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Title: Ripening-specific stigmasterol increase in tomato fruit is associated with increased sterol 22-desaturase (CYP710A7) gene expression

item Whitaker, Bruce

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2008
Publication Date: 5/28/2008
Citation: Whitaker, B.D., Gapper, N.E. 2008. Ripening-specific stigmasterol increase in tomato fruit is associated with increased sterol 22-desaturase (CYP710A7) gene expression. Journal of Agricultural and Food Chemistry. 56(10):3828-3835.

Interpretive Summary: The last phase of ripening, called senescence, involves changes in fruit metabolism that result in rapid deterioration of quality. Processes that promote water loss are critical, causing shriveling and undesirable changes in texture. In senescing tomato fruit, modification of a major membrane component similar to cholesterol by a single enzyme is thought to increase membrane leakiness and thereby enhance water loss. This study showed that the gene encoding the key enzyme (sterol 22-desaturase) is markedly up-regulated late in ripening. Hence, the resulting increase in sterol 22-desaturase activity accounts for modification of the critical membrane component. This knowledge can be applied by plant biologists in efforts to develop new tomato lines bearing fruit with longer shelf life and improved quality.

Technical Abstract: Phytosterol content and composition and sterol 22-desaturase (LeSD1; CYP710A7) transcript levels in pericarp tissue of 'Rutgers' tomato fruit were compared in the wild-type (wt) and isogenic lines of the non-ripening mutants nor and rin at four stages of ripening/aging. Fruit of wt were harvested at the mature-green (MG), breaker (BK), breaker plus 3 days (B+3) and breaker plus 6 days (B+6) stages, whereas nor and rin fruit were harvested at comparable chronological ages (days after pollination). At the MG stage, wt and mutant fruit had closely similar sterol content, composition and distribution, with over 91% of the total sterols in the acylated steryl glycoside plus steryl glycoside (ASG+SG) fraction. During ripening/aging, there were substantial increases in total sterols and the percentage of sterols in the free plus esterified (FS+SE) fraction. Both changes were greater in wt than in nor or rin. In fruit of wt, nor and rin, respectively, the increase in total sterols between MG and B+6 was 2.1-, 1.9- and 1.5-fold, and at B+6 the percentage of sterols in FS+SE was 42, 21 and 24. Among all sterol lipids (ASG, SG, FS and SE), the ratio of stigmasterol (stigmasta-5,22-dien-3ß-ol) to sitosterol (stigmast-5-en-3ß-ol), the two major sterols in tomato, increased 2.3-fold during ripening of wt fruit but declined slightly during comparable aging of nor and rin fruit. In accord with these changes, the abundance of LeSD1 transcript increased 4-fold in pericarp of ripening wt fruit, peaking at B+3, whereas transcript levels in nor and rin fruit fluctuated but never exceeded the abundance in wt fruit at the MG stage. These findings indicate that the ripening-specific increase in stigmasterol in wt fruit results from a marked increase in LeSD1 transcription and translation, which accelerates 22-desaturation of the precursor sterol, sitosterol.