Auxin-induced Growth Research Articles

Auxin Uptake and the Rapid Auxin-Induced Growth in Isolated Sections of Helianthus annuus

When a sunflower (Helianthus annuus L.) hypocotyl section is placed in a solution of 10 micromolar 1-naphthylacetic acid, the majority of the auxin enters via the cut surfaces. However, it is the minority entering radially which causes the typical growth response. Auxin supplied only to the apical cut surface gives a weak, slower response.

Osmoregulation in the Avena Coleoptile

Peeled Avena sativa coleoptile sections (i.e. sections from which the epidermis has been removed) have been used to study the control of solute uptake under conditions where the uptake is not limited by the cuticular barrier. In the presence of 2% sucrose, auxin enhances the rate at which the total osmotic solutes increase, but this appears to be a response to the increased growth rate, inasmuch as the auxin effect is eliminated when growth is inhibited osmotically. When sections are incubated in sucrose or in 20 millimolar NaCl, the osmotic concentration increases until a plateau is reached after 8 to 24 hours. Auxin has no effect on the initial rate of increase in osmotic concentration but causes the osmotic concentration to reach a plateau earlier and at a lower osmotic conentration value. This difference in steady-state osmotic concentration is, in part, a response to auxin itself, as it persists when auxin-induced growth is inhibited osmotically. The upper limit for osmotic concentration does not appear to be determined by the turgor pressure, inasmuch as a combination of sucrose and NaCl gave a higher plateau osmotic concentration than did either solute alone. We suggest that the rate of solute uptake is determined by the availability of absorbable solutes and by the surface area exposed to the solutes. Each absorbable solute reaches a maximum internal concentration independent of other absorbable solutes; the steady-state osmotic concentration is simply the sum of these individual internal concentrations.

Synthesis and biological activity of brassinolide and its 22 beta, 23 beta-isomer: novel plant growth-promoting steroids.

Brassinolide (2 alpha, 3 alpha, 22 alpha, 23 alpha-tetrahydroxy-24 alpha-methyl -B-homo-7-oxa-5 alpha-cholestan-6-one), a novel plant growth-promoting steroid isolated from rape pollen, and its hitherto unknown 22 beta, 23 beta-isomer were synthesized from a C-24 epimeric 60:40 mixture of 22-dehydroxampesterol (24 alpha-methyl) and brassicasterol (24 beta-methyl) from oysters. The method of synthesis favored the formation of the 22 beta, 23 beta-isomer by better than 4:1. Comparative plant growth-promoting capabilities of brassinolide, both natural and synthetic, and its three side chain cis-glycolic isomers in the bean second internode bioassay showed that the natural and synthetic brassinolides were equally active and caused splitting of the internode at the 0.1 microgram level. The least active was the 22 beta, 23 beta-isomer of brassinolide. The isomers with the 22 alpha, 23 alpha and 24 alpha, and the 22 beta, 23 beta and 24 beta configurations were highly active and were required at about 10 times the concentration of brassinolide to cause the same physiological response. In the bean first internode bioassay, an auxin-induced growth test system which employs isolated bean plant segments, the isomer with 22 beta, 23 beta and 24 beta configuration caused a greater response than brassinolide. Two of the four tetrahydroxy ketones obtained in the synthesis of the isomers were also active in both assays.

Osmoregulation in the Avena Coleoptile in Relation to Auxin and Growth

A study has been made of the effects of auxin and growth on the ability of Avena coleoptile sections to osmoregulate, i.e. to take up solutes so as to maintain their osmotic concentration, turgor pressure, and growth rate. The high auxin-induced growth rate of Avena coleoptiles is maintained when cells are provided sucrose, glucose, NaCl, or KCl as a source of absorbable solutes, but not when 2-deoxy-d-glucose or 3-O-methyl-d-glucose is used. In the absence of auxin, cells take up solutes from a 2% sucrose solution and the osmotic concentration increases. The rate of solute uptake is even greater in the presence of auxin or fusicoccin, but the osmotic concentration rises only slightly because of the water taken up during growth. Solute uptake is not stimulated by auxin when growth is inhibited osmotically or by calcium ions. Solute uptake appears to have two components: a basal rate, independent of auxin or growth, and an additional uptake which is proportional to growth. Osmoregulation of sections may be limited by the rate of entry of solutes into the tissue rather than by their rate of uptake into the cells.

