Elongation Of Coleoptile Segments Research Articles

Interactive effects of temperature and heavy metals (Cd, Pb) on the elongation growth in maize coleoptiles

The effect of Cd and Pb on endogenous and IAA-induced elongation growth and medium pH of maize coleoptile segments incubated at 20, 25 and 30°C was studied. It was found that the elongation of coleoptile segments and proton extrusion increased with the temperature and reached its maximum at 30°C. For Cd, the maximal inhibition of endogenous and IAA-induced growth as well as medium acidification of coleoptile segments was observed at 25°C. Meanwhile, Pb, irrespective of the temperature, diminished the growth of the segments by ca. 20%, increasing the acidification of the incubation medium. It was also found that in contrast to Cd, Pb accumulation in maize coleoptile segments did not correlate with temperature. The results suggest that the toxic effect of Cd on elongation growth of coleoptile segments is connected with the decrease of the PM H+-ATPase activity and probably with Cd-induced high acivity of IAA oxidase, whereas the effect of Pb did not depend on activity of any of the enzymes.

Stimulatory effects of potassium iodide on auxin action in the coleoptile segments of maize (Zea mays)

Potassium iodide (KI) was found to stimulate IAA-induced elongation of coleoptile segments in maize (Zea mays L.). The promoting effects of KI on coleoptile elongation, which were optimal at 1 mM in the presence of IAA, did not occur as a result of better conservation of IAA in the incubation medium. In addition, KI did not affect fusicoccin- or epibrassinolide-induced elongation. Additionally, sodium iodide (NaI) induced similar stimulatory effects on IAA-induced elongation, however, potassium chloride (KCl) showed no effect, suggesting that iodide is the active component. KI also enhanced IAA-induced ethylene biosynthesis in maize coleoptile segments. Taken together, these results suggest the involvement of KI-sensitive step(s) in auxin action before effectors of the signal transduction pathway split to elongation growth and ethylene biosynthesis.

Specificity patterns indicate that auxin exporters and receptors are the same proteins.

A study of transport and action of synthetic auxin analogues can help to identify transporters and receptors of this plant hormone. Both aspects--transportability and action on growth--were tested with 2-naphthoxyacetic acid (2-NOA) and compared across several plant species. 2-NOA stimulates elongation effectively at low concentrations in petioles of the gymnosperm Ginkgo biloba L., in hypocotyls or internodes of the dicot legumes, mung bean (Vigna mungo L.) and pea (Pisum sativum L.), in cotyledons of onion (Allium cepa L.) and in leaf bases of chive (Allium schoenoprasum L.), the latter two of the monocot order Asparagales. In contrast, elongation of coleoptile segments of maize (Zea mays L.) is poorly responsive to 2-NOA. Significant auxin-like transport of 2-NOA was observed in segments of mung bean hypocotyls, pea internodes, and chive leaf bases, but not in segments of the grass coleoptiles. Thus, for the two assays, elongation and polar transportability, the same difference in ligand specificity was observed between the grass and all other species assayed. This finding supports the hypothesis that a common protein mediates auxin efflux as well as auxin action on elongation.

The Auxin-induced Maize Gene ZmSAUR2 Encodes a Short-lived Nuclear Protein Expressed in Elongating Tissues

By differential screening of a cDNA library from auxin-induced maize coleoptiles we have isolated and characterized a SAUR gene, designated ZmSAUR2, belonging to a not yet characterized subtype of the SAUR family. ZmSAUR2 encodes a 15.3-kDa protein and is specifically induced by auxin in elongating coleoptile tissue but not in primary leaves or in roots. The transcript level rapidly increased within minutes and preceded auxin-stimulated elongation of coleoptile segments. Cycloheximide also induced ZmSAUR2 transcription, as has been shown for other early auxin-induced genes, whereas abscisic acid, brassinolide, ethylene, gibberellic acid, kinetin, and methyl jasmonate did not provoke an increase in ZmSAUR2 mRNA abundance. In pulse-chase experiments using auxin-induced coleoptiles and an anti-ZmSAUR2 antibody we were able to precipitate a protein of the expected molecular mass and to determine a half-life of about 7 min, which is among the shortest known in eukaryotes. In gel shift assays binding of calmodulin to ZmSAUR2 was demonstrated, suggesting the possibility of post-transcriptional regulation. Upon transformation of onion epidermal cells with a ZmSAUR2::GUS construct the corresponding chimeric protein was detected in the nucleus. The results suggest that ZmSAUR2 encodes a short-lived nuclear protein that might be involved in auxin-mediated cell elongation.

