Effects of Indoleacetic Acid on the Quantity of Mitochondria, Microbodies, and Plastids in the Apical and Expanding Cells of Dark-grown Oat Coleoptiles

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Abstract
The number of mitochondria, microbodies and plastids in dark-grown oat (Avena sativa) coleoptiles was determined following incubation in IAA for a period of 60 min at 6 min intervals. In the apical outer epidermis of coleoptiles, the mitochondria increased from 31.4-35 per cell section with a 6 min incubation in IAA and this trend persisted over the 60 min incubation. Neither the microbodies, plastids, nor the dicytosomes responded to the hormone. The apical parenchyma showed no change in quanitty of any of the organelles including the dictyosomes during IAA incubation. The quick response of mitochondria in the coleoptile tip could be interpreted as an association of this organelle with hormone transport, growth, or perhaps with gravity perception. In the subapical expansion region, IAA caused significant reductions of mitochondria, microbodies and dictyosomes in the outer epidermis compared to the control, the timing of which preceded the IAA-induced elongation and geotropism. The fast response of organelles in the various cells is probably a change in organelle volume rather than number. That microbodies show a response to the plant hormone in the permanently achlorophyllous epidermis indicates that these organelles, in addition to their peroxisomal functions in green leaves, also may have a growth regulation function. IAA treatment was without effect on the quantity of the various types of plastids (including the amyloplasts) in the different oat coleoptile cells.