The Role of Gibberellin in the Formation of Onion Bulbs
Onion plants form bulbs in response to the stimulus of long-day conditions. More than forty years ago, Heath1 reported that the bulb is formed by the swelling of leaf-sheath cells and that cell division is not involved in this phenomenon. Under short-day conditions, leaf-sheath cells elongate longitudinally, whereas under long-day conditions they expand laterally. This means that leaf-sheath cells respond to a long-day stimulus by a change in direction of cell expansion. The direction of cell expansion depends largely on the orientation of the cellulose microfibrils in the cell wall.2 The orientation of cellulose microfibrils, in turn, is considered to be controlled by cortical microtubules.3 Since changes in the direction of cell expansion are part of bulb development, the cortical microtubules may be involved in the regulation of this development. Therefore, we examined the behavior of cortical microtubules in the leaf-sheath cells of onion plants during bulb development. We have reported previously that the suppression by gibberellin A3 (GA3) of the lateral expansion of cells in azuki bean epicotyls is correlated with an increased number of transversely oriented cortical microtubules.4 This is followed by the accumulation of transversely oriented cellulose microfibrils in the cell wa11.5 Thus, in the work reported here, we examined the effects of GA3 on the arrangement of microtubules during the development of onion bulbs.
KeywordsLeaf Sheath Lateral Expansion Cellulose Microfibril Cortical Microtubule Cell Axis
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