Auxin-regulated Wall Loosening and Sustained Growth in Elongation

It is proposed that auxin regulates and coordinates both wall loosening and the supply of wall materials in elongation. The tenets of the proposal allowed testable predictions. It was determined that, if the cell walls of Glycine max L. var. Wayne hypocotyl segments are maintained in a loosened state (by excising the segments directly into pH 4 medium), exogenous auxin induced only the second response. It was also predicted and confirmed that elongating systems, e.g. pea epicotyl, with certain early auxin-induced growth kinetics (an initial high non-steady-state rate followed immediately by a drop to a lower steady-state rate) would show a transient second response (in addition to the usual transient first response) when stimulated by pH 4 medium. Finally, it is pointed out that recent results which establish the existence of auxin-induced elongation-associated proteins support the proposition that auxin coordinates wall loosening and the supply of wall materials in elongation.

Response of Isolated Plant Membranes to Auxins: Calcium Release

Membranes from etiolated hypocotyls of soybean (Glycine max [L] Merr.) were loaded with 45Ca in the presence of the ionophore A23187. Upon subsequent treatment with 2,4 dichlorophenoxyacetic acid or indole-3-acetic acid, calcium was released A weak or inactive auxin analogue (2,3-D) did not promote release from the hypocotyl membranes, nor did red blood cells treated with either auxin release calcium The concentration dependency of auxin-promoted calcium release paralleled that of auxin-induced growth and was biphasic with an optimum at 1 μM Membrane vesicles from mung bean hypocotyls and pea epicotyls also released calcium in response to auxin. The similar concentration dependency of auxin-induced growth and of calcium release, and the fact that most of the in vitro responses of plant plasma membranes to auxin are duplicated by appropriate modulation of membrane calcium, suggest a role of calcium ions in the responses of plant membranes to auxins.

An Improved Method for Detecting Auxin-induced Hydrogen Ion Efflux from Corn Coleoptile Segments

Conditions necessary to detect maximal auxin-induced H(+) secretion using a macroelectrode have been investigated using corn coleoptile segments. Auxin-induced H(+) secretion is strongly dependent upon oxygenation or aeration when the tissue to volume ratio is high. Cuticle disruption or removal is also necessary to detect substantial auxin-induced H(+) secretion. The auxin-induced decrease in pH of the external medium is stronger when the hormone is applied to tissue in which the cuticle has been disrupted with an abrasive than when the hormone is applied to tissue from which the cuticle and epidermis have been removed by peeling. The lower detectable acidification of the external medium when using peeled segments appears to be due in part to the leakage of buffers into the medium and in part to the removal of the auxin-sensitive epidermal cells.The sensitivity of corn coleoptile segments to auxin, as measured by H(+) secretion, increases about 2-fold during the first 2 hours after excision. This change in apparent sensitivity to auxin as reflected by H(+) secretion is paralleled by a time-dependent change in the growth response to auxin. Under optimal conditions for detecting H(+) efflux (oxygenation, abrasion, hormone application 2 hours after excision), the latent period in auxin-induced H(+) efflux (about 7 or 8 minutes) is only half as great as the latent period in auxin-induced growth (about 18 to 20 minutes). These observations are consistent with the acid growth hypothesis of auxin action.

