Are auxin and calcium movements in corn coleoptiles linked processes?

In this paper we present evidence for a tight linkage of calcium and auxin fluxes in corn coleoptile tissue. Imposed lateral gradients of auxin concentration induce a movement of calcium ions to the side of lower auxin concentration in the tissue. Imposing a lateral gradient of calcium concentration in turn leads to a movement of auxin to the side of lower calcium concentration. Whether these fluxes are using symplastic or apoplastic pathways or are transcellular cannot be decided from the results.Countercurrent fluxes of calcium ions and auxin could be demonstrated in polar transport experiments. This might be a further indication of a complex carrier system in the plasma membrane extruding auxin out of the cell with a concomitant calcium influx from the cell wall space into the cytoplasm.     

Effects of Cations on Hormone Transport in Primary Roots of Zea mays

We examined the influence of aluminum and calcium (and certain other cations) on hormone transport in corn roots. When aluminum was applied unilaterally to the caps of 15 mm apical root sections the roots curved strongly away from the aluminum. When aluminum was applied unilaterally to the cap and ³H-indole-3-acetic acid was applied to the basal cut surface twice as much radioactivity (assumed to be IAA) accumulated on the concave side of the curved root as on the convex side. Auxin transport in the apical region of intact roots was preferentially basipetal, with a polarity (basipetal transport divided by acropetal transport) of 6.3. In decapped 5 mm apical root segments, auxin transport was acropetally polar (polarity = 0.63). Application of aluminum to the root cap strongly promoted acropetal transport of auxin reducing polarity from 6.3 to 2.1. Application of calcium to the root cap enhanced basipetal movement of auxin, increasing polarity from 6.3 to 7.6. Application of the calcium chelator, ethylene-glycol-bis-(β-aminoethylether)-N,N,N′, N′-tetraacetic acid, greatly decreased basipetal auxin movement, reducing polarity from 6.3 to 3.7. Transport of label after application of tritiated abscisic acid showed no polarity and was not affected by calcium or aluminum. The results indicate that the root cap is particularly important in maintaining basipetal polarity of auxin transport in primary roots of corn. The induction of root curvature by unilateral application of aluminum or calcium to root caps is likely to result from localized effects of these ions on auxin transport. The findings are discussed relative to the possible role of calcium redistribution in the gravitropic curvature of roots and the possibility of calmodulin involvement in the action of calcium and aluminum on auxin transport.     

The transport of indole-3-acetic Acid in boron- and calcium-deficient sunflower hypocotyl segments

Transfer of sunflower (Helianthus annuus L. cv Russian Mammoth) seedlings from complete nutrient solution to solutions deficient in either boron or calcium resulted in a steady decline in the rate of auxin transport, compared to seedlings that remained in the complete solution. In seedlings transferred to solutions deficient in both B and Ca, the decline in auxin transport was greater than seedlings deficient in only one element. The transfer of B- or Ca-deficient seedlings back to the complete solution prevented further decline in auxin transport, but auxin transport did not increase to the same level as seedlings maintained in complete solution. The significant reduction in auxin transport during the early stages of B or Ca deficiency was not related to (a) reduced growth rate of the hypocotyl, (b) increased acropetal movement of auxin, or (c) lack of respiratory substrates in the hypocotyl. In addition, no difference was found in the water-extractable total and ionic Ca in B-deficient and control nondeficient hypocotyls, indicating a direct effect of B on auxin transport, rather than indirectly by affecting Ca absorption. The rate of auxin transport in hypocotyls deficient in either B or Ca, was inversely correlated with K⁺ leakage and rate of respiration. The data presented strongly support the view that there are separate sites for B and Ca in the basipetal transport of the plant hormone indoleacetic acid.     

