Cell wall modifications and ethylene-induced tolerance to non-chilling peel pitting in citrus fruit
Introduction
The plant hormone ethylene is usually associated with fruit ripening regulation, senescence and it is involved in the development of some physiological disorders such as blackheart of pineapple [1], toughening of asparagus [2] and apple scald [3]. However, it is well established that it may protect other horticultural commodities from stresses causing pathological and physiological disorders and that the responses to ethylene are affected by hormone concentrations and treatment durations and also by tissue susceptibility and by the organ physiological stage [4], [5], [6], [7].
Mature citrus fruits, in which the chloroplast to chromoplast transition has been completed, tolerate ethylene levels that may reduce infection caused by Penicillium digitatum [8], chilling injury [9] as well as peel collapse occurring in fruit held under non-chilling conditions (22 °C, 90–95% RH) [7]. This disorder, manifested as collapsed areas of the flavedo and part of the albedo (Supplementary data, Fig. S1), is known as non-chilling peel pitting (NCPP). A transcriptomic approach on mature oranges highlighted the molecular basis of the ethylene-induced resistance of citrus fruits to P. digitatum infection [8]. However, the biological basis of the ethylene-induced tolerance to NCPP in citrus fruits is still unknown [7]. Water stress may favour this disorder [10], [11], but NCPP may be also developed in harvested citrus fruit held under non-stressful environmental conditions [12]. Given that fruit detachment induces fast sucrose depletion [13], [14] and changes in proteins related to starvation-induced ageing in citrus peel [15], it has been suggested that the lack of carbon sources originated by fruit detachment may be involved in peel collapse [16]. Moreover, on view of ultrastructural changes, it was suggested that ethylene-induced modifications in cell wall might participate in the beneficial effect of the hormone reducing NCPP [16].
The plant cell wall provides mechanical support to individual cells, tissues, and organs. However, its perception as a solely rigid structure providing mechanical support is long past and it is now accepted that it has a key role in a plant's interactions with pathogens and responses to abiotic factors. Dynamic changes in wall polymers occur during normal development and in response to hormonal and environmental conditions [17]. These modifications may have large effects on tissue biomechanical properties [18], [19]. In addition, wall turnover may generate biologically active oligogalacturonides (OGs) able to induce defensive and developmental responses [20] and to induce ethylene biosynthesis in climacteric fruits like tomato [21]. Solubilized pectin and/or other carbohydrate pools derived from the bulk degradation of cell wall have been considered as alternative sources for fuelling metabolism and maintaining cellular homeostasis under shortage conditions [17]. Although we have recently found that ethylene treatments reducing NCPP caused cell wall ultrastructural modifications [16], the cell wall changes induced by these treatments in the flavedo and albedo tissues of citrus fruit are still unknown. The aim of this study was to characterize compositional changes induced by ethylene treatments that reduce NCPP in cell wall of the flavedo and albedo of mature ‘Navelate’ (Citrus sinensis, L. Osbeck) sweet orange.
Section snippets
Fruit material, ethylene treatments and storage
Mature ‘Navelate’ orange fruits were harvested by the end of February (2 months after fruit colour change) from a commercial orchard at Valencia, Spain and immediately delivered to the laboratory. A total of 540 oranges free of visual defects were sorted on the basis of uniform size and divided into three groups, each containing 180 fruit, which were stored under: (a) a continuous flow of air for up to 16 days (control); (b) a continuous flow of air after being conditioned for 4 days with air
Effect of ethylene on the susceptibility of ‘Navelate’ oranges to NCPP
‘Navelate’ sweet orange fruit harvested after fruit colour change can be considered as a model system to study NCPP because of its high susceptibility to this disorder and its tolerance to ethylene levels required for NCPP reduction [7]. Depressed peel areas (NCPP) affecting the inner and outer part of the peel were first observed by day 8 in fruit held at 20 °C and 90–95% RH and increased over time (Fig. 1). Treating the fruits for 4 days with ethylene (2 μL L−1) considerably reduced NCPP. This
Conclusions
Ethylene exposure decreased NCPP in ‘Navelate’ oranges. Continuous storage under ethylene (2 μL L−1) did not increase pectin deposition, but affected the cell wall composition and disassembly, increasing the solubility of both pectin and hemicelluloses, and also the UA content of the CSF, after long storage periods. The effect was more marked in the albedo than in the flavedo. The moderate modification in cell wall solubilization in fruit discontinuously treated with ethylene, in which NCPP was
Acknowledgements
We gratefully acknowledge the technical assistance of Mrs. D. Arocas for helping us with the ethylene treatments and sampling. This work was supported by the Spanish Ministry of Science and Technology (research grant AGL2009-11969) and by the Generalitat Valenciana (grant PROMETEO/2010/010). This study was partially supported by an internal grant (EX-038) of Cyprus University of Technology.
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