A vena Coleoptile Segments: Hyperelongation Growth after Anaerobic Treatment

Abstract Avena sativa coleoptile segments show an anomalous increase in elongation growth following a short period of oxygen deprivation (tested between 0 and 60 min) lasting 20-30 min (Anaero-biosis-Aerobiosis transition effect = ANA effect). The increase in growth rate is 600% and is commensurate with that observable following an auxin treatment. This hyperelongation growth, in contrast to the auxin-induced growth, begins without a lag phase. The growth “burst” following anaerobiosis is similarly to auxin-induced elongation growth, and is suppressed increasingly by neutral or more alkaline buffers. Hyperelongation growth is suppressed by respiratory inhibitors and uncouplers. A complete inhibition is effected with KCN (0.5 mM) sodium azide (0.5 mM) and CCCP (1 μM); amytal (in the range 0.5 to 1 mM) and sodium arsenate (0.1 to 1 mM) are strong inhibitors. Some of these compounds (KCN, arsenate, amytal) cause a slight increase of the ANA effect in very low concentrations, which is probably due to the K+ or Na+ ions present; on their own, these ions have a strong positive influence on the ANA effect. During anaerobiosis the ATP level sinks around 75% and almost returns to the old value, following the supply of air, within one minute. The cell sap pH drops from 6.3 to 5.9 during anaerobiosis within 20 min. This lowering is mainly due to an increase in lactic acid concentration. Other acids such as citric, malic, and aspartic acids show insignificant changes in concentration. The NADH content increases during anaerobiosis, whereas that of NADPH drops almost as much. The mentioned changes in concentration of lactic acid, NADH and NADPH return to the control value within 20-30 min; thus the differences exist as long as hyperelongation growth is under way. Possible relationships between the mentioned chemical changes and hyperelongation growth are discussed. One of the possible explanations is the following: the lowering of the cytoplasmic pH (normally around pH 7) during anaerobiosis due to the formation of lactic acid causes an activation of H+-ATPases in the plasmalemma and ER, since their optimum activity occurs in a pH of 5.5 to 6.5. This activation causes a greater H+-excretion into the cell wall compartment, and thus hyperelongation growth following supply of air and of ATP.

Evidence that Auxin-induced Growth of Soybean Hypocotyls Involves Proton Excretion

The role of H(+) excretion in auxin-induced growth of soybean hypocotyl tissues has been investigated, using tissues whose cuticle was rendered permeable to protons or buffers by scarification (scrubbing). Indoleacetic acid induces both elongation and H(+) excretion after a lag of 10 to 12 minutes. Cycloheximide inhibits growth and causes the tissues to remove protons from the medium. Neutral buffers (pH 7.0) inhibit auxin-induced growth of scrubbed but not intact sections; the inhibition increases as the buffer strength is increased. Both live and frozen-thawed sections, in the absence of auxin, extend in response to exogenously supplied protons. Fusicoccin induces both elongation and H(+) excretion at rates greater than does auxin. These results indicate that H(+) excretion is involved in the initiation of auxin-induced elongation in soybean hypocotyl tissue.

Studies on the Chemical Basis of Auxin-Induced Cell Wall Loosening

Chemical changes accounting for the increase in wall extensibility of tissue grown in the presence of auxin were sought on the basis of chain scission mechanisms. Incorporation of glucose-14C into the hotwater extractable constituents from cell walls of basal sections of soybean hypocotyl was increased 15 min after the addition of auxin. However, this increase represented only 1%-2% of the total incorporation of radioactivity into the cell walls. With pea internode sections, both auxin-induced growth and incorporation of radioactivity from glucose-14C were inhibited at supraoptimal auxin concentrations that resulted in increased wall extensibility. No consistent correlation was found between wall loosening and rate of incorporation of glucose-14C into cell wall fractions, average length of wall polymers (based on number of free reducing groups), or the ratio of semicrystalline to amorphous wall materials (based on deuterium exchange). It is unlikely that wall loosening is mediated to a significant extent ...

