Experimental Studies on Lateral Root Formation in Radish Seedling Roots: II. Analysis of the Dose-Response to Exogenous Auxin.

Application of indoleacetic acid (IAA) and other auxins causes cultured radish (Raphanus sativus L. ;Scarlet Globe') seedling root segments to produce an increased frequency (FR, no. cm(-1)) of lateral roots (LR); in the absence of auxin, segments spontaneously form about 6 LR cm(-1). A dose-response study has revealed that the increase in FR follows a biphasic Michaelis-Menten relationship with the medium concentration of the undissociated form of IAA ([IAAH](m)). The fitted curve for phase I has a maximum response level (R(max)) of 5.2 LR per centimeter above the spontaneous FR; the [IAAH](m) giving half-maximal response (C(1/2)) is 21 nanomolar. For phase II, the values for R(max) and C(1/2) are 56 LR per centimeter and 11 micromolar, respectively. The response is variable in the transition concentration region between the two phases; in that region (but not, or much less commonly, at higher or lower [IAAH](m)), LR initiation may resume or continue after the first day. At and above 100 micromolar [IAAH](m), the roots are hyperstimulated and generally fail to respond. The developmental stage of LR formed in medium with very low [IAAH](m) (10 nanomolar) is enhanced compared to LR formed in medium lacking auxin; the stage is diminished at higher auxin levels, in inverse correlation with FR. Trends in the responses to NAA and IBA were similar, but NAA required only 0.03 times the dose of IAA, while IBA required 6 times the dose of IAA. These findings may be of use in a search for possible auxin receptors involved with LR initiation.