Harry J. Klee

SUMMARY The unique flavors of different fruits depend upon complex blends of soluble sugars, organic acids, and volatile organic compounds. 2‐Phenylethanol and phenylacetaldehyde are major contributors to flavor in many foods, including tomato. In the tomato fruit, glucose, and fructose are the chemicals that most positively contribute to human flavor preferences. We identified a gene encoding a tomato aldo/keto reductase, Sl‐AKR9 , that is associated with phenylacetaldehyde and 2‐phenylethanol contents in fruits. Two distinct haplotypes were identified; one encodes a chloroplast‐targeted protein while the other encodes a transit peptide‐less protein that accumulates in the cytoplasm. Sl‐AKR9 effectively catalyzes reduction of phenylacetaldehyde to 2‐phenylethanol. The enzyme can also metabolize sugar‐derived reactive carbonyls, including glyceraldehyde and methylglyoxal. CRISPR‐Cas9‐induced loss‐of‐function mutations in Sl‐AKR9 significantly increased phenylacetaldehyde and lowered 2‐phenylethanol content in ripe fruit. Reduced fruit weight and increased soluble solids, glucose, and fructose contents were observed in the loss‐of‐function fruits. These results reveal a previously unidentified mechanism affecting two flavor‐associated phenylalanine‐derived volatile organic compounds, sugar content, and fruit weight. Modern varieties of tomato almost universally contain the haplotype associated with larger fruit, lower sugar content, and lower phenylacetaldehyde and 2‐phenylethanol, likely leading to flavor deterioration in modern varieties.

Significance Aroma is an important factor in consumer perception and acceptance of fresh tomatoes and involves a cocktail of several dozen compounds. Tomato fruits produce uncommon nitrogen-containing volatiles derived mainly from the amino acids leucine and phenylalanine. These volatiles have strong positive correlations with consumer liking. We show that an enzyme active in ripening tomatoes is responsible for the production of all nitrogenous volatiles in tomato fruit, at the expense of substrates derived from cysteine and volatile aldehydes. This discovery defines a cysteine-dependent route to nitrogenous volatiles in plants, prompting a reconsideration of the impact of sulfur metabolism on tomato flavor and quality.