Molecular and biochemical differences underlying the efficacy of lovastatin in preventing the onset of superficial scald in a susceptible and resistant Pyrus communis L. cultivar
Introduction
By using cold storage in combination with controlled atmosphere or other postharvest strategies, pears (Pyrus communis) can be commercialized throughout the year, similarly to apple and other fleshy fruits (Little and Holmes, 2000). Unlike other rosaceae fruit, most pear cultivars are distinguished by the requirement of a chilling period or ethylene treatment for the completion of the ripening process (El-Sharkawy et al., 2004; Lelièvre et al., 1997; Villalobos-Acuña and Mitcham, 2008). However, prolonged low temperature storage can induce several physiological disorders, among which superficial scald is one of the most dramatic in terms of economical losses in pome fruit (Lurie and Watkins, 2012; Wang, 2016; Whitaker, 2008).
The symptoms of superficial scald are characterized by the development of brown patches on the fruit skin generally appearing after the fruit is removed from cold storage and placed at room temperature conditions (Lurie and Watkins, 2012) and caused by the oxidation of chlorogenic acid through the action of polyphenol oxidase (PPO) (Busatto et al., 2014; Giné-Bordonaba et al., 2020). In detail, the reaction between PPO and chlorogenic acid leads to the accumulation of quinones in the cytoplasm, reacting together to form the brown pigment melanin (Busatto et al., 2014). Despite the deep comprehension of the symptom appearance, mainly investigated in apples (Lurie and Watkins, 2012), the mechanism related to the etiological cause leading to the scald development in pears is still not completely elucidated. Recent studies shed light on the physiological details related to the scald development and on the molecular mechanism underlying the basis of the scald resistance induced by 1-Methylcyclopropene (1-MCP) treatment in apple (Busatto et al., 2018). 1-MCP, a competitive inhibitor of ethylene, is among the most effective strategies to prevent the development of superficial scald (Lurie and Watkins, 2012; Watkins, 2006). The regulation of superficial scald through the action of ethylene is supposed to rely on the ability of this hormone to mediate the expression of α-farnesene synthase 1 gene (AFS1), the limiting step in the production of α-farnesene. Therefore, the effectiveness of 1-MCP in preventing the onset of superficial scald was initially accounted to the inhibition of the ethylene perception induced by this ethylene analog (Lurie and Watkins, 2012). However, it has recently been shown that 1-MCP treatment is also able to promote a deep transcriptional reprogramming inducing a specific group of genes involved in the cold stress response, finally leading to the establishment of a cold tolerance phenotype (Busatto et al., 2018). 1-MCP is also routinely used in the post-harvest management to increase the fruit storability, slowing down softening as well as other multiple ripening associated events (Ikiz et al., 2018; Watkins, 2006). The application of 1-MCP in pear can, however, dramatically impair the progression of the fruit ripening and affect several ethylene-dependent fruit quality related processes, such as the production of volatile organic compounds (VOCs) and fruit softening thereby compromising consumer acceptance. Indeed, while juiciness and crispiness are generally the most important apple quality traits in terms of consumer acceptance, consumers demand pears with a buttery and juicy texture. In this context, several strategies have been employed in the past to prevent the irreversible block of ethylene caused by 1-MCP yet achieving unsuccessful results (Chiriboga et al., 2011).
Consequently, the search of novel treatments using specific compounds able to reduce the impact of post-harvest physiological disorders, such as superficial scald, without impairing the pear ripening capability is a key factor for an innovative pear post-harvest management. Even if the etiology of superficial scald is still a matter of speculation, a positive correlation between superficial scald onset and the presence of 6-Methyl-5-hepten-2-one (6-MHO) is well documented in the literature. 6-MHO, together with the conjugated trienes hydroperoxides, are thought to be the major products of the α-farnesene autoxidation (Farneti et al., 2015; Rowan, 2011; Rowan et al., 2001) leading to the appearance of superficial scald symptoms. Therefore, the possibility of reducing the incidence of this disorder disrupting the accumulation of α-farnesene without interfering with the ethylene signaling, could represent a valuable strategy to promote or better maintain fruit quality.
