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Effects of Indoleacetic Acid on the Quantity of Mitochondria, Microbodies, and Plastids in the Apical and Expanding Cells of Dark-grown Oat Coleoptiles ¹

^a Division of Biological and Medical Research, Argonne National Laboratory, Argonne, Illinois 60439

We determined the number of mitochondria, microbodies, and plastids in dark-grown oat (Avena sativa) coleoptiles following incubation in indoleacetic acid (IAA) for a period of 60 minutes at 6-minute intervals. In the apical outer epidermis of coleoptiles, the mitochondria increased from 31.4 to 35 per cell section with a 6-minute incubation in IAA, and this trend persisted over the 60-minute incubation. Neither the microbodies, plastids, nor the dicytosomes (Gawlik and Miller 1974 Plant Physiol 54:217-221) responded to the hormone. The apical parenchyma showed no change in quantity 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 of 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.