Up- and down-regulation of Fragaria x ananassa O-methyltransferase: impacts on furanone and phenylpropanoid metabolism.

A complex mixture of hundreds of substances determines strawberry (Fragaria x ananassa) aroma, but only approximately 15 volatiles are considered as key flavour compounds. Of these, 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) is regarded as the most important, but it is methylated further by FaOMT (Fragaria x ananassa O-methyltransferase) to 2,5-dimethyl-4-methoxy-3(2H)-furanone (DMMF) during the ripening process. It is shown here that transformation of strawberry with the FaOMT sequence in sense and antisense orientation, under the control of the cauliflower mosaic virus 35S promoter, resulted in a near total loss of DMMF, whereas the levels of the other volatiles remained unchanged. FaOMT repression also affected the ratio of feruloyl 1-O-beta-D-glucose and caffeoyl 1-O-beta-D-glucose, indicating a dual function of the enzyme in planta. Thus, FaOMT is involved in at least two different biochemical pathways in ripe strawberry fruit.

Isolation, cloning and expression of a multifunctional O-methyltransferase capable of forming 2,5-dimethyl-4-methoxy-3(2H)-furanone, one of the key aroma compounds in strawberry fruits.

Strawberry fruits contain an uncommon group of key aroma compounds with a 2,5-dimethyl-3(2H)-furanone structure. Here, we report on the methylation of 2,5-dimethyl-4-hydroxy-3(2H)-furanone (DMHF) to 2,5-dimethyl-4-methoxy-3(2H)-furanone (DMMF) by a S-adenosyl-L-methionine dependent O-methyltransferase, the cloning of the corresponding cDNA and characterization of the encoded protein. Northern-hybridization indicated that the Strawberry-OMT specific transcripts accumulated during ripening in strawberry fruits and were absent in root, petiole, leaf and flower. The protein was functionally expressed in E. coli and exhibited a substrate specificity for catechol, caffeic acid, protocatechuic aldehyde, caffeoyl CoA and DMHF. A common structural feature of the accepted substrates was a o-diphenolic structure also present in DMHF in its dienolic tautomer. FaOMT is active as a homodimer and the native enzyme shows optimum activity at pH 8.5 and 37 degrees C. It does not require a cofactor for enzymatic activity. Due to the expression pattern of FaOMT and the enzymatic activity in the different stages of fruit ripening we suppose that FaOMT is involved in lignification of the achenes and the vascular bundles in the expanding fruit. In addition, it is concluded that the Strawberry-OMT plays an important role in the biosynthesis of strawberry volatiles such as vanillin and DMMF.