Soluble Auxin-Binding: Is there a Correlation between Growth-Stage Dependent High-Affinity Auxin-Binding and Auxin Competence?

Abstract

It is a general observation in plant tissue culture experiments that explants, depending on type, age, physiological state or genotype show specific and different reactions to the application of phytohormones in the medium: Tissues seem to possess different competence to react to the phytohormones applied, in most cases auxins and/or cytokinins. In large seeded legumes, for instance, high concentrations of NAA, 2,4-D or Picloram induce the formation of somatic embryos from immature zygotic embryos (Lazzari et al., 1985, Barwale et al. 1985, Ranch et al. 1986, Kysely et al. 1986), while the same concentrations applied to leaf - or epicotyl explants will lead to poor or nearly no callus formation. From numerous physiological experiments on the action of phytohormones the concept has been evolved that there might be changes in the “growth substance sensitivity” (see Firn 1986 and the literature cited in this paper). It was argued by Trewavas (1981 a,b, 1982, 1983) that the number of receptors may be altered thus causing changes in the sensitivity. Basing the discussion on our own data on soluble auxin binding proteins in pea and soybean cell suspensions as well as on the observations mentioned above, we will discuss the possibility that, besides changes in the number of receptors or modifications of the affinity of the receptor, also differences in the expression of different receptor sites may contribute to the observed phenomena of differential phytohormone sensitivity.