Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2004
Publication Date: 3/1/2005
Citation: Lurie, S., Lers, A., Shaham, Z., Sonego, L., Burd, S., Whitaker, B.D. 2005. Expression of a-farnesene synthase and hmg-co a reductase genes in relation to levels of a-farnesene and conjugated trienols in peel tissue of 'granny smith' apples treated with heat or 1-mcp to prevent scald. Journal of the American Society for Horticultural Science. 130(2):232-236. Interpretive Summary: Fruit of many popular apple varieties such as Granny Smith, Red Delicious, and Law Rome are prone a costly storage disorder called superficial scald. Oxidation products of the volatile compound alpha-farnesene, which accumulate in the peel tissue of apples during storage, are thought to promote scald. To control scald, apples are routinely drenched after harvest with a solution that includes an antioxidant chemical plus a fungicide. This is expensive and results in unwanted chemical waste and residue on the fruit. Our research is aimed at understanding, at the genetic and biochemical level, why certain apple varieties are highly susceptible to the scald disorder. In this study, we examined the effects of two treatments known to delay or prevent scald development on genes involved in alpha-farnesene synthesis, and on production and oxidation of alpha-farnesene. Treatment with 1-MCP, which blocks fruit responses to the ripening hormone ethylene, inhibited alpha-farnesene synthesis by shutting down a key gene, AFS1. Heat treatment, on the other hand, appeared to delay production of the AFS1 enzyme that performs the final step of alpha-farnesene synthesis. The long-range goal of this work is to limit alpha-farnesene synthesis in apples, and thereby prevent scald development, by molecular genetic means. This outcome will benefit both the apple industry and consumers by creating new scald-resistant lines that do not require chemical treatment prior to storage.
Technical Abstract: Nontreated control, 1-methylcyclopropene(1-MCP)-treated, and heated apples of the scald-susceptible 'Granny Smith' cultivar were compared with respect to scald incidence, ethylene production, a-farnesene metabolism, and expression of the genes AFS1, which encodes a-farnesene synthase, the final, rate-limiting enzyme in the a-farnesene biosynthetic pathway, and HMG2 and HMG3, which encode isozymes of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, the proposed rate limiting enzyme in the mevalonate pathway of isoprenoid synthesis. The incidence of scald in nontreated 'Granny Smith' apples after 16 weeks at 0°C plus 1 week at 20°C was 100%; 1-MCP treatment prevented scald development, while heat treatment prevented scald development for the first 12 weeks. 1-MCP also inhibited both a-farnesene and ethylene production, suggesting that ethylene induces transcription of key genes involved in a-farnesene biosynthesis. Heat treatment reduced levels of a-farnesene and conjugated trienols, but not to the extent of 1-MCP. Internal ethylene concentrations in heated apples did not differ from those in the controls. In both control and heated fruit, a sharp increase in AFS1 mRNA during the first 4 weeks of storage preceded an increase in a-farnesene and a subsequent increase in CTols. AFS1 transcript was absent from 1-MCP-treated apples for the first 10 weeks of storage, and even at 16 weeks was much lower than in heated and control fruit. Levels of the HMG2 and HMG3 transcripts varied during storage and among treatments, but were not correlated with the incidence of scald. HMG2 mRNA was very low at harvest and increased in abundance during storage in all treatments, with the greatest increase occurring in 1-MCP-treated fruit. In contrast, HMG3 transcript was constitutively present at all storage times, although it too was marginally more abundant in 1-MCP-treated fruit.