Effects of osmotic stress on polar auxin transport in Avena mesocotyl sections

Segments of mesocotyls of Avena sativa L. transported [1-¹⁴C]indol-3yl-acetic acid (IAA) with strictly basipetal polarity. Treatment of the segments with solutions of sorbitol caused a striking increase in basipetal auxin transport, which was greatest at concentrations around 0.5 M. Similar effects were observed with mannitol or quebrachitol as osmotica, but with glucose or sucrose the increases were smaller. Polar transport was still detectable in segments treated with 1.2 M sorbitol. The effects of osmotic stress on the polar transport of auxin were reversible, but treatment with sorbital solutions more concentrated than 0.5 M reduced the subsequent ability of mesocotyl segments to grow in response to IAA. The increased transport of auxin in the osmotically stressed segments could not be explained in terms of an increased uptake from donor blocks. The velocity of transport declined with higher concentrations of osmoticum. The reasons for the enhancement of auxin transport by osmotic stress are not known.     

Dim-Red-Light-Induced Increase in Polar Auxin Transport in Cucumber Seedlings : I. Development of Altered Capacity, Velocity, and Response to Inhibitors

We have developed and characterized a system to analyze light effects on auxin transport independent of photosynthetic effects. Polar transport of [³H]indole-3-acetic acid through hypocotyl segments from etiolated cucumber (Cucumis sativus L.) seedlings was increased in seedlings grown in dim-red light (DRL) (0.5 μmol m⁻² s⁻¹) relative to seedlings grown in darkness. Both transport velocity and transport intensity (export rate) were increased by at least a factor of 2. Tissue formed in DRL completely acquired the higher transport capacity within 50 h, but tissue already differentiated in darkness acquired only a partial increase in transport capacity within 50 h of DRL, indicating a developmental window for light induction of commitment to changes in auxin transport. This light-induced change probably manifests itself by alteration of function of the auxin efflux carrier, as revealed using specific transport inhibitors. Relative to dark controls, DRL-grown seedlings were differentially less sensitive to two inhibitors of polar auxin transport, N-(naphth-1-yl) phthalamic acid and 2,3,5-triiodobenzoic acid. On the basis of these data, we propose that the auxin efflux carrier is a key target of light regulation during photomorphogenesis.     

Calcium-dependence of sugar transport in rat small intestine

The involvement of Ca2+ in the theophylline action on sugar transport was investigated in isolated rat small intestinal mucosa. Theophylline significantly increased cell water free sugar accumulation and reduced mucosal to serosal sugar fluxes both in the presence and absence of calcium, but the effects of theophylline were significantly less in calcium free media. In theophylline untreated tissues, calcium-deprived bathing solutions decreased tissue galactose accumulation and increased mucosal to serosal sugar flux. The calcium-channel blocker verapamil produced similar effects on intestinal galactose transport to those induced by low extracellular calcium activity. RMI 12330A and the calmodulin antagonist trifluoperazine abolished the theophylline-effects on intestinal galactose transport. Both drugs also affected sugar transport in basal conditions. These studies suggest that calcium might modulate sugar permeability across the basolateral boundary of rat enterocytes, and that its effect may be mediated by calmodulin.     

Mechanisms of active transport in isolated bacterial membrane vesicles. XII. Active transport by a mutant of Escherichia coli uncoupled for oxidative phosphorylation

Amino acid and β-galactoside transport activity catalyzed by whole cells and membrane vesicles prepared from an Escherichia coli mutant uncoupled for oxidative phosphorylation is comparable to the activity of analogous preparations from the parent strain. Valinomycin-induced rubidium uptake is also similar in membrane vesicles prepared from wild-type and mutant cells. The properties of the transport systems in mutant vesicles are the same as those of wild-type vesicles with respect to electron donors which stimulate transport, and with respect to inhibition by anoxia, cyanide, and 2,4-dinitrophenol.Magnesium ion markedly stimulates the ATPase activity of wild-type membrane vesicles and ethylenediaminetetraacetate markedly inhibits. However, these compounds have relatively slight effects on either the initial rate or extent of transport. Dicyclohexylcarbodiimide does not inhibit respiration-dependent transport despite inhibition of the calcium, magnesium-activated ATPase activity of wild-type vesicles.These results confirm earlier observations indicating that oxidative phosphorylation is not involved in respiration-linked active transport.     

