GIBBERELLIN-ETHYLENE INTERACTION CONTROLS RADIAL EXPANSION IN CITRUS ROOTS

The involvement of gibberellins (GAs) and ethylene in the process of r oot radial expansion was studied in young seedlings of Carrizo citrang e [Citrus sinensis (L.) Osb. x Poncirus trifoliata (L.) Raf.]. The GA inhibitors cycocel, paclobutrazol, and tetcyclacis enhanced radial exp ansion of the root tip (up to 2.3-fold) as a result of increases in st ele diameter and inner cortex width. The GA deficiency increased cell number and width, and changed the polarity of growth, generating wider and shorter cortical cells in the elongation zone. In the presence or absence of GA inhibitors, GA(3) decreased root tip width and reduced all parameters related to radial expansion. The ethylene inhibitors (a minooxyacetic acid; cobalt ions, CoCl2; silver thiosulfate) suppressed swelling induced by GA deficiency, generating thinner cells just as G A(3) did. In contrast to GA(3), ethylene inhibitors produced longer ce lls strongly resembling those of the untreated seedlings. Ethylene rel eased by ethephon did not modify root tip width in control plants, whi le root diameter behind the root tip was increased. In the presence of low and ineffective concentrations of cycocel, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid increased radial expansion of r oot tips (1.3-fold) and changed the polarity of growth, producing wide r and shorter inner cortical cells as GA inhibitors did. These observa tions imply, first, that ethylene is the hormonal effector of the proc ess of root radial expansion and, second, that the endogenous GAs modu late the promotive response of ethylene.