Expression of ACC oxidase genes differs among sex genotypes and sex phasesin cucumber

Ethylene has been implicated as a sex-determining hormone in cucumber: its exogenous application increases femaleness, and gynoecious genotypes were r eported to produce more ethylene. In this study, three full-length ACC oxid ase cDNAs were isolated from cucumber floral buds. RFLP analysis of a popul ation that segregates for the F (femaleness) locus indicated that CS-ACO2 i s linked to F at a distance of 8.7 cM. Expression of two of the genes, CS-A CO2 and CS-ACO3, was monitored in flowers, shoot tips and leaves of differe nt sex genotypes. In situ mRNA hybridization indicated different patterns o f tissue- and stage-specific expression of CS-ACO2 and CS-ACO3 in developin g flowers. CS-ACO3 expression in mid-stage female flowers was localized to the nectaries, pistil and in the arrested staminoids, whereas CS-ACO2 trans cript levels accumulated later and were found in placental tissue, ovary an d staminoids. In male flowers, petals and nectaries expressed both genes, w hereas ACO2 expression was strong in pollen of mature flowers. In young bud s, strong expression was observed along developing vascular bundles. Four s ex genotypes were compared for CS-ACO2 and CS-ACO3 expression in the shoot apex and young leaf. FF genotypes had higher transcript levels in leaves bu t lower levels in the shoot apex and in young buds, as compared to ff genot ypes; the shoot-tip pattern is, therefore, inversely correlated with female ness, and the possibility of a feedback inhibition mechanism underlying suc h correlation is discussed. The two CS-ACO genes studied displayed a differ ential response to ethrel treatment in different organs and sex genotypes, further demonstrating the complexity of the mechanisms controlling ethylene production during cucumber floral development.