Seedlings of tomato fruit ripening mutants were screened for their abi
lity to respond to ethylene. Ethylene induced the triple response in e
tiolated hypocotyls of all tomato ripening mutants tested except for o
ne, Never ripe (Nr). Our results indicated that the lack of ripening i
n this mutant is caused by ethylene insensitivity. Segregation analysi
s indicated that Nr-associated ethylene insensitivity is a single codo
minant trait and is pleiotropic, blocking senescence and abscission of
flowers and the epinastic response of petioles. In normal tomato flow
ers, petal abscission and senescence occur 4 to 5 days after the flowe
r opens and precede fruit expansion. If fertilization does not occur,
pedicel abscission occurs 5 to 8 days after petal senescence. If unfer
tilized, Nr flowers remained attached to the plant indefinitely, and p
etals remained viable and turgid more than four times longer than thei
r normal counterparts. Fruit development in Nr plants was not preceded
by petal senescence; petals and anthem remained attached until they w
ere physically displaced by the expanding ovary. Analysis of engineere
d 1-aminocyclopropane-1-carboxylate (ACC) synthase-overexpressing plan
ts indicated that they are phenotypic opposites of Nr plants. Constitu
tive expression of ACC synthase in tomato plants resulted in high rate
s of ethylene production by many tissues of the plant and induced peti
ole epinasty and premature senescence and abscission of flowers, usual
ly before anthesis. Them were no obvious effects on senescence in leav
es of ACC synthase overexpressers, suggesting that although ethylene m
ay be important, it is not sufficient to cause tomato leaf senescence;
other signals are clearly involved.