Submitted to: Postharvest Biology and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/21/2004
Publication Date: 2/1/2005
Citation: Pechous, S.W., Watkins, C.B., Whitaker, B.D. 2005. Expression of alpha-farnesene synthase gene afs1 in relation to levels of alpha-farnesene and conjugated trienols in peel tissue of scald-susceptible 'law rome' and scald-resistant 'idared' apple fruit. Postharvest Biology and Technology. 35:125-132. Interpretive Summary: Superficial scald is a costly storage disorder of apples that affects fruit of many popular varieties such as Red Delicious, Granny Smith, and Law Rome. Scald is thought to be induced by oxidation products of a volatile compound, alpha-farnesene, which is produced in the peel tissue of apples during cold storage. To control scald, apples are routinely drenched after harvest with a solution including 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 some apples are highly susceptible and others are resistant to the scald disorder. In this study we compared fruit of scald-prone Law Rome and scald-resistant Idared apples with respect to genetic control of a-farnesene production, the rate of a-farnesene oxidation, and the incidence of scald after storage. We found that a key gene involved in a-farnesene synthesis, AFS1, was turned on by the apple ripening hormone ethylene and was much more active in Law Rome than in Idared fruit. Consequently, Law Rome fruit accumulated high levels of a-farnesene and its oxidation products and scalded badly, whereas Idared fruit had low levels and did not scald. The long-range goal of this work is to limit a-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: Fruit of different apple cultivars vary widely in susceptibility to the physiological storage disorder superficial scald. The genetic and biochemical factors involved in this variation are not yet known, despite many years of investigation. Conjugated trienol (CTol) oxidation products of the sesquiterpene alpha-farnesene are thought to play a primary role in scald induction. Typically, a-farnesene accumulates to high levels in peel tissue of scald-susceptible apples early in storage, which results in later accumulation of high levels of CTols as a-farnesene is oxidized. Pre-storage treatment of scald-susceptible fruit with 1-methylcyclopropene (1-MCP), a blocker of ethylene perception, strongly inhibits a-farnesene synthesis and largely prevents scald. In this study, nontreated control and 1-MCP-treated fruit of the scald-susceptible Law Rome and scald-resistant Idared cultivars were compared with respect to scald incidence, internal ethylene concentration (IEC), a-farnesene metabolism, and expression of the gene encoding a-farnesene synthase (AFS1), the final, rate-limiting enzyme in the a-farnesene biosynthetic pathway. Scald incidence in nontreated 'Law Rome' apples after 20 weeks at 0.5C plus 1 week at 20C averaged 86%; 1-MCP treatment reduced increases in IEC and reduced the incidence of scald to less than 1%. Fruit of 'Idared' showed no scald regardless of the treatment. 1-MCP also inhibited a-farnesene production, suggesting that ethylene induces transcription of key genes involved in its biosynthesis. In both 'Law Rome' and 'Idared', a sharp increase in AFS1 mRNA during the first 4 to 8 weeks of storage preceded a proportional rise in a-farnesene and a subsequent increase in CTols. However, maximum levels of AFS1 transcript, a-farnesene, and CTols were, respectively, 2.5-, 4-, and 33-fold greater in 'Law Rome' than in 'Idared' apples. 1-MCP suppressed the increases in AFS1 transcript and a-farnesene early in storage, although AFS1 expression and a-farnesene synthesis had recovered in treated 'Law Rome' fruit after 20 weeks, consistent with increasing IEC in these fruit.