Analysis of the ethylene response in the epinastic mutant of tomato

Ethylene can alter plant morphology due to its effect on cell expansion. Th e most widely documented example of ethylene-mediated cell expansion is pro motion of the "triple response" of seedlings grown in the dark in ethylene. Roots and hypocotyls become shorter and thickened compared with controls d ue to a reorientation of cell expansion, and curvature of the apical hook i s more pronounced. The epinastic (epi) mutant of tomato (Lycopersicon escul entum) has a dark-grown seedling phenotype similar to the triple response e ven in the absence of ethylene. In addition, in adult plants both the leave s and the petioles display epinastic curvature and there is constitutive ex pression of an ethylene-inducible chitinase gene. However, petal senescence and abscission and fruit ripening are all normal in epi. A double mutant ( epi/epi;Nr/Nr) homozygous for both the recessive epi and dominant ethylene- insensitive Never-ripe loci has the same dark-grown seedling and vegetative phenotypes as epi but possesses the senescence and ripening characteristic s of Never-ripe. These data suggest that a subset of ethylene responses con trolling vegetative growth and development may be constitutively activated in epi. In addition, the epi locus has been placed on the tomato RFLP map o n the long arm of chromosome 4 and does not demonstrate linkage to reported tomato CTR1 homologs.