Changes in the Activities and Polypeptide Levels of Exo- and Endoglucanases in Cell Walls during Developmental Growth of Zea mays Coleoptiles

Exo- and endoglucanases present in cereal coleoptile cell walls are capable of mediating hydrolysis of non-cellulosic /7-(l,3)(l,4)-glucan in situ. To assess the relationship with cell elongation, glucanase activities and the respective polypeptide abundance were determined as a function of Zea mays coleoptile development. Both exo- and endoglucanase activities were quite low initially, but increased to achieve maximum levels by days 5 or 6. Western blots revealed that the density of the protein bands increased with coleoptile development generally in correspondence to activity levels. However, in bioassays with 3 d old coleoptile segments we found that auxin stimulation of glucanase activities did not result from increased glucanase polypeptide levels. Hence, there was no evidence for de novo protein synthesis in excised coleoptiles in response to added auxin. While glucanase antibodies strongly inhibited IAAinduced elongation of coleoptile segments on days 2-4, these same antibodies had little effect on day 1. We conclude that glucanases contribute to auxin mediated coleoptile growth only during a limited developmental interval. We propose that when elongation is dominate, the physical properties of the cell wall adjust in response to metabolism of cell wall /?-(l,3)(l,4)-glucans but the enhancement of such activity is governed by factors other than glucanase protein levels.

Conversion of D‐tryptophan to indole‐3‐acetic acid in coleoptiles of a normal and a semi‐dwarf barley (Hordeum vulgare) strain

Exogenously applied D‐tryptophan (D‐Trp) was more effective than L‐Trp in inducing elongation of coleoptile segments of a normal barley (Hordeum vulgare L. cv. Akashinriki) strain and a semi‐dwarf strain with lower endogenous indole‐3‐acetic acid (IAA) level. D‐cycloserine, an inhibitor of D‐aminotransferase, completely inhibited both the D‐ and L‐Trp‐induced elongation of both strains. Addition of D‐Trp increased IAA levels in both strains 4‐fold over endogenous levels. The increase in IAA level was completely inhibited by D‐cycloserine. The endogenous L‐Trp level of semi‐dwarf coleoptiles was similar to that of normal ones. These results suggested that IAA is synthesized by the conversion of L‐Trp to indole‐3‐pyruvic acid via D‐Trp in both strains, and that the lower IAA level of the semi‐dwarf strain probably is a result of the impeded IAA biosynthesis involved in D‐Trp.

Role of cell-wall biogenesis in the initiation of auxin-mediated growth in coleoptiles of Zea mays L.

The involvement of cell-wall polymer synthesis in auxin-mediated elongation of coleoptile segments from Zea mays L. was investigated with particular regard to the growth-limiting outer epidermis. There was no effect of indole acetic acid (IAA) on the incorporation of labeled glucose into the major polysaccharide wall fractions (cellulose, hemicellulose) within the first 2 h of IAA-induced growth. 2,6-Dichlorobenzonitrile inhibited cellulose synthesis strongly but had no effect on IAA-induced segment elongation even after a pretreatment period of 24 h, indicating that the growth response is independent of the apposition of new cellulose microfibrils at the epidermal cell wall. The incorporation of labeled leucine into total and cell-wall protein of the epidermis was promoted by IAA during the first 30 min of IAA-induced growth. Inhibition of IAA-induced growth by protein and RNA-synthesis inhibitors (cycloheximide, cordycepin) was accompanied by an inhibition of leucine incorporation into the epidermal cell wall during the first 30 min of induced growth but had no effect on the concomitant incorporation of monosaccharide precursors into the cellulose or hemicellulose fractions of this wall. It is concluded that at least one of the epidermal cell-wall proteins fulfills the criteria for a 'growth-limiting protein' induced by IAA at the onset of the growth response. In contrast, the synthesis of the polysaccharide wall fractions cellulose and hemicellulose, as well as their transport and integration into the growing epidermal wall, appears to be independent of growth-limiting protein and these processes are therefore no part of the mechanism of growth control by IAA.

Effect of lectins on auxin‐induced elongation and wall loosening in oat coleoptile and azuki bean epicotyl segments