Modifications of cell wall polysaccharides during auxin-induced growth in azuki bean epicotyl segments

Changes in sugar compositions and the distribution pattern of the molecular weight of cell wall polysaccharides during indole-3-acetic acid (IAA)-induced cell elongation were investigated. Differential extraction of the cell wall and gel permeation chromatography of wall polysaccharides indicated that galactans, polyuronides, xylans, glucans and cellulose were present in the azuki bean epicotyl cell wall. When segments were incubated in the absence of sucrose, IAA enhanced the degradation of galactans in both the pectin and hemicellulose fractions, whereas to some extent it enhanced the polymerization of xylans and glucans in the hemicellulose fraction and an increase in the amounts of polyuronides in the pectin fraction and of a-cellulose. In the presence of 50 nw sucrose, IAA caused large increases in the amounts of all the wall polysaccharides, and enhanced the polymerization of galactans, xylans and glucans in the hemicellulose fraction. These results and an important role of galactan turnover in cell wall extension are discussed.

Invertases in Oat Seedlings

The soluble invertase activity in etiolated Avena seedlings was highest at the apex of the coleoptile and much lower in the primary leaf, mesocotyl, and root. The activity in all parts of the seedling consisted of two invertases (I and II) which were separated by chromatography on diethylaminoethylcellulose. Both enzymes appeared to be acid invertases, but they differed in molecular size, pH optimum, and the kinetic parameters K(m) and V(max) of their action on sucrose, raffinose, and stachyose. Invertase II had low stability at pH 3.5 and below, and exhibited high sensitivity to Hg(2+), with complete inhibition by 2 micromolar HgCl(2). Segments of coleoptiles incubated in water lost about two-thirds of the total invertase activity after 16 hours. The loss of activity was due primarily to a decrease in the level of invertase II. The loss of invertase was decreased by indoleacetic acid, 2,4-dichlorophenoxyacetic acid, and alpha-naphthaleneacetic acid but not by beta-naphthaleneacetic acid and p-chlorophenoxyisobutyric acid. Conditions that inhibited auxin-induced growth of the segments (20 millimolar CaCl(2) and 200 millimolar mannitol) also blocked the auxin effect on invertase loss.

Cyanide Inhibition of Acid-induced Growth in Avena Coleoptile Segments

The comparative effects of metabolic inhibitors on acid- and auxininduced growth in oat (Avena sativa L. var. Victory) coleoptile segments have been examined. Acid (pH 4)-induced growth in both peeled and unpeeled segments is inhibited by 1 millimolar KCN when added at the time of acidification. KCN inhibits total acid-induced growth by 59 and 76%, respectively, in peeled and nonpeeled segments during the first 60 minutes. The growth rate of cyanide-treated tissue drops to zero or near zero in both peeled and nonpeeled segments during this period. Cyanide inhibition of total acid-induced growth in peeled segments at pH 5 is even more severe, amounting to about 80% during the first 60 minutes. The possibility that inhibition by cyanide may be caused by some nonspecific effect of the inhibitor on a process other than respiration, e.g. turgor reduction due to membrane damage, has not been ruled out. Acid-induced growth is also inhibited by 3 millimolar sodium fluoride and by anoxia. In unpeeled segments total pH 4-induced growth is inhibited 73% by sodium fluoride and 38% by anoxia during the 1st hour. Possible corrections to the above inhibition percentages which may be necessary due to the sensitivity of basal growth to inhibitors are discussed. Cyanide was found to inhibit auxin-induced growth much more rapidly than acid-induced growth. These data suggest that acid growth may be dependent on respiratory metabolism but to a lesser degree than is auxin-induced growth. If the acid growth theory of auxin action is correct, it appears that there may be two steps in the growth process which are dependent on respiratory metabolism: (a) auxin-induced proton pumping which is highly sensitive to respiratory inhibitors; and (b) acid-mediated wall loosening which is moderately and perhaps indirectly sensitive to respiratory inhibitors.

