Effects of Dimethyl Sulfoxide and pH on Indoleacetic Acid-Induced Lateral Root Formation in the Radish Seedling Root

Abstract

ABSTRACI Segments (2.5 cm) cut from 3day-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.A characteristic action of IAA is to stimulate the formation of LR.3 This IAA effect has been studied using simple experimental systems employing excised or intact seedling roots (1, 10-1 3, and references cited therein). Using our system (1) based on the radish (Raphanus sativus L. 'Scarlet Globe') seedling root, we find that the response to exogenous auxin can be substantially modulated by chemical or physical factors known or presumed to influence diffusional IAA uptake by plant cells. The observations reported here indicate that there is an IAA permeation barrier between the external medium and the auxin-responsive sites involved with LR formation within the radish seedling root. MATERIALS AND METHODSThese studies were performed with roots of seedlings germinated and grown in the dark at 24C for 3 d, during which time the roots grew to an average length of 6 cm. Excised 2.5 cm segments, cut starting 0.5 to 1.5 cm behind the seedling root tip, were cultured 4 d in medium containing Murashige-Skoog salts (7), 88 mol m-3 sucrose, and 10 mol m-3 each of the buffers Mes and succinic acid. IAA RESULTS AND DISCUSSION Exogenous IAA greatly increased the frequency (i.e. the number/cm) of LRP formation, which varied from 5 cm-' in the absence of IAA to about 60 cm-' under optimal conditions. Excess IAA inhibited LRP formation. Figure 1 illustrates the effect of 30 mmol m-3 IAA, and the modulation ofthe response by pH and DMSO. Typical treatment mean frequencies of LRP formation in medium containing IAA were: 49.7 ± 3.6 SE LRP cmn' at pH 4.5 and 19.0 ± 1.9 SE LRP cm ' at pH 6. Treatment with DMSO increased the response to IAA at pH 6; a typical treatment mean frequency was 48.1 ± 3.8 SE LRP cm-'.DMSO is known to increase plasmalemma permeability to small molecules (3). In our experience, it was found most effective when given as a pulse treatment at a medium concentration of 5% (v/v) during the first 1 to 4 h of culture. Longer exposure to DMSO, or higher concentrations of DMSO, were inhibitory. DMSO by itself (i.e. with no exogenous auxin) had no promotive effects on the frequency of LRP formation. Administration of a 5% DMSO 4 h pulse treatment when medium containing 30 mmol m-3 IAA was buffered at pH 4.5, rather than 6, inhibited LRP formation.The effect of pH on IAA uptake by plant cells has been well characterized (4, 9). Since the membrane permeability coefficient to the lipophilic IAAH is on the order of 103 times that of IAA-(9), lowe...

A Two-Dimensional Vibrating Probe Study of Currents around Lateral Roots of Raphanus sativus Developing in Culture

“…A representative root segment cultured in IAA-supplemented medium for 5 d is shown in the inset in Figure 1A. This effect of IAA on root initiation and growth is well known (4). On the other hand, the root segments cultured in IAA-free medium produced a smaller number of lateral roots (3.7 lateral roots/cm of primary root) that elongated faster at an average rate of 1250 Mm.d-'.…”

“…Lateral roots can also be induced on root segments growing in culture (4 The use ofa circularly oscillating probe, based on the design of Nuccitelli (14), permitted us to obtain a two-dimensional picture of the current pattern in the horizontal plane. With a conventional one-dimensional probe, two measurements at each point are required to detect the two orthogonal components of the current vector.…”

Pseudomonas fluorescens and Pseudomonas putida for Promoting Growth of Jatropha curcas Seedling Root

“…The absorption response of exogenous IAA by sprout seed will increase followed by the decrease of pH auxine solution, and optimum on pH 5.5-6.0 [14]. IAA exogen can increase quantity of lateral root, with optimum concentration 11 μM [17].…”

Effect of naphthaleneacetic acid on the formation of lateral roots in the adventitious root of Allium cepa: number and arrangement of laterals along the parent root

“…Auxin-treated roots of the monocot Zea mays produce a considerable number of LRs in response to treatment (7). Nevertheless, auxin is ineffective in promoting LRs in other monocots (2,5).…”

“…According to Blakely et al (2) there is an indoleacetic acid permeability barrier between the external medium and the auxin-responsive sites involved with LR formation within the radish seedling root. According to Blakely et al (2) there is an indoleacetic acid permeability barrier between the external medium and the auxin-responsive sites involved with LR formation within the radish seedling root.…”