PbSRT1 and PbSRT2 regulate pear growth and ripening yet displaying a species-specific regulation in comparison to other Rosaceae spp.

Epigenetic regulation is crucial to ensure a coordinated control of the different events that occur during fruit development and ripening. Sirtuins are NAD+-dependent histone deacetylases involved in the regulation of gene expression of many biological processes. However, their implications in the Rosaceae family remains unexplored. Accordingly, in this work, we demonstrated the phylogenetic divergence of both sirtuins among Rosaceae species. We then characterized the expression pattern of both SRT1 and SRT2 in selected pome and stone fruit species. Both SRT1 and SRT2 significantly changed during the fruit development and ripening of apple, nectarine and pear fruit, displaying a different expression profile. Such differences could explain in part their different ripening behaviour. To further unravel the role of sirtuins on the fruit development and ripening processes, a deeper analysis was performed using pear as a fruit model. In pear, PbSRT1 gene expression levels were negatively correlated with specific hormones (i.e. abscisic acid, indole-3-acetic acid, gibberellin A1 and zeatin) during the first phases of fruit development. PbSRT2 seemed to directly mediate pear ripening in an ethylene-independent manner. This hypothesis was further reinforced by treating the fruit with the ethylene inhibitor 1-methylcyclopropene (1-MCP). Instead, enhanced PbSRT2 along pear growth/ripening positively correlated with the accumulation of major sugars (R2 > 0.94), reinforcing the idea that sugar metabolism may be a target of epigenetic modifications during fruit ripening. Overall, the results from this study point out, for the first time, the importance that sirtuins have in the regulation of fruit growth and ripening of pear fruit by likely regulating hormonal and sugar metabolism.

Elucidating the involvement of ethylene and oxidative stress during on- and off-tree ripening of two pear cultivars with different ripening patterns.

Scarce information is available about the ripening process of European pears attached and detached from the tree. Accordingly, this study aimed to investigate the physiological and biochemical processes underlying both on- and off-tree fruit ripening in a summer ('Conference') vs. a winter ('Flor d'Hivern') pear cultivar. For each cultivar, a batch of fruit was harvested at the commercial harvest date and ripened at 20 °C and another batch was left to ripen on the tree. In both cultivars the inability of the fruit to soften on-tree, was related to a very limited ethylene metabolism but also associated to high content of H2O2 and low lipid peroxidation levels. In contrast, ripening in detached fruit was cultivar-dependent. In 'Conference' pears, the sharp firmness loss and colour changes observed during off-tree ripening were not strictly associated to an enhanced ethylene production but rather triggered by an oxidative related process preceding the climacteric rise. In contrast, 'Flor d'Hivern' pears experienced limited softening and degreening during off-tree ripening not being related to the action of ethylene or oxidative stress. Collectively our results showed that pear ripening was not exclusively dependent of ethylene production and that the fruit potential to limit oxidative damage may be involved with the inability of some European pear cultivars to ripen on-tree.

Investigation of the transcriptomic and metabolic changes associated with superficial scald physiology impaired by lovastatin and 1-methylcyclopropene in pear fruit (cv. "Blanquilla").

To elucidate the physiology underlying the development of superficial scald in pears, susceptible "Blanquilla" fruit was treated with different compounds that either promoted (ethylene) or repressed (1-methylcyclopropene and lovastatin) the incidence of this disorder after 4 months of cold storage. Our data show that scald was negligible for the fruit treated with 1-methylcyclopropene or lovastatin, but highly manifested in untreated (78% incidence) or ethylene-treated fruit (97% incidence). The comparison between the fruit metabolomic profile and transcriptome evidenced a distinct reprogramming associated with each treatment. In all treated samples, cold storage led to an activation of a cold-acclimation-resistance mechanism, including the biosynthesis of very-long-chain fatty acids, which was especially evident in 1-methylcyclopropane-treated fruit. Among the treatments applied, only 1-methylcyclopropene inhibited ethylene production, hence supporting the involvement of this hormone in the development of scald. However, a common repression effect on the PPO gene combined with higher sorbitol content was found for both lovastatin and 1-methylcyclopropene-treated samples, suggesting also a non-ethylene-mediated process preventing the development of this disorder. The results presented in this work represent a step forward to better understand the physiological mechanisms governing the etiology of superficial scald in pears.

Interplay between hormones and assimilates during pear development and ripening and its relationship with the fruit postharvest behaviour.

The ability of European pears (Pyrus communis L.) to ripen immediately after harvest is cultivar-dependent and relies on a range of physiological and biochemical events occurring during fruit growth and development that remain largely unknown. To gain further knowledge on these events, changes in the content of sugars, acids, major hormones and ethylene precursors or related enzymes were studied in two pear varieties ('Blanquilla' and 'Conference') with known differences in their postharvest ripening behaviour. In both cultivars, low contents of abscisic acid (ABA) seemed to be a prerequisite to initiate on-tree fruit ripening including sugar accumulation and softening. In 'Blanquilla' pears, the enhanced potential to produce ethylene and thereby to ripen upon harvest was associated to a late increase in ABA content paralleled by an accumulation of indole 3-acetic acid (IAA). In turn, the inability of 'Conference' fruit to produce ethylene upon harvest appeared to be related to a coordinated action of gibberellins (more specifically GA1), salicylic acid (SA) and jasmonic acid (JA), which remained at high concentrations during the latest phases of fruit growth. Collectively, our results highlight that a complex hormonal cross-talk during the development and on-tree ripening of pear fruit may finally determine the ability of the fruit to ripen upon harvest.