The effects of calcium deficiency on Cucurbita pepo L. hypocotyl cells: A 31P nuclear-magnetic-resonance study

Calcium deficiency in zucchini (Cucurbita pepo L.) is associated with reduced growth and a reduced ability to transport auxin (Allan and Rubery, 1991, Planta 183, 604–612). An investigation of the effects of calcium-deficiency on zucchini hypocotyl cells was made using weak-acid uptake and ³¹P-nuclear-magneticresonance (³¹P-NMR) spectroscopy in vivo and in tissue extracts. Calcium-deficient tissue had the same cytoplasmic and vacuolar pHs as normal tissue when extracellular pH was near neutral. At acidic external pH the vacuolar pH was lower in deficient tissue. Adenine nucleotides were present predominantly as ATP in both control and calcium-deficient tissues. Addition of calcium to calcium-deficient tissue, under conditions which cause recovery of auxin transport induced no changes in the ³¹P-NMR spectra of deficient tissue. The content of mobile, phosphorylated metabolites was reduced in calcium-deficient tissue in comparison to control tissue. However, a substantial increase in the content of phosphorylcholine occurs in calcium-deficient tissues compared with controls; this may reflect changes in lipid turnover in calcium-stressed cells. We wish to thank Drs. Terry Moore and Jamie Vandenberg for technical assistance and Professor Peter Morris for providing the gated oxygen device. A.C.A. thanks the Cambridge Commonwealth Trust for a Prince of Wales Scholarship and the O.R.S. Awards Scheme for an award.     

Modeling Auxin Transport and Plant Development

The plant hormone auxin plays a critical role in plant development. Central to its function is its distribution in plant tissues, which is, in turn, largely shaped by intercellular polar transport processes. Auxin transport relies on diffusive uptake as well as carrier-mediated transport via influx and efflux carriers. Mathematical models have been used to both refine our theoretical understanding of these processes and to test new hypotheses regarding the localization of efflux carriers to understand auxin patterning at the tissue level. Here we review models for auxin transport and how they have been applied to patterning processes, including the elaboration of plant vasculature and primordium positioning. Second, we investigate the possible role of auxin influx carriers such as AUX1 in patterning auxin in the shoot meristem. We find that AUX1 and its relatives are likely to play a crucial role in maintaining high auxin levels in the meristem epidermis. We also show that auxin influx carriers may play an important role in stabilizing auxin distribution patterns generated by auxin-gradient type models for phyllotaxis.     

Novel auxin transport inhibitors phenocopy the auxin influx carrier mutation aux1

The hormone auxin is transported in plants through the combined actions of diffusion and specific auxin influx and efflux carriers. In contrast to auxin efflux, for which there are well documented inhibitors, understanding the developmental roles of carrier-mediated auxin influx has been hampered by the absence of specific competitive inhibitors. However, several molecules that inhibit auxin influx in cultured cells have been described recently. The physiological effects of two of these novel influx carrier inhibitors, 1-naphthoxyacetic acid (1-NOA) and 3-chloro-4-hydroxyphenylacetic acid (CHPAA), have been investigated in intact seedlings and tissue segments using classical and new auxin transport bioassays. Both molecules do disrupt root gravitropism, which is a developmental process requiring rapid auxin redistribution. Furthermore, the auxin-insensitive and agravitropic root-growth characteristics of aux1 plants were phenocopied by 1-NOA and CHPAA. Similarly, the agravitropic phenotype of inhibitor-treated seedlings was rescued by the auxin 1-naphthaleneacetic acid, but not by 2,4-dichlorophenoxyacetic acid, again resembling the relative abilities of these two auxins to rescue the phenotype of aux1. Further investigations have shown that none of these compounds block polar auxin transport, and that CHPAA exhibits some auxin-like activity at high concentrations. Whilst results indicate that 1-NOA and CHPAA represent useful tools for physiological studies addressing the role of auxin influx in planta, 1-NOA is likely to prove the more useful of the two compounds.     

