Metabolic profiling of a range of peach fruit varieties reveals high metabolic diversity and commonalities and differences during ripening.

Peach (Prunus persica) fruits from different varieties display differential organoleptic and nutritional properties, characteristics related to their chemical composition. Here, chemical biodiversity of peach fruits from fifteen varieties, at harvest and after post-harvest ripening, was explored by gas chromatography-mass spectrometry. Metabolic profiling revealed that metabolites involved in organoleptic properties (sugars, organic and amino acids), stress tolerance (raffinose, galactinol, maltitol), and with nutritional properties (amino, caffeoylquinic and dehydroascorbic acids) displayed variety-dependent levels. Peach varieties clustered into four groups: two groups of early-harvest varieties with higher amino acid levels; two groups of mid- and late-harvest varieties with higher maltose levels. Further separation was mostly dependent on organic acids/raffinose levels. Variety-dependent and independent metabolic changes associated with ripening were detected; which contribute to chemical diversity or can be used as ripening markers, respectively. The great variety-dependent diversity in the content of metabolites that define fruit quality reinforces metabolomics usage as a tool to assist fruit quality improvement in peach.

Deciphering the metabolic pathways influencing heat and cold responses during post-harvest physiology of peach fruit.

Peaches are highly perishable and deteriorate quickly at ambient temperature. Cold storage is commonly used to prevent fruit decay; however, it affects fruit quality causing physiological disorders collectively termed 'chilling injury' (CI). To prevent or ameliorate CI, heat treatment is often applied prior to cold storage. In the present work, metabolic profiling was performed to determine the metabolic dynamics associated with the induction of acquired CI tolerance in response to heat shock. 'Dixiland' peach fruits exposed to 39 °C, cold stored, or after a combined treatment of heat and cold, were compared with fruits ripening at 20 °C. Dramatic changes in the levels of compatible solutes such as galactinol and raffinose were observed, while amino acid precursors of the phenylpropanoid pathway were also modified due to the stress treatments, as was the polyamine putrescine. The observed responses towards temperature stress in peaches are composed of both common and specific response mechanisms to heat and cold, but also of more general adaptive responses that confer strategic advantages in adverse conditions such as biotic stresses. The identification of such key metabolites, which prime the fruit to cope with different stress situations, will likely greatly accelerate the design and the improvement of plant breeding programs.

Transcriptomic profiling during the post-harvest of heat-treated Dixiland Prunus persica fruits: common and distinct response to heat and cold.

Cold storage is extensively used to slow the rapid deterioration of peach (Prunus persica L. Batsch) fruit after harvest. However, peach fruit subjected to long periods of cold storage develop chilling injury (CI) symptoms. Post-harvest heat treatment (HT) of peach fruit prior to cold storage is effective in reducing some CI symptoms, maintaining fruit quality, preventing softening and controlling post-harvest diseases. To identify the molecular changes induced by HT, which may be associated to CI protection, the differential transcriptome of peach fruit subjected to HT was characterized by the differential display technique. A total of 127 differentially expressed unigenes (DEUs), with a presence-absence pattern, were identified comparing peach fruit ripening at 20°C with those exposed to a 39°C-HT for 3 days. The 127 DEUs were divided into four expression profile clusters, among which the heat-induced (47%) and heat-repressed (36%) groups resulted the most represented, including genes with unknown function, or involved in protein modification, transcription or RNA metabolism. Considering the CI-protection induced by HT, 23-heat-responsive genes were selected and analyzed during and after short-term cold storage of peach fruit. More than 90% of the genes selected resulted modified by cold, from which nearly 60% followed the same and nearly 40% opposite response to heat and cold. Moreover, by using available Arabidopsis microarray data, it was found that nearly 70% of the peach-heat responsive genes also respond to cold in Arabidopsis, either following the same trend or showing an opposite response. Overall, the high number of common responsive genes to heat and cold identified in the present work indicates that HT of peach fruit after harvest induces a cold response involving complex cellular processes; identifying genes that are involved in the better preparation of peach fruit for cold-storage and unraveling the basis for the CI protection induced by HT.

