Saturable Uptake of Indol-3yl-Acetic Acid by Maize Roots

Abstract

The uptake of 5-[3Hlindol-3yl-acetic acid (IAA*) by segments of Zea mays L. roots was measured in the presence of nonradioactive indol-3yl-acetic acid (IAA°) at different concentrations. IAA uptake was found to have a nonsaturable component and a saturable part with (at pH 5.0) an apparent K, of 0.285 micromolar and apparent V,,,, 55.0 picomoles per gram fresh mass per minute. These results are consistent with those which might be expected for a saturable carrier capable of regulating IAA levels. High performance liquid chromatography analyses showed that very little metabolism of IAA* took place during 4 minute uptake experiments. Whereas nonsaturable uptake was similar for all 2 millimeter long segments prepared within the 2 to 10 millimeter region, saturable uptake was greatest for the 2 to 4 millimeter region. High levels of uptake by stelar (as compared with cortical) segments are partly attributable to the saturable carrier, and also to a high level of uptake by nonsaturable processes. The carrier may play an essential role in controlling IAA levels in maize roots, especially the accumulation of IAA in the apical region. The increase in saturable uptake toward the root tip may also contribute to the acropetal polarity of auxin transport.The natural occurrence of IAA in maize roots was unequivocally established by 5,26). There is evidence that IAA plays a part in the regulation of root growth and gravireaction, though it is not clear how this control is exercised. The growth rate of maize roots was found to be inversely correlated with the level of endogenous IAA in the elongating zone (24). Applied IAA has been shown to inhibit root growth (2) but stimulates the growth of auxin-depleted roots (23).When roots are treated with inhibitors of auxin transport, gravireaction is strongly retarded and growth is inhibited (6, 12). When triodobenzoic acid was applied only to the caps of pea roots (11) gravireaction was inhibited but growth appeared to be unaffected. The effect of auxin on growth and gravireaction may therefore depend on the level of IAA in the elongating zone or, more specifically, on the action of the carrier for IAA efflux.Transport between tissues and subcellular compartments is one of the factors influencing the levels of IAA in different tissues and in various parts of the cell. The transport of auxin in the root tip is acropetally polarized (20,21,30,31) and involves movement from cell to cell. The transfer of IAA across cell membranes comprises uptake and efflux, both of which may be partly by diffusion and partly via carriers.According to the chemiosmotic polar diffusion theory (25, 28) energy is expended creating a gradient of pH and of electrical potential across the plasmalemma and IAA, which is a weak acid (pK 4.7), accumulates as the anion inside the cytoplasm. Polarity of auxin transport is conferred by a carrier for the efflux of IAA across the plasmalemma, located preferentially at the apical end of the cell (basal end in the case of shoots). This carrier has been shown to be...

Auxin Transport in Suspension-Cultured Soybean Root Cells

“…The transition between the two phases is relatively rapid and occurs within the first 1 min of uptake. The nature of the curve is in marked contrast to the curve obtained by Martin and Pilet (14) with corn roots at a comparable IAA concentration (10-M) which appears to show an initial linear phase lasting at least 4 min. Rubery and Sheldrake (20) found a pattern more similar to that in Figure 3 with crown gall cells, although the earliest time point was 1 min.…”

Non-invasive quantification of endogenous root auxin transport using an integrated flux microsensor technique

“…Uptake of exogenously applied IAA across the root tissue surface has been attributed to a combination of diffusion and carrier mediated transport (Martin and Pilet, 1986; Bennett et al. , 1996).…”

“…However, a drawback of the induced IAA experimental approach is that application of 5–15 μ m IAA inhibits root growth (Pilet and Saugy, 1985; Martin and Pilet, 1986). To study the effect of exogenous IAA on root metabolism, oxygen influx was measured in 4‐day B73 and br2 roots during and after IAA addition using a fiber optic SR oxygen sensor (Chatni and Porterfield, 2009).…”

“…Induced IAA flux in Zea mays roots Uptake of exogenously applied IAA across the root tissue surface has been attributed to a combination of diffusion and carrier mediated transport (Martin and Pilet, 1986;Bennett et al, 1996). A previously reported study using noninvasive SR microsensors used application of external IAA to investigate saturation of transport (Mancuso et al, 2005).…”

Simultaneous Gas Chromatography-Mass Spectrometry Quantification of Endogenous [12C]- and Applied [13C]Indole-3yl-Acetic Acid Levels in Growing Maize Roots

“…Research has so far been oriented into three main directions: (a) IAA labeled with radioactive isotopes (3H, 14C) was applied to plant tissue and the pattern of IAA metabolism analyzed (11,17); (b) enzymes, which oxidize IAA (15), hydrolyze IAA conjugates (4,10) and bind IAA to conjugates (12) were also partially characterized; (c) the products of these in vitro enzymatic reactions and also some of the endogenous indolic compounds were isolated and identified (16,23,28). Moreover, the level of IAA in a particular time and part of the root reflects the steady-state balance (9), on the one hand, between the rates of anabolism (including hydrolyses of IAA conjugates) (6,24) and the rates of catabolism (oxidation and conjugation) (18,28) and, on the other hand, ofthe rates and directions oftransport fluxes (acropetal and basipetal transport) (13,19) of IAA.…”