Cell specificity in auxin- and ethylene-induced 'supergrowth' in Riella helicophylla

In gemmalings of Riella helicophylla, auxin and ethylene stimulate elongation growth, especially of pillar cells. When the two hormones are supplied simultaneously, the effects are additive, i.e. the result is 'supergrowth'. In the cells of the meristem, elongation is enhanced by auxin, but not by ethylene when given alone. However, these cells also respond with 'supergrowth' to a combined treatment with auxin and ethylene. The antiauxin p-chlorophenoxyisobutyric acid suppresses both the ethylene stimulation of cell growth and the additive 'supergrowth'. The results support the concept that auxin pre-conditions the cells to the ethylene-dependent growth event. We suggest that the response elicited by the specific cell types could be related to differences in their level of endogenous auxin.