Ripening-specific stigmasterol increase in tomato fruit is associated with increased sterol C-22 desaturase...

Phytosterol content and composition and sterol C-22 desaturase (LeSD1; CYP710A11) transcript levels in pericarp tissue of 'Rutgers' tomato fruit were compared in the wild-type (wt) and isogenic lines of the nonripening mutants nor and rin at four stages of ripening/aging. Wild-type fruit 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 fruits were harvested at comparable chronological ages (days after pollination). At the MG stage, wt and mutant fruits had closely similar sterol contents, compositions, and conjugations, with >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, rin, and nor, respectively, the increases in total sterols between MG and B + 6 were 2.1-, 1.9-, and 1.5-fold, and at B + 6 the percentages of total sterols in FS + SE were 42, 21, and 24. Among all sterol lipids (ASG, SG, FS, and SE), the ratio of stigmasterol (stigmasta-5,22-dien-3beta-ol) to beta-sitosterol (stigmast-5-en-3beta-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 fruits. 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 fruits 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 C-22 desaturation of the precursor sterol, beta-sitosterol.