The role of hormones and gradients in the initiation of cortex proliferation and nodule formation in Pisum sativum L.

The effect of exogenous phytohormones on proliferation of the root cortex, and their relation to the division factors from Rhizobium which participate in the initiation of root nodules, were studied using explants of root-cortex tissue from 7-day-old, sterile pea plants. The explants were cultured for 7 days on a synthetic nutrient medium supplemented with auxin, or auxin and cytokinin. With only auxin present in the medium, ca. 10% of the explants showed cell proliferation. With both auxin and cytokinin this percentage was much higher (ca. 80%). The active explants showed proliferation patterns which were similar to or could be derived from a pattern with three predominant meristematic areas in the inner cortex opposite the three xylem radii of the excised central cylinder. These proliferation patterns were similar to the initial proliferative stages in root-nodule formation in seedling intact roots. From this restricted division response of the explants to the hormones, a hypothesis of endogenous division factors is proposed. To test this hypothesis, extractions of root tissue were performed. The addition of a crude alcoholic extract from the central cylinder or the cortex to the medium resulted in cell divisions throughout the cortex. The results are interpreted as evidence for the presence of a transverse gradient system of (an) unknown cell-division factor(s) in the root cortex which may control the induction of cell divisions in nodule initiation brought about by the release of auxin and cytokinin from Rhizobium.

The complex kinetics of auxin-binding to a particulate fraction from tobacco-pith callus.

The kinetics of binding of 1-naphthylacetic acid to particulate fractions from tobacco-pith callus were studied. This binding site does not bind auxin at 0° C. Binding experiments performed at 25° C demonstrated an apparent K a of approx. 6.5·10(6) M(-1). A filtration method was developed in order to study non-equilibrium kinetics of this binding. Dissociation of the complex of auxin and binding site indicates the presence of at least two binding components with dissociation rate constants (k off) of 6.1·10(-3) min(-1) and 6.0·10(-2) min(-1). This binding behaviour was not independent, indicating that the binding of auxin to the particulate fractions was more complex than binding of one hormone molecule to one binding site. This complexity was further confirmed by experiments in which the initial velocity of complex formation was measured. A model was worked out into which our data fit without contradictions. It involves the binding of four hormone molecules to one receptor molecule.