Heat treatment of peach fruit: modifications in the extracellular compartment and identification of novel extracellular proteins.

Ripening of peach (Prunus persica L. Batsch) fruit is accompanied by dramatic cell wall changes that lead to softening. Post-harvest heat treatment is effective in delaying softening and preventing some chilling injury symptoms that this fruit exhibits after storage at low temperatures. In the present work, the levels of twelve transcripts encoding proteins involved in cell wall metabolism, as well as the differential extracellular proteome, were examined after a post-harvest heat treatment (HT; 39 °C for 3 days) of "Dixiland" peach fruit. A typical softening behaviour, in correlation with an increase in 1-aminocyclopropane-1-carboxylic acid oxidase-1 (PpACO1), was observed for peach maintained at 20 °C for 3 days (R3). Six transcripts encoding proteins involved in cell wall metabolism significantly increased in R3 with respect to peach at harvest, while six showed no modification or even decreased. In contrast, after HT, fruit maintained their firmness, exhibiting low PpACO1 level and significant lower levels of the twelve cell wall-modifying genes than in R3. Differential proteomic analysis of apoplastic proteins during softening and after HT revealed a significant decrease of DUF642 proteins after HT; as well as an increase of glyceraldehyde-3-phosphate dehydrogenase (GAPC) after softening. The presence of GAPC in the peach extracellular matrix was further confirmed by in situ immunolocalization and transient expression in tomato fruit. Though further studies are required to establish the function of DUF642 and GAPC in the apoplast, this study contributes to a deeper understanding of the events during peach softening and after HT with a focus on this key compartment.

Metabolic profiling during peach fruit development and ripening reveals the metabolic networks that underpin each developmental stage.

Fruit from rosaceous species collectively display a great variety of flavors and textures as well as a generally high content of nutritionally beneficial metabolites. However, relatively little analysis of metabolic networks in rosaceous fruit has been reported. Among rosaceous species, peach (Prunus persica) has stone fruits composed of a juicy mesocarp and lignified endocarp. Here, peach mesocarp metabolic networks were studied across development using metabolomics and analysis of key regulatory enzymes. Principal component analysis of peach metabolic composition revealed clear metabolic shifts from early through late development stages and subsequently during postharvest ripening. Early developmental stages were characterized by a substantial decrease in protein abundance and high levels of bioactive polyphenols and amino acids, which are substrates for the phenylpropanoid and lignin pathways during stone hardening. Sucrose levels showed a large increase during development, reflecting translocation from the leaf, while the importance of galactinol and raffinose is also inferred. Our study further suggests that posttranscriptional mechanisms are key for metabolic regulation at early stages. In contrast to early developmental stages, a decrease in amino acid levels is coupled to an induction of transcripts encoding amino acid and organic acid catabolic enzymes during ripening. These data are consistent with the mobilization of amino acids to support respiration. In addition, sucrose cycling, suggested by the parallel increase of transcripts encoding sucrose degradative and synthetic enzymes, appears to operate during postharvest ripening. When taken together, these data highlight singular metabolic programs for peach development and may allow the identification of key factors related to agronomic traits of this important crop species.

Peach (Prunus persica) fruit response to anoxia: reversible ripening delay and biochemical changes.