Auxin movement in corn coleoptiles

Summary Movement of radioactive auxins was analysed in corn coleoptile sections. The results support the idea that processes involved in the transport of indoleacetic acid (IAA) are specific for growth-promoting auxins.Inhibition of IAA transport by triiodobenzoic acid is caused by a reversible block of the exit; the auxin held back remains in the transport pool. The observed increase in immobilization may be a secondary effect caused by the increased concentration of free IAA in the tissue.Auxin molecules are most likely transported by anon-covalent mechanism. IAA and naphthaleneacetic acid (NAA) move through the cell and exit as free molecules. A search for a transient auxin complex, chaseable as required for any transport carrier intermediate, yielded negative results. No¹⁸O was lost from NAA labeled with¹⁸O in the carboxyl group during transport of the auxin through coleoptile tissue.After application of IAA to auxin-depleted tissue, the transport rate undergoes oscillations with a period length of ca. 25 min.The movement of the auxin 2.4-dichlorophenoxyacetic acid which is usually sluggish, increased several times if some IAA was added. Auxin, thus, stimulates its own transport.A model is discussed in which auxin-binding to the plasma membrane and reversible changes of membrane conformation may provide a basis for active secretion and for the observed cooperativity. Leo Brauner zum 70. Geburtstag gewidmet.     

Regulation of auxin transport polarity by AGC kinases

The plant hormone auxin controls plant development through gradients and maxima that are generated by PIN efflux carrier driven polar auxin transport. PIN proteins direct this cell-to-cell auxin transport, and thus orient plant development through their asymmetric subcellular distribution. PIN polarity is regulated by PINOID and the phototropins, members of the AGC protein serine/threonine kinase family. Here we review the signaling pathways of these kinases and the role of calcium and BTB proteins in translating both internal and external signals into developmental responses via PIN relocalization, to adapt plant development to changing environmental conditions.     

Ethylene-induced opposite redistributions of calcium and auxin are essential components in the development of tomato petiolar epinastic curvature.

Calcium has been suggested as an important mediator of gravity signaling transduction within the root cap statocyte. In a horizontally-placed root, it is redistributed in the direction of the gravity vector (i.e. it moves downward) and its redistribution is closely correlated with auxin downward movement. However, the involvement of calcium in the regulation of ethylene-induced epinasty and auxin movement is not known. In this report, we examined the involvement of calcium in lateral auxin transport during ethylene-induced epinasty in an effort to understand the relationship among calcium, auxin, and ethylene. Ethylene-induced epinasty was further stimulated by exogenously applied Ca2+, the calcium effect being the strongest among divalent cations tested. Pretreatment with NPA, an auxin transport inhibitor, negated the promotive effect of calcium ions on the petiolar epinasty. Ethylene caused redistribution/differential accumulation of 45Ca2+ toward the morphologically lower (abaxial) side of the leaf petioles, an effect opposite to that of 14C-IAA redistribution. Verapamil, a Ca2+ channel blocker, inhibited ethylene-induced epinasty, as well as the redistribution of 14C-IAA and 45Ca2+. When the petiole was inverted in the presence or absence of ethylene, the direction of 45Ca2+ differential accumulation was still toward the morphologically abaxial side of the petiole during epinastic movement regardless of gravitational direction. These results suggest that gravity-insensitive, ethylene-induced Ca2+ redistribution and accumulation toward the abaxial side are closely coupled to the adaxial auxin redistribution/accumulation and, in turn, to the petiolar epinasty.     

