Biochemical and molecular characterization of [alpha]-farnesene biosynthesis in relation to superficial scald development in apple (Malus x domestica Borkh.)
Metabolism of [alpha]-farnesene, a sesquiterpene that accumulates in apple skin during cold storage, has been implicated in the development of superficial scald in pome fruits. Biosynthesis of [alpha]-farnesene occurs through the classical mevalonate pathway, and is formed directly from ' trans,trans'-farnesyl pyrophosphate. This step is catalyzed by ' trans,trans'-[alpha]-farnesene synthase, an enzyme located mainly in hypodermal and epidermal cells of apple fruit. The enzyme was purified seventy-fold from the cytosolic fraction, where activity was highest among sub-cellular fractions. The enzyme required a divalent metal (Mg2+ or Mn2+) for activity and exhibited allosteric kinetics; S(0.5) for FPP was 84 [mu]mol·L-1. The Hill coefficient (nH) indicated that the native protein was a trimer. Activity of [alpha]-farnesene synthase was not evident in apple skin at harvest, but was induced by low temperature storage and preceded the accumulation of [alpha]-farnesene. In contrast, high activity of 3-hydroxy-3-methylglutaryl CoA reductase (HMGR) was present in the skin at harvest, but declined during the first 8 weeks in storage and then remained unchanged. The inhibition of ethylene (C2H4) action in apple fruit by 1-methylcyclopropene (1-MCP) revealed that C2H4 was required for [alpha]-farnesene synthesis and the development of superficial scald. However, activity of [alpha]-farnesene synthase was not affected by C2H4. Since fruit respiration was suppressed significantly by 1-MCP, the regulation of [alpha]-farnesene biosynthesis by C2H4 may be through control of glycolysis; e.g., acetyl CoA availability limits isoprene synthesis and HMGR activity. A full-length ('hmg1') and a fragment ('hmg2') of two cDNA clones comprising the HMGR gene family were isolated from apple skin. The transcription product of 'hmg1' cDNA has an open reading frame of 1767 nucleotides and encodes a protein of 589 polypeptide residues of 62.7 kD. The presence of two highly hydrophobic domains near the amino terminus, a unique feature of plant HMGR genes, was recognized. The two genes were expressed differentially in response to developmental stimuli; ' hmg1' being expressed relatively constitutively, and 'hmg2 ' being highly sensitive to low temperatures and C2H 4. The synthesis of [alpha]-farnesene possibly occurs through a complex of sequential metabolic enzymes or "metabolon" located in the cytosol/ER boundary, where 'hmg2' also may be involved.