The use of modified atmospheres has been successfully applied in different fruits to delay the ripening process and to prevent physiological disorders. In addition, during normal ripening, hypoxic areas are generated inside the fruit; moreover, anaerobic conditions may also arise during fruit post-harvest storage and handling. In consequence, the fruit is an interesting model to analyze the metabolic modifications due to changes in oxygen levels. In this work, a 72 h anoxic treatment by using an N(2) storage atmosphere was applied to peaches (Prunus persica L. Batsch) after harvest. Ripening was effectively delayed in treated fruits, preventing fruit softening, color changes and ethylene production. Metabolic changes induced by anoxia included induction of fermentative pathways, glycolysis and enzymes involved in both sucrose synthesis and degradation. Sucrose, fructose and glucose contents remained unchanged in treated fruit, probably due to sucrose cycling. Sorbitol was not consumed and citrate was increased, correlating with citric acid cycle impairment due to O(2) deprivation. Malate content was not affected, indicating compensation in the reactions producing and consuming malate. Changes in malic enzymes and pyruvate orthophosphate dikinase may provide pyruvate for fermentation or even act to regenerate NADP. After fruit transfer to aerobic conditions, no signs of post-anoxia injury were observed and metabolic changes were reversed, with the exception of acetaldehyde levels. The results obtained indicate that peach fruit is an organ with a high capacity for anoxic tolerance, which is in accord with the presence of hypoxic areas inside fruits and the fact that hypoxic pre-treatment improves tolerance to subsequent anoxia.

Biochemical and proteomic analysis of 'Dixiland' peach fruit (Prunus persica) upon heat treatment.

Shipping of peaches to distant markets and storage require low temperature; however, cold storage affects fruit quality causing physiological disorders collectively termed 'chilling injury' (CI). In order to ameliorate CI, different strategies have been applied before cold storage; among them heat treatment (HT) has been widely used. In this work, the effect of HT on peach fruit quality as well as on carbon metabolism was evaluated. When fruit were exposed to 39 degrees C for 3 d, ripening was delayed, with softening inhibition and slowing down of ethylene production. Several differences were observed between fruit ripening at ambient temperature versus fruit that had been heat treated. However, the major effects of HT on carbon metabolism and organoleptic characteristics were reversible, since normal fruit ripening was restored after transferring heated peaches to ambient temperature. Positive quality features such as an increment in the fructose content, largely responsible for the sweetness, and reddish coloration were observed. Nevertheless, high amounts of acetaldehyde and low organic acid content were also detected. The differential proteome of heated fruit was characterized, revealing that heat-induced CI tolerance may be acquired by the activation of different molecular mechanisms. Induction of related stress proteins in the heat-exposed fruits such as heat shock proteins, cysteine proteases, and dehydrin, and repression of a polyphenol oxidase provide molecular evidence of candidate proteins that may prevent some of the CI symptoms. This study contributes to a deeper understanding of the cellular events in peach under HT in view of a possible technological use aimed to improve organoleptic and shelf-life features.

Carbon metabolism of peach fruit after harvest: changes in enzymes involved in organic acid and sugar level modifications.

Peach (Prunus persica L. Batsch) is a climacteric fruit that ripens after harvest, prior to human consumption. Organic acids and soluble sugars contribute to the overall organoleptic quality of fresh peach; thus, the integrated study of the metabolic pathways controlling the levels of these compounds is of great relevance. Therefore, in this work, several metabolites and enzymes involved in carbon metabolism were analysed during the post-harvest ripening of peach fruit cv 'Dixiland'. Depending on the enzyme studied, activity, protein level by western blot, or transcript level by quantitative real time-PCR were analysed. Even though sorbitol did not accumulate at a high level in relation to sucrose at harvest, it was rapidly consumed once the fruit was separated from the tree. During the ripening process, sucrose degradation was accompanied by an increase of glucose and fructose. Specific transcripts encoding neutral invertases (NIs) were up-regulated or down-regulated, indicating differential functions for each putative NI isoform. Phosphoenolpyruvate carboxylase was markedly induced, and may participate as a glycolytic shunt, since the malate level did not increase during post-harvest ripening. The fermentative pathway was highly induced, with increases in both the acetaldehyde level and the enzymes involved in this process. In addition, proteins differentially expressed during the post-harvest ripening process were also analysed. Overall, the present study identified enzymes and pathways operating during the post-harvest ripening of peach fruit, which may contribute to further identification of varieties with altered levels of enzymes/metabolites or in the evaluation of post-harvest treatments to produce fruit of better organoleptic attributes.