Growth and cell wall changes in rice coleoptiles growing under different conditions II. Auxin-induced growth in coleoptile segments

Growth and cell wall changes in rice coleoptiles growing under different conditions II. Auxin-induced growth in coleoptile segments

Effect of cytokinins and nojirimycin on auxin-induced growth in rice coleoptiles

Effect of cytokinins and nojirimycin on auxin-induced growth in rice coleoptiles

Growth and cell wall changes in azuki bean epicotyls II. Changes in wall polysaccharides during auxininduced growth of excised segments

Changes in cell wall polysaccharides and mechanical properties of the cell wall were examined during IAA-induced elongation growth of excised azuki bean epicotyl segments under different growth conditions. Sucrose promoted IAA-induced cell elongation, but had very little effect on IAA-induced cell wall loosening. In the absence of sucrose, the amount of galactose in the cell wall decreased during the incubation period. IAA enhanced the decrease in the galactose level. In the presence of sucrose, on the other hand, IAA induced increases in the amounts of cellulose, galactose and xylose in noncellulosic polysaccharides. These IAA-induced increases were not observed in the presence of mannitol at concentrations higher than 0.1 M, although cell wall loosening was induced by IAA even in the presence of 0.2 M mannitol.

Protein synthesis and auxin-induced growth: Inhibitor studies

We have compared the effects of cycloheximide (CHI) and two other rapid and effective inhibitors of protein synthesis, pactamycin and 2-(4-methyl-2,6-dinitroanilino)-N-methyl proprionamide (MDMP), on protein synthesis, respiration, auxin-induced growth and H(+)-excreation of Avena sativa L. coleoptiles. All three compounds inhibit protein synthesis without affecting respiration. The effectiveness of the inhibitors against H(+)-excretion and growth correlates with their ability to inhibit protein synthesis. Both CHI and MDMP inhibit auxin-induced H(+)-excretion after a latent period of 5-8 min, and inhibit growth after a 8-10-min lag. These results support the idea that continued protein synthesis is required in the initial stages of the growth-promoting action of auxin.

Ethylene and auxin-induced cell growth in relation to auxin transport and metabolism and ethylene production in the semi-aquatic plant, Regnellidium diphyllum

Cell elongation in the rachis of the semiaquatic fern Regnellidium diphyllum is induced by the addition of ethylene or indoleacetic acid (IAA). Experiments with whole plants or rachis segments have shown that ethylene-induced growth requires the presence of auxin. Ethylene does not cause a modification in either endogenous auxin levels or in the extent of auxin metabolism but auxin transport is reduced. Rates of ethylene production in Regnellidium are not altered by either mechanical excitation or by the addition of auxin. A two-hormone control of cell expansion is proposed in which an initial, auxin-dependent growth event pre-conditions the cells to a further subsequent (or synchronous) ethylene-dependent growth event.

The surface pH of Avena coleoptiles and its correlation with growth rates

The surface ph of Avena coleoptiles of different lengths and at different positions was measured. A close correlation between the pH's measured and the growth rates of Avena coleoptiles reported in the literature was found. In the fast growing zones of the coleoptiles a high hydrogen ion concentration was observed. When comparing coleoptiles of different lengths the widths of the zones with relatively high hydrogen ion concentrations correlate positively with the overall growth rates of the coleoptiles. In old coleoptiles in which growth had ceased the hydrogen ion concentration was low. The measurements reported in this paper were performed without addition of auxins. Therefore the growth was mainly dependent on the internal auxin concentrations. The results of this investigation strongly support the theory of auxin-induced growth mediated by hydrogen ions.

Naturally occuring modifiers of auxin-receptor interaction in corn: Identification as benzoxazolinones

A naturally-occurring material termed 'supernatant factor' [Ray, P.R., Dohrmann, U., Hertel, R.: Plant Physiol. 59. 357-364 (1977)], which has the property of modifying the binding affinity of auxins to receptor sites, has been isolated from corn (Zea mays L.) and characterised as a mixture of 6-methoxy-2-benzoxazolinone (MBOA) and 6,7-dimethoxy-2-benzoxazolinone (DMBOA). DMBOA is about 50 times more active than MBOA in inhibiting binding of the auxin 1-naphthylacetic acid to membrane-bound or solubilised receptors. The activity of these compounds and the parent analogue in inhibiting auxin binding is correlated with their ability to inhibit auxin-induced growth.