Physiological evidence that the primary site of auxin action in maize coleoptiles is an intracellular site

To locate functionally the primary site of auxin action in growing cells, the pool of auxin relevant to induction of growth in maize (Zea mays L.) coleoptile sections was determined. A positive correlation was consistently noted between growth and intracellular levels of indole-3-acetic acid (IAA), i.e. growth appears to be relatively independent of the external level of IAA. N-1-Naphthylphthalamic acid (NPA), a potent inhibitor of auxin transport, was used to enhance accumulation of IAA in coleoptile cells. From the use of NPA, it is shown that: 1) increasing the accumulation of IAA in cells, while the external concentration is held constant, resulted in a concomitant increase in growth, and 2) blocking the exit of IAA from cells with NPA sustained an IAA-induced growth response in the absence of externally applied IAA. Furthermore, the absence of any alterations in auxin binding to microsomal fractions by NPA indicates that the action of NPA in causing enhancement of auxin-induced growth is based upon its inhibition of efflux of IAA from the cells.