Some studies have investigated the effects of lovastatin treatment on α-farnesene and ethylene biosynthesis, VOC production, and fruit color changes during apple ripening showing that lovastatin is capable to reduce the production of α-farnesene and sesquiterpenes without affecting the ethylene synthesis and the ripening progression (Ju and Curry, 2001; Kader, 1999; Pechous and Whitaker, 2004; Rudell et al., 2009; Savran and Koyuncu, 2016). Lovastatin is a statin inhibitor of the 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase), an enzyme devoted to the conversion of HMG-CoA to mevalonate and a potent cholesterol-lowering pharmaceutical in animals. In higher plants, the biosynthesis of the C5 universal sesquiterpene precursor, isopentenyl diphosphate (IPP), is synthetized, in the cytosol, through the mevalonate pathway (Ju and Curry, 2001; Vranová et al., 2013). IPP is, in turn, converted to the α-farnesene precursor, farnesyl diphosphate (FPP) and then accumulated in the wax layer of the pear skin during cold storage, where undergoes progressive autoxidation processes (Giné Bordonaba et al., 2013; Larrigaudière et al., 2016).
In this work, we investigated the role of lovastatin in reducing the development of superficial scald and the treatment effect on major fruit quality traits of two pear cultivars, ‘Blanquilla’ and ‘Conference’, characterized by a distinct superficial scald susceptibility (Lindo-García et al., 2020b). For comparative purposes, fruit were treated with lovastatin, 1-MCP and ethylene prior to storage and gene expression and secondary metabolite analysis were done on fruit after removing the fruit from cold storage and further shelf-life.
Section snippets
Plant materials, storage protocols and treatments
‘Blanquilla’ and ‘Conference’ pears were harvested in a commercial orchard located in Lleida (Spain). Trees, at the time of the analysis, were in the full bearing stage, trained and grown following standard horticultural practice for canopy management, pruning, fruit thinning and pest-disease control. Homogeneous fruit, in terms of both ripening stage and shape, were sampled at commercial maturity based on local grower standards mainly based on firmness and starch index values (6.3 for
Effect of the treatments on scald incidence and fruit quality in ‘Blanquilla’ and ‘Conference’ pears
After four months of cold storage and shelf-life the susceptibility of the fruit to superficial scald was significantly different for ‘Blanquilla’ and ‘Conference’ pears (Fig. 1a). Prolonged cold storage severely affected the scald development in untreated ‘Blanquilla’ fruit (78 %) and almost entirely the ethylene treated fruit (96 %) upon shelf-life. The application of both 1-MCP and lovastatin efficiently alleviated the scald development, with a complete reduction of the symptoms (0%) in the
The occurrence of superficial scald in pear is governed by the contribution of several metabolite pathways acting in a cultivar specific manner
The development of superficial scald was strongly influenced by the type of treatment (1-MCP or lovastatin) as well as by the cultivar. In fact, while 78 % of untreated ‘Blanquilla’ pears showed superficial scald symptoms, very low incidence (5%) was observed in ‘Conference’ fruit (Fig. 1a), confirming the differential susceptibility to superficial scald among cultivars reported in the literature (Larrigaudière et al., 2016; Lindo-García et al., 2020a) and suggesting a specific genetic control
Conclusion
The use of 1-MCP to prevent superficial scald development in pear, despite its effectiveness, may represent for some cultivars, undesirable side-effects such as the inability of the fruit to properly ripen after cold storage thereby reducing the general fruit quality. Lovastatin is well known to interfere with the mevalonate pathway, and therefore with the production of α-farnesene, a sesquiterpene thought to be involved in the superficial scald etiology. Our results suggest that lovastatin can
Declaration of Competing Interest
The authors report no declarations of interest.
CRediT authorship contribution statement
Nicola Busatto: Investigation, Writing - original draft, Formal analysis, Conceptualization. Jordi Giné-Bordonaba: Investigation, Writing - review & editing, Formal analysis, Conceptualization. Christian Larrigaudière: Writing - review & editing. Violeta Lindo-Garcia: Investigation. Brian Farneti: Investigation, Writing - review & editing. Franco Biasioli: Investigation. Urska Vrhovsek: Investigation. Fabrizio Costa: Supervision, Writing - review & editing, Conceptualization.
Acknowledgements
This work was partially supported by the CERCA programme from the Generalitat de Catalunya and by the Ministerio de Economía y Competitividad (MINECO; grant AGL2017-87923-R). JGB was the recipient of a José Castillejo Mobility Fellowship (CAS18/00186).
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These authors contributed equally to this work.