Biosynthesis And Regulation Of Fruit Aroma-related Volatiles Derived From The LOX Pathway

Melting flesh peach (Prunus persica L. Batsch cv. Yulu) and tomato(Solanum lycopersicum) fruits were used to study the biosynthesis and regulation of fruit aroma-related volatiles derived from LOX pathway. Volatiles from fruits were identified by using SPME-GC-MS, and genes related to biosynthesis of those volatiles were isolated based on ESTree database (http://www.itb.cnr.it/estree/). Real time quantitative PCR (qPCR) and transgenic techniques were applied to investigate expression patterns and functions of genes. The main results are as follow:1. Changes in characteristic aroma compounds during postharvest peach fruit ripening at20℃. Firmness of fruit rapidly decreased and the emission of ethylene increased during fruit ripening. Concentrations of hexanal,(E)-2-hexenal,(E)-2-hexenol and (Z)-3-hexenol decreased after1d of ripening, then were maintained at a low level; whereas the contents of (Z)-3-hexenyl acetate, y-hexalactone, y-decalactone,δ-decalactone significantly accumulated during ripening, peaking at about7d after harvest. Emission of y-octalactone increased to the highest level of7.36μg g-1FW at3d after harvest. Generally, green sensory volatiles such as C6aldehydes and alcohols were replaced by fruity-note esters and lactones during postharvest peach fruit ripening.2. LOX pathway genes showed different expression patterns during peach fruit ripening. PpLOX2and PpLOX3had relatively high transcript levels at harvest followed by a decline, while levels of PpLOXl and PpLOX4transcripts were increased along with accumulated ethylene production after harvest. PpHPLl and PpADHl-3displayed similar decreasing patterns during ripening, whereas the levels of PpAAT1showed a rapid increase during the first2days of postharvest ripening followed by a gradual decrease. The increased levels of unsaturated fatty acids such as linolenic and linoleic acids closely paralleled increasing expressions of PpFAD1and PpFAD2. Principle component analysis (PCA) result showed that C6aldehydes and alcohols were clustered with PpLOX2, PpLOX3, PpHPLl and PpADH1-3.3. TomloxC was involved in the biosynthesis of C5volatiles. Suppressing expression of TomloxC in tomato fruit resulted in significant reduction in emissions of C6volatiles, including hexanal, hexanol,(Z)-3-hexenal and (Z)-3-hexen-l-ol, to1%of those of wild-type controls, while levels of1-penten-3-ol,1-penten-3-one and pentanal were reduced to22-31%of the controls. Concentrations of (Z)-2-penten-l-ol and1-pentanol were also decreased but not significantly. Knock-down of LeHPL expression led to reduced C6volatile emissions in fruits relative to the controls, but about four-fold high concentrations of C5volatiles. Gene expression data showed that TomloxC and LeHPL were significantly suppressed in ripe tomato fruits, but depletion of TomloxC caused no reduction in expressions of the two closest homologs, TomloxD and TomloxF. Thus, biosynthesis of C5volatiles is dependent upon TomloxC activity and is independent on LeHPL.4. The role of TomloxC in biosynthesis of C5volatiles was also observed in tomato leaves by using stimulants such as mechanical wounding and pathogen infection. The levels of C5and C6volatiles in TomloxC-AS lines were significantly lower relative to wild-type controls. The emissions of C5and C6volatiles were significantly induced by mechanical wounding, but did not increase in response to infection with Xanthomonas campestris pv. vesicatoria (Xcv). The reductions of C5and C6volatiles in TomloxC-AS tomato leaves caused significant accumulation of linolenic acid, but did not alter the defense of tomato plant to Xcv.