic control of fruit softening with the goal of controlling this process as a means to enhance both fruit quality and shelf-life. Current models of fruit softening focus on cell wall disassembly as the major biochemical event regulating fruit softening. Examination of the sequence of cell wall disassembly in ripening Charentais melon fruit suggested that softening could be divided into two distinct phases. The early stage of fruit softening was associated with the regulated disassembly of xyloglucan polymers and the later softening that accompanies over-ripe deterioration was associated with pectin depolymerization. Characterization of cell wall changes in other fruit, including tomato, suggest that this may represent a general model of sequential cell wall disassembly in ripening fruit. Interestingly, the early events of xyloglucan disassembly were not associated with the activation or expression of xyloclucan hydrolases but were associated with the expression of a ripening-regulated expansin gene. Analysis of transgenic tomato fruit with suppressed expansin gene expression or with suppressed polygalacturonase gene expression supports a general model of sequential disassembly of xyloglucan and pectin that control the early and late phases of fruit softening, respectively.

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Supreetha Hegde and Niels Maness

Peach fruit softening appears to be associated with changes in cell wall polymers, particularly pectins and hemicelluloses. To determine changes of cell wall polymers associated with peach fruit softening, we conducted sequential extractions of pectin and hemicellulose from softening fruit. A more tightly bound hemicellulose fraction contained considerable amounts of pectin associated sugars. This fraction was separated into charged and neutral fractions, using anion exchange chromatography, and then fractionated into two apparent molecular weight classes by size exclusion chromatography. Virtually all of the charged fraction eluted in the higher apparent molecular weight fraction. The neutral sugar fraction segregated into both apparent molecular weight size classes, with a redistribution from the large to the small size class during softening. This redistribution was accompanied by changes in neutral sugar composition. A possible relationship between changes in this fraction and fruit softening will be discussed. Supported by USDA grant 92-34150-7190 and the Oklahoma Agricultural Experiment Station.

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Ann M. Callahan, Peter H. Morgens, Reuben A. Cohen, Ken E. Nichols Jr., and Ralph Scorza

We are interested in identifying and isolating genes which affect the rate of softening in peach fruit. It may be possible through the engineering of these genes to delay or extend the softening. This could ultimately allow for the harvest and transport of more mature, higher quality fruit. The clone, pch313, was isolated from a ripe peach fruit cDNA library. RNA homologous to this clone is detected at a low abundance in fruit until softening when a >100 fold increase in abundance of the RNA occurs. Pch313 RNA is also detected 30 min after wounding leaf or fruit tissue and peaks in accumulation within 2-8 hours. Wound ethylene was measured from the same tissue and its rate of evolution paralleled the accumulation of the RNA. The cDNA was sequenced and found to have 78% sequence identity with pTom13, a tomato gene that is expressed during fruit ripening and wounding (Holdsworth et al., NAR 15:731-739, 1987). To determine the universality of pch313 related gene expression, RNA accumulation was measured in other fruits during softening, and in leaf tissue upon wounding.

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Supreetha Hegde and Niels O. Maness

Softening to a normal melting flesh texture in peaches involves a combined participation between polymers located in the middle lamella and primary cell wall. Pectins located in the primary cell wall polysaccharide matrix which cosolubilize when hemicellulose is extracted with KOH have received less attention than the chelator or sodium carbonate soluble pectin likely to be associated with the middle lamella. We conducted a series of extractions for cell walls prepared from softening peach fruit (47, 30, and 15 N firmness) using 0.5 m imidazole, sodium carbonate and a graded series of KOH. Hemicellulose-associated pectin was a substantial proportion of most KOH extracts (30 to 50 mole percent) and fractionated on size exclusion chromatography as a high apparent molecular weight peak which became more prominent as fruit softened and could be separated from two lower apparent molecular weight peaks by anion exchange chromatography. The nature of a hemicellulose-pectin interaction in peach was apparently by physical entrapment, versus covalent cross-linking. Softening related changes in hemicellulose-associated pectin will be addressed.

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Ann M. Callahan, Peter H. Morgens, Reuben A. Cohen, Ken E. Nichols Jr., and Ralph Scorza

We are interested in identifying and isolating genes which affect the rate of softening in peach fruit. It may be possible through the engineering of these genes to delay or extend the softening. This could ultimately allow for the harvest and transport of more mature, higher quality fruit. The clone, pch313, was isolated from a ripe peach fruit cDNA library. RNA homologous to this clone is detected at a low abundance in fruit until softening when a >100 fold increase in abundance of the RNA occurs. Pch313 RNA is also detected 30 min after wounding leaf or fruit tissue and peaks in accumulation within 2-8 hours. Wound ethylene was measured from the same tissue and its rate of evolution paralleled the accumulation of the RNA. The cDNA was sequenced and found to have 78% sequence identity with pTom13, a tomato gene that is expressed during fruit ripening and wounding (Holdsworth et al., NAR 15:731-739, 1987). To determine the universality of pch313 related gene expression, RNA accumulation was measured in other fruits during softening, and in leaf tissue upon wounding.

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D.M. Dawson, C.B. Watkins, and L.D. Melton

Cell wall changes in `Fantasia' nectarines [Prunus persica (L) Batsch var. nectarina (Ait) maxim] were determined after storage at 0C with or without intermittent warming (at 20C at 2-week intervals) and after ripening. For comparison, fruit were examined at harvest and after ripening without storage. Fruit stored continuously at 0C for 6 weeks became mealy during ripening, whereas fruit subjected to intermittent warming ripened normally. Ripening immediately after harvest was associated with solubilization and subsequent depolymerization of pectic polymers and a net loss of galactosyl residues from the cell wall. No solubilization of pectic polymers from the cell wall occurred during storage of fruit at 0C. Mealy fruit, ripened after continuous storage at 0C, showed only limited solubilization of pectins and depolymerization, high relative molecular weight (M) polymers being predominant. During ripening after storage, pectic polymer solubilization was not as extensive in intermittently warmed fruit as in fruit undergoing normal ripening but solubilized polymers were depolymerized, low M uronic acid-rich polymers becoming predominant. Intermittent warming of fruit resulted in significant softening during storage, alleviating the development of mealiness by promotion of cell wall changes associated with normal ripening.

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Ann M. Callahan, Peter H. Morgens, and Reuben A. Cohen

Peach [Prunus persica (L.) Batsch] cDNA libraries have been constructed from RNA isolated from immature (30 days after bloom) and ripe fruit. cDNA clones of interest have been identified by differential hybridization among the cDNAs of various peach cultivars or from several stages in fruit development. In addition, several clones were isolated by low stringency hybridizations with oligonucleotides derived from a tomato polygalacturonase cDNA sequence and a cucumber peroxidase amino acid sequence. The pattern of accumulation of the corresponding mRNAs during fruit development was examined by RNA gel-blot analyses in the commercial cultivar Suncrest. Three cDNA clones, pch201, pch307, and pch313, were related to mRNAs that accumulate during the softening stage of fruit development. cDNA clones pchl03, pch205, and pch306 were related to an mRNAs that increase in abundance throughout development, with maximum levels in ripe fruit. cDNA clones pch104 and pch202 were related to mRNAs that exhibit maximum abundance in midfruit development, and clone pch108 was related to mRNA that decreases as the fruit matures. Southern analyses indicated that seven of the cDNAs are represented by only a few genes, while pch104 detects a repetitive family, and pch307 detects a small family of genes. These clones will provide the initial source of genes to manipulate and affect fruit development.