Ethylene-auxin crosstalk regulates postharvest fruit ripening process in apple

The ripening of climacteric fruits, such as apple, is represented by a series of genetically programmed events orchestrated by the action of several hormones. In this study, we investigated the existence of a hormonal crosstalk between ethylene and auxin during the post-harvest ripening of three internationally known apple cultivars: 'Golden Delicious', 'Granny Smith' and 'Fuji'. The normal climacteric ripening was impaired by the exogenous application of 1-methylcyclopropene (1-MCP) that affected the production of ethylene and the physiological behaviour of specific ethylene-related quality traits, such as fruit texture and the production of volatile organic compounds. The application of 1-MCP induced, moreover, a de-novo accumulation of auxin. The RNA-Seq wide-transcriptome analysis evidenced as the competition at the level of the ethylene receptors induced a cultivar-dependent transcriptional re-programming. The DEGs annotation carried out through the KEGG database identified as most genes were assigned to the plant hormone signaling transduction category, and specifically related to auxin and ethylene. The interplay between these two hormones was further assessed through a candidate gene analysis that highlighted a specific activation of GH3 and ILL genes, encoding key steps in the process of the auxin homeostasis mechanism. Our results showed that a compromised ethylene metabolism at the onset of the climacteric ripening in apple can stimulate, in a cultivar-dependent fashion, an initial de-novo synthesis and de-conjugation of auxin as a tentative to restore a normal ripening progression

Climacteric ripening of apple fruit is regulated by transcriptional circuits stimulated by cross-talks between ethylene and auxin

Apple is a fleshy fruit distinguished by a climacteric type of ripening, since most of the relevant physiological changes are triggered and governed by the action of ethylene. After its production, this hormone is perceived by a series of receptors to regulate, through a signaling cascade, downstream ethylene related genes. The possibility to control the effect of ethylene opened new horizons to the improvement of the postharvest fruit quality. To this end, 1-methylcyclopropene (1-MCP), an ethylene antagonist, is routinely used to modulate the ripening progression increasing storage life. In a recent work published in The Plant Journal, the whole transcriptome variation throughout fruit development and ripening, with the adjunct comparison between normal and impaired postharvest ripening, has been illustrated. In particular, besides the expected downregulation of ethylene-regulated genes, we shed light on a regulatory circuit leading to de-repressing the expression of a specific set of genes following 1-MCP treatment, such as AUX/IAA, NAC and MADS. These findings suggested the existence of a possible ethylene/auxin cross-talk in apple, regulated by a transcriptional circuit stimulated by the interference at the ethylene receptor leve

Wide transcriptional investigation unravel novel insights of the on-tree maturation and postharvest ripening of \u2018Abate Fetel\u2019 pear fruit

To decipher the transcriptomic regulation of the on-tree fruit maturation in pear cv. \u2018Abate Fetel\u2019, a RNA-seq transcription analysis identified 8939 genes differentially expressed across four harvesting stages. These genes were grouped into 11 SOTA clusters based on their transcriptional pattern, of which three included genes upregulated while the other four were represented by downregulated genes. Fruit ripening was furthermore investigated after 1 month of postharvest cold storage. The most important variation in fruit firmness, production of ethylene and volatile organic compounds were observed after 5 days of shelf-life at room temperature following cold storage. The role of ethylene in controlling the ripening of \u2018Abate Fetel\u2019 pears was furthermore investigated through the application of 1-methylcyclopropene, which efficiently delayed the progression of ripening by reducing fruit softening and repressing both ethylene and volatile production. The physiological response of the interference at the ethylene receptor level was moreover unraveled investigating the expression pattern of 12 candidate genes, initially selected to validate the RNA-seq profile. This analysis confirmed the effective role of the ethylene competitor in downregulating the expression of cell wall (PG) and ethylene-related genes (ACS, ACO, ERS1, and ERS2), as well as inducing one element involved in the auxin signaling pathway (Aux/IAA), highlighting a possible cross-talk between these two hormones. The expression patterns of these six elements suggest their use as molecular toolkit to monitor at molecular level the progression of the fruit on-tree maturation and postharvest ripening

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A Multidisciplinary Approach Providing New Insight into Fruit Flesh Browning Physiology in Apple (<i>Malus x domestica</i> Borkh.)

<div><p>In terms of the quality of minimally processed fruit, flesh browning is fundamentally important in the development of an aesthetically unpleasant appearance, with consequent off-flavours. The development of browning depends on the enzymatic action of the polyphenol oxidase (PPO). In the ‘Golden Delicious’ apple genome ten <i>PPO</i> genes were initially identified and located on three main chromosomes (2, 5 and 10). Of these genes, one element in particular, here called <i>Md-PPO</i>, located on chromosome 10, was further investigated and genetically mapped in two apple progenies (‘Fuji x Pink Lady’ and ‘Golden Delicious x Braeburn’). Both linkage maps, made up of 481 and 608 markers respectively, were then employed to find QTL regions associated with fruit flesh browning, allowing the detection of 25 QTLs related to several browning parameters. These were distributed over six linkage groups with LOD values spanning from 3.08 to 4.99 and showed a rate of phenotypic variance from 26.1 to 38.6%. Anchoring of these intervals to the apple genome led to the identification of several genes involved in polyphenol synthesis and cell wall metabolism. Finally, the expression profile of two specific candidate genes, up and downstream of the polyphenolic pathway, namely phenylalanine ammonia lyase (PAL) and polyphenol oxidase (PPO), provided insight into flesh browning physiology. <i>Md-PPO</i> was further analyzed and two haplotypes were characterised and associated with fruit flesh browning in apple.</p> </div