Wall fibrils and microtubules in normal and gibberellic-acid-induced growth of lettuce hypocotyl cells

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Abstract
The orientations of microtubules and wall fibrils in primary walls of epidermal and cortical cells of gibberellic-acid(GA3)-treated and untreated lettuce hypocotyls were studied during a 72-h growth period. In both treatments, the epidermal and some cortical cells have thick polylamellate walls in which lamellae with longitudinal orientation of fibrils alternate with those having transverse orientation of fibrils. The microtubules generally parallel the innermost wall fibrils, but the orientations of both these structures change during the 72-h period presumably in association with deposition of new wall material. In normal growth, there is considerable variability among individual cells with respect to orientation of these structures at any one time. When the cells are boosted to elongate several-fold by GA3, the variability is minimal and the cortical cells show preeminently a horizontal arrangement and the epidermal cells show a longitudinal arrangement of microtubules and innermost wall fibrils. These results suggest that cell elongation and wall deposition are separate processes but may occur simultaneously, and implicate cortical cells in control of directionality of growth of multicellular organs. They also necessitate a modification in the multinet hypothesis of cell growth for cells with polylamellate walls.

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