EFFECTS OF GIBBERELLIN AND ABSCISIC-ACID ON THE CORTICAL MICROTUBULE ORIENTATION IN HYPOCOTYL CELLS OF LIGHT-GROWN CUCUMBER SEEDLINGS

Effects of gibberellin (GA4) and abscisic acid (ABA) on the orientation of cortical microtubules (MTs) in the epidermal and cortex cells of light-grown cucumber (Cucumis sativus L. cv Aonagajibai) hypocotyls were examined by immunofluorescence microscopy. MTs were observed in the 0-1 mm region (region A) and 3-4 mm region (region D) below the cotyledonary node. GA, promoted and ABA inhibited hypocotyl elongation. In the control, the predominant pattern of MT arrangement in the tangential surface of epidermal cells (Ep-t) was oblique in region A and oblique/longitudinal in region D, while in the radial surface of epidermal cells (Ep-r), the MT arrangement was mostly transverse in both regions. The MT arrangement in the radial surface of cortex cells (C-r) was all transverse in both regions. GA, increased transversely oriented MTs and decreased obliquely and longitudinally oriented MTs, whereas ABA increased obliquely and longitudinally oriented MTs and decreased transversely oriented MTs in Ep-t and Ep-r in both regions. ABA counteracted GA4-induced effects on MT orientation as well as elongation. The frequency of the occurrence of transverse MTs in Ep-t but not in Ep-r and C-r was found to be highly correlated to the growth rate of the hypocotyl, suggesting that the epidermis is the essential tissue that limits the growth of the hypocotyl.