Change of cell structure during tropistic responses

1.4.1 Change of cell structure during tropistic responses

Circular arrays of microtubules and microfilaments, as well as the microfibril pattern at the subapical portion of protonemata, change during phototropism (Wada et al., 1990;Kadota and Wada, 1992a, 1992b). When polarotropism was induced by polarized red light vibrating 45. to thecell axis, the cortical array of microtubules became oblique within 30 minutes after irradiation to the direction of bending, but if the vibration plane was 70.,themicrotubule array disappeared. After 1 hour, the tropistic response could be observed using a microscope. By 2 hours after polarotropism induction, the microfibril rearrangement of the innermost layer of the cell wall became oblique to the former growing axis (Wada et al., 1990). During phototropism, reorganization of the microfilament structure precedes that of the microtubule structure (Kadota and Wada, 1992a), suggesting that the microtubule array is influenced by the microfilament array. Interestingly, this hypothesis was confirmed by experiments using cytoskeletal inhibitors (Kadota and Wada, 1992b). Colchicine and amiprophosmethyl disrupted the microtubule array but not the microfilament array. In contrast, cytochalasin B disrupted both arrays, indicating that the microtubule array depends on the array of microfibrils. Taken together, phototropism and polarotropism must occur through sequential changes: the microfilament array controls the direction of the microtubule array, which controls the direction of microfibril arrangement, and finally microfibrils restrict the cell diameter and the direction of cell growth.

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