Auxin‐induced elongation of coleoptile segments of oat (Avena sativa L. cv. Victory) was suppressed by agglutinin from Canavalia ensiformis DC. (concanavalin A), Triticum vulgaris Vill. germ, Dolichos biflorus L., Glycine max Merr. and Limulus polyphemus L. at 0.5 mg ml−1, while that of epicotyl segments of azuki bean (Vigna angularis Ohwi and Ohashi cv. Takara) was suppressed by agglutinin from Canavalia ensiformis DC., Tetragonolobus purpureus Moench and Ulex europaeus L. I when segments were incubated at pH 6.0. These lectins suppressed elongation without any prominent lag period. This suppression of elongation by the lectins was not apparent at pH 4.0. When a ligand for lectins, for instance methyl‐α‐D‐mannopyranoside, was added simultaneously with the lectin, elongation of segments was completely or partly restored. The lectins that suppressed elongation were bound to the cell walls of the segments and inhibited the auxin‐induced cell wall loosening (decrease in the minimum stress‐relaxation time of the cell wall). Thus, the lectins may, at least partly, suppress elongation by inhibiting cell wall loosening. Our results demonstrate that the wall components containing α‐D‐mannose (α‐D‐glucose), N‐acetyl‐D‐glucosamine, N‐acetyl‐D‐galactosamine or D‐glucuronic acid play an important role in wall loosening of oat coleoptiles, while the wall components containing α‐D‐mannose (α‐D‐glucose) or L‐fucose are important for that of azuki epicotyls.

Inhibition of auxin‐induced cell elongation by galactose

Galactose at concentrations higher than 3 mM inhibited specifically auxin‐induced elongation of oat, wheat and rice coleoptile segments but not of pea and mung bean stem segments. Glucose, arabinose, rhamnose, xylose, mannose and glucosamine did not inhibit auxin‐induced elongation of coleoptile segments. Galactose inhibited auxin‐induced but not hydrogen ion‐induced growth.

Mode of dinitroaniline herbicide action I. Analysis of the colchicine-like effects of dinitroaniline herbicides

Colchicine and a variety of dinitroaniline herbicides (DNHs) produce a similar pattern of inhibition of elongation, induction of swelling in the elongation zone, depolarization of cell enlargement, and induction of multiple nuclei in corn seedling roots. However, a 1000-fold higher concentration of colchicine is needed to produce effects quantitatively similar to those of oryzalin. Both colchicine- and DNH-inhibition of elongation start at about 3 hr. Since these compounds cause swelling and inhibition of elongation in γ-seedling roots, segments from the root elongation zone and intact roots in the presence of cell division inhibitors (all growing without cell division), it appears that the inhibition of root elongation is caused in part by their effect on cell elongation independent of their effect on cell division. Since the growth (increase in fresh weight) of γ-seedling roots and excised root segments is not inhibited by these compounds, their effect on the polarity of cell enlargement must be fairly specific. Unlike colchicine, oryzalin applied to the roots did not cause any significant, visible effect on shoot (mesocotyl and coleoptile) growth. These organs are not resistant to oryzalin, for the IAA-induced elongation of coleoptile segments is inhibited when they are floated in oryzalin solutions. As expected, when coleoptile segments are incubated in 14C-oryzalin, it is taken up rapidly and not degraded. The failure of root-applied oryzalin to affect seedling shoot growth is due to lack of transport to the shoots.

Some Growth Promotive Effects of Abscisic Acid

Abscisic acid promoted increase in frond number and fresh weight in Lernna polyrhiza when applied at low concentrations although higher concentrations inhibited both parameters. Elongation of coleoptile segments of oats and wheat was also promoted by low ABA concen trations when the coleoptile tip was not excised, although when the tip was removed no signifi cant promotion was detected. Elongation of mesocotyl segments of oats and maize was pro moted by ABA and in these cases the maximum promotion occurred at higher concentrations with maximum elongation of maize mesocotyl segments occurring at the highest concentration tested.

Structure-activity relationship in the auxin activity of mono-substituted phenylacetic acids.

The analysis of substituent constants for the lipophilic and electronic factors in the auxin activity of substituted phenylacetic acids in elongation of coleoptile segments shows that these factors parallel those for the phenoxyacetic acids but assign reactivity in growth promotion to the meta position of phenylacetic acid. The inhibitory effects with supra-optimal concentrations are highly dependent on the lipophilic character of the molecules.

Studies on Growth Regulators. I. Improved Avena Coleoptile Elongation Test for Auxin

A modified procedure is presented for the bioassay of auxin using Avena coleoptile segments. The modifications introduced result in a substantial improvement of the commonly used coleoptile elongation tests.The proposed test retains the simplicity of physical requirements and of manipulation characteristic of all elongation tests. The main feature of the proposed test is that it permits the measurement of the elongation of coleoptile segments as being directly proportional to the concentration rather than to the logarithm of the concentration of indoleacetic acid. With an accuracy and a range comparable to those of the standard Avena curvature test, the proposed bioassay overcomes the limitations for the quantitative assessment of auxin inherent in other coleoptile elongation tests. An additional advantage is provided by the present procedure: two 5 mm segments from each coleoptile can be utilized thus doubling the number of assays hitherto possible with a given number of coleoptiles.