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Title: Ethylene and a-farnesene metabolism in green and red skin of three apple cultivars in response to 1-methylcyclopropene (1-MCP)

item Gapper, Nigel
item Whitaker, Bruce

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2007
Publication Date: 6/27/2007
Citation: Tsantili, E., Gapper, N.E., Arquiza, J., Whitaker, B.D., Watkins, C.B. 2007. Ethylene and a-farnesene metabolism in green and red skin of three apple cultivars in response to 1-methylcyclopropene (1-MCP). Journal of Agricultural Food & Chemistry. 55:5267-5276.

Interpretive Summary: A storage disorder called scald occurs in many popular varieties of apples, appearing as sunken brown patches on the skin that make the fruit unmarketable, thus resulting in great economic loss. This study addressed the questions of why some apple varieties are much more prone to scald than others, and why scald symptoms only develop on the shaded ('green') side of the fruit. Our results showed that during cold storage much higher levels of natural toxic compounds linked with scald development accumulate in skin of scald-prone compared with scald-resistant apples. Moreover, after 4-5 months of storage levels of these toxic compounds are twice as high in 'green' than in 'red' skin of scald-prone fruit. It was also found that production of the precursor from which the toxic compounds are derived is induced by the fruit ripening hormone ethylene via gene activation. This work provided information that can be used by plant scientists to elucidate the genetic basis of scald susceptibility and to develop new scald-resistant varieties of apple fruit.

Technical Abstract: Relationships among alpha-farnesene synthesis and oxidation, ethylene production and perception, antioxidative enzyme activities, and superficial scald development in fruit of three commercial apple cultivars were investigated at the biochemical and gene transcriptional levels. Scald-susceptible Cortland (CL) and Law Rome (LR) and scald-resistant Idared (IR) apples were untreated or treated with the ethylene action inhibitor 1-methylcyclopropene (1-MCP) and stored up to 25 weeks at 0.5C. Separate blushed (red) and unblushed (green) peel tissue samples were taken at harvest and after 2, 4, 6, 10, 15, 20 and 25 weeks of storage. Large increases in peel tissue concentrations of a-farnesene and its conjugated trienol (CTol) oxidation products occurred in untreated CL and LR, and were about 4- to 9-fold greater than those in IR. In both CL and LR, accumulation of CTols in green peel was nearly twice that in red peel. 1-MCP treatment delayed and attenuated a-farnesene and CTol accumulation in each cultivar. After rising during the first 4-6 weeks, internal ethylene concentrations (IECs) in control fruit were high in CL, intermediate in LR, and low in IR. In 1-MCP treated fruit, IECs increased gradually to modest levels by 25 weeks in CL and LR but were almost nil in IR. Expression patterns of the a-farnesene synthase gene AFS1, the ethylene receptor gene MdERS, and the ethylene biosynthetic genes MdACS and MdACO were generally in accord with the patterns of a-farnesene and ethylene production. In particular, AFS1 and MdACS showed similar patterns of expression in each cultivar; among the controls, transcript levels increased more rapidly in CL and LR than in IR during the first few weeks of storage, and in 1-MCP treated fruit, transcript abundance in CL and LR rose to untreated control levels after 10-15 weeks but remained low in IR. Scald symptoms were restricted to unblushed skin and the incidence in controls after 25 weeks was nearly 100% in CL and LR compared with 1% in IR. 1-MCP treatment reduced scald incidence to 14%, 3%, and 0% in CL, LR, and IR, respectively. Overall, our results support the proposed role of CTols in scald induction and indicate that a-farnesene synthesis is tightly regulated by ethylene. However, gene transcription alone does not account for the big differences in ethylene and a-farnesene production in CL, LR, and IR apples.