RESUMO
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.
RESUMO
Segments (2.5 cm) cut from 3-day-old seedling roots of radish (Raphanus sativus L. ;Scarlet Globe') were cultured in medium with or without indoleacetic acid (IAA). Lateral root primordia frequency, determined for the central centimeter of segments, was dependent on IAA concentration and on conditions affecting IAA uptake. Dimethyl sulfoxide treatment, or a relatively low medium pH, greatly enhanced the response to exogenous IAA. It was concluded that a permeation barrier exists between the external medium and the hormone responsive sites within the radish seedling root.
RESUMO
Branch root formation required only the presence of minerals, sucrose as a carbon source, and an auxin. The number of primordia formed was a function of auxin concentration. With naphthaleneacetic acid at 0.1 mg/l, up to 60 or more branches were formed per centimeter of Haplopappus ravenii root segment. Under our conditions, pea root segments formed only five or six branches per centimeter, but tomato and radish, like H. ravenii, formed large numbers of branches. Cytokinin inhibited branch formation, while gibberellic acid was without effect. Vitamins were not required for branch formation, although they enhanced elongation. Up to 5 days were required for the maximum number of stable branch primordia to form under the influence of naphthaleneacetic acid. If naphthaleneacetic acid was withdrawn earlier, fewer branch primordia developed. The requirement for a lengthy exposure to naphthaleneacetic acid, the kinetics of the response, and the ease with which naphthaleneacetic acid could be rinsed out of the tissue with consequent cessation of branch root formation, were similar to other hormone-regulated developmental systems. Anatomical and cytological studies were made of segments exposed for various times to auxin. The segments were mostly diarch, and branches formed obliquely to protoxylem poles. While primarily only pericycle-endodermis cells divided, both these and cortex cells responded in the first 24 hours exposure to naphthaleneacetic acid with enlarged nuclei and nucleoli, and a few cortical cells divided. Maximum nucleus and nucleolus size was reached approximately 9 hours after exposure to naphthaleneacetic acid. Branches rarely elongated more than 5 cm before their meristems died. The H. ravenii culture is maintained only by the frequent formation of new naphthaleneacetic acid-induced branches.