The Arabidopsis MAX pathway controls shoot branching by regulating auxin transport

BACKGROUND: Plants achieve remarkable plasticity in shoot system architecture by regulating the activity of secondary shoot meristems, laid down in the axil of each leaf. Axillary meristem activity, and hence shoot branching, is regulated by a network of interacting hormonal signals that move through the plant. Among these, auxin, moving down the plant in the main stem, indirectly inhibits axillary bud outgrowth, and an as yet undefined hormone, the synthesis of which in Arabidopsis requires MAX1, MAX3, and MAX4, moves up the plant and also inhibits shoot branching. Since the axillary buds of max4 mutants are resistant to the inhibitory effects of apically supplied auxin, auxin and the MAX-dependent hormone must interact to inhibit branching. RESULTS: Here we show that the resistance of max mutant buds to apically supplied auxin is largely independent of the known, AXR1-mediated, auxin signal transduction pathway. Instead, it is caused by increased capacity for auxin transport in max primary stems, which show increased expression of PIN auxin efflux facilitators. The max phenotype is dependent on PIN1 activity, but it is independent of flavonoids, which are known regulators of PIN-dependent auxin transport. CONCLUSIONS: The MAX-dependent hormone is a novel regulator of auxin transport. Modulation of auxin transport in the stem is sufficient to regulate bud outgrowth, independent of AXR1-mediated auxin signaling. We therefore propose an additional mechanism for long-range signaling by auxin in which bud growth is regulated by competition between auxin sources for auxin transport capacity in the primary stem.     

Flavonoids act as negative regulators of auxin transport in vivo in arabidopsis

Polar transport of the plant hormone auxin controls many aspects of plant growth and development. A number of synthetic compounds have been shown to block the process of auxin transport by inhibition of the auxin efflux carrier complex. These synthetic auxin transport inhibitors may act by mimicking endogenous molecules. Flavonoids, a class of secondary plant metabolic compounds, have been suggested to be auxin transport inhibitors based on their in vitro activity. The hypothesis that flavonoids regulate auxin transport in vivo was tested in Arabidopsis by comparing wild-type (WT) and transparent testa (tt4) plants with a mutation in the gene encoding the first enzyme in flavonoid biosynthesis, chalcone synthase. In a comparison between tt4 and WT plants, phenotypic differences were observed, including three times as many secondary inflorescence stems, reduced plant height, decreased stem diameter, and increased secondary root development. Growth of WT Arabidopsis plants on naringenin, a biosynthetic precursor to those flavonoids with auxin transport inhibitor activity in vitro, leads to a reduction in root growth and gravitropism, similar to the effects of synthetic auxin transport inhibitors. Analyses of auxin transport in the inflorescence and hypocotyl of independent tt4 alleles indicate that auxin transport is elevated in plants with a tt4 mutation. In hypocotyls of tt4, this elevated transport is reversed when flavonoids are synthesized by growth of plants on the flavonoid precursor, naringenin. These results are consistent with a role for flavonoids as endogenous regulators of auxin transport.     

Effects of ethylene on auxin transport

The effect of ethylene on the uptake, distribution and polar transport of C¹⁴ from indole-3-acetic acid-2-C¹⁴ and naphthalene acetic acid-1-C¹⁴ in tissue sections was studied. Test species were cotton (Gossypium hirsutum, L.) and cowpea (Vigna sinensis, Endl.). Generally, incubation of tissue or intact plants with ethylene reduced the degree of polar auxin transport. Ethylene inhibited the movement of both auxins in stem tissue and IAA in petiole tissue of cotton. The effect of ethylene on auxin movement in cow-peas was more complex. Ethylene apparently inhibited transport in younger petiole and stem tissue, but stimulated the process to a small but significant degree in basal petiole segments.

Ethylene, in some experiments, reduced C¹⁴ (auxin) uptake. This reduction was consistently smaller than the inhibition of transport. Effects upon transport were observed when uptake was not different. Differences in uptake declined as the period of incubation with auxin was lengthened, but transport was inhibited for up to 23 hours.

It is proposed that ethylene may, through its effect on transport, cause localized shortages and surpluses of auxin which in turn contribute to symptoms now associated with the response of sensitive species to ethylene.