1-MCP regulates ethylene biosynthesis and fruit softening during ripening of ‘Tegan Blue’ plum

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Abstract

To investigate the effects of postharvest application of 1-MCP on ethylene production and fruit softening, activities of ethylene biosynthesis and fruit softening enzymes were measured during postharvest ripening of plum (Prunus salicina Lindl. cv. Tegan Blue) fruit after being exposed to 1-MCP (0, 0.5, 1.0 or 2.0 μL L−1) at 20 ± 1 °C for 24 h. Following the treatments, fruit were allowed to ripen at ambient temperature (20 ± 1 °C), and ethylene production in fruit, activities of ACS and ACO, ACC content and fruit softening enzymes (PE, EGase, exo-PG and endo-PG) in fruit skin and pulp were recorded at different intervals. Postharvest application of 1-MCP significantly delayed and suppressed the climacteric ethylene production with reduction in the activities of ethylene biosynthesis enzymes (ACS, ACO) and ACC content, and fruit softening enzymes (PE, EGase, exo-PG and endo-PG) in the skin as well as in pulp tissues. The reduction was more pronounced with increased concentrations of 1-MCP. 1-MCP treated fruit showed different rates of fruit softening and activities of ethylene biosynthesis enzymes in the skin and pulp tissues which warrant further investigation on regulation of gene expression related to these enzymes with the inhibitory effect of 1-MCP.

Introduction

The fruit ripening process in plum is regulated by endogenous ethylene biosynthesis (Lelievre et al., 1997a). Fruit softening is one of the indices of fruit maturity and ripening, which is partially controlled by endogenous ethylene biosynthesis (Menniti et al., 2004). The exogenous application of ethylene has also been reported to hasten the fruit softening in plum (Abdi et al., 1997). The non-toxic ethylene action inhibitor 1-methylcyclopropene (1-MCP), which blocks the ethylene receptors (Sisler et al., 1996), has been reported to delay fruit softening and improve postharvest quality of climacteric fruit (Sisler and Serek, 1997), including different cultivars of plum (Abdi et al., 1998, Martinez-Romero et al., 2003b, Salvador et al., 2003a, Skog et al., 2003, Valero et al., 2003, Khan and Singh, 2004, Menniti et al., 2004), peach and nectarine (Fan et al., 2002, Liguori et al., 2004), apricot (Dong et al., 2002) and cherry (Gong et al., 2002). 1-MCP application has also been reported to retard ethylene production and fruit softening in plum depending upon the cultivar (Abdi et al., 1998, Martinez-Romero et al., 2003a), maturity stage at harvest (Valero et al., 2003), concentration applied (Martinez-Romero et al., 2003a, Salvador et al., 2003a, Salvador et al., 2003b, Valero et al., 2003), duration of fruit exposure to 1-MCP (Abdi et al., 1998, Dong et al., 2001b, Dong et al., 2002), and fruit temperature at the time of 1-MCP application (Dong et al., 2001b).

Inhibitory effects of 1-MCP on ethylene biosynthesis with reduced activities of 1-amino-cyclopropane carboxylic acid synthase (ACS) and 1-amino-cyclopropane carboxylic acid oxidase (ACO) enzymes and their respective gene transcription have been reported in banana (Pathak et al., 2003) tomatoes (Nakatsuka et al., 1997) and peach (Mathooko et al., 2001). A higher activity of ACS was observed in control and 1-MCP treated ‘Red Rosa’ plum fruit as compared to ethylene treated fruit during ripening (Dong et al., 2001b).

Fruit softening is associated with cell wall disassembly (Seymour and Gross, 1996), and during fruit softening, pectin and hemicellulose in cell walls undergo solubilization and depolymerisation which contribute to cell wall loosening (Fischer and Bennett, 1991). Most of the studies on ripening-related cell wall hydrolyses have examined the activities of pectin esterase, pectin lyase, polygalacturonase (PG) and endo-1,4-β-d-glucanase (EGase) or cellulase enzymes in various fruit (Jeong and Huber, 2004, Lohani et al., 2004). A dramatic increase in the activity of these enzymes, protein and mRNA levels has been observed during ripening in several climacteric fruit including tomato (Lashbrook et al., 1994), pear (Hiwasa et al., 2003) and avocado (Jeong et al., 2002). 1-MCP application has been reported to affect the trends in activities of cell wall enzymes in avocado fruit and to completely suppress the activity of PG enzymes during ripening (Jeong et al., 2002). A difference in the cell wall enzyme activities in fruit pulp during the ripening of 1-MCP treated ‘Red Rosa’ plum was observed without any correlation with differences in fruit softening (Dong et al., 2001b).

Application of 1-MCP is known to modulate the physiology of fruit softening in different cultivars of plum during ripening. However, the effects of 1-MCP on the ethylene biosynthesis and cell wall hydrolysis enzymes during fruit ripening in the skin have not been investigated and warrant further investigation. Therefore, the aim of the present study was to investigate how 1-MCP application retards ethylene biosynthesis and fruit softening, including the activities of ethylene biosynthetic enzymes such as ACS and ACO, and ACC content, as well as fruit softening enzymes including PE, exo-PG, endo-PG and EGase in the skin and pulp of the fruit during plum fruit ripening.

Section snippets

Plant material

Plum (Prunus salicina Lindl. cv. Tegan Blue) fruit were harvested from Casuarina Valley Orchard at Manjimup (lat. 34°15′S; long. 116°09′E), in the South West region of Western Australia. Experimental trees were grafted on myrobalan (Prunus cerasifera Ehrh.) root stock, planted in 1987 in a north-south direction, maintaining a row distance of 4.5 m × 4.5 m and a plant distance of 2 m × 2 m, trained on a palmette training system and with commercial cultural practices. Plum fruit at commercial maturity

Ethylene production and activities of ethylene biosynthesis enzymes

1-MCP-treated fruit exhibited delayed and suppressed ethylene production, as compared to untreated fruit. On day 1 of ripening, untreated fruit showed 11.8-fold higher ethylene production than 1-MCP-treated fruit. In the control fruit, climacteric ethylene production peaked on day 6 of ripening and declined gradually to 0.414 μmol kg−1 h−1 on day 16 (Fig. 1). While none of the 1-MCP-treated fruit showed any increase in ethylene production until day 8 of ripening, 0.5 μL L−1 1-MCP-treated fruit

Ethylene production and ethylene biosynthesis enzymes

As expected, postharvest application of 1-MCP (1.0 and 2.0 μL L−1) reduced ethylene production during ripening at ambient temperature (Fig. 1). The reduction in ethylene production during fruit ripening in 1-MCP-treated fruit may be due to 1-MCP interferring with the autocatalytic production of ethylene, as ethylene binding sites have been irreversibly blocked by 1-MCP (Sisler et al., 1996). Reduction in ethylene production with postharvest application of 1-MCP has previously been reported in

Acknowledgements

We are thankful to Dr. G. Seymour, Horticultural Research International, Warwick, UK for his constructive review and comments on the manuscript and Dr J. Dawson, Curtin University of Technology, Perth, Western Australia for editing the manuscript. A.S. Khan acknowledges the financial support from Higher Education Commission, Pakistan for awarding scholarship under the programme “Partial Support for Ph.D. Studies Abroad” and Curtin University of Technology for granting International Postgraduate

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