Intact Maize Seedlings Research Articles

Induction of phenylalanine ammonia-lyase (PAL) in insect damaged and neighboring undamaged cotton and maize seedlings

ABSTRACTThere has been scepticism about plants responding to information from their damaged neighbor plants since it was first reported, although a growing body of evidence indicates that interplant communication may play a key role in the resistance of plants. Cotton aphids and corn borer are important insects in cotton and maize, respectively. The resistance of healthy cotton and maize plants induced by their neighbor damaged plants was investigated in this study. Phenylalanine ammonia-lyase (PAL) is a chemical marker of induced resistance in many plants. The enzymatic activity and gene expression of PAL were determined in cotton and maize seedlings damaged by wounding, aphid and corn borer attack and in intact undamaged maize and cotton seedlings growing nearby. The enzymatic activity and gene expression of PAL were significantly increased in the cotton and corn seedlings damaged by wounding or by aphid and corn borer. Furthermore, undamaged neighboring cotton and maize seedlings also showed elevated enzymatic activity and gene expression of PAL. These inductions of PAL enzymatic activity and gene expression in the damaged and undamaged seedlings suggest inter-species communication between cotton and maize.

Growth-limiting proteins in maize coleoptiles and the auxin-brassinosteroid hypothesis of mesocotyl elongation.

The shoot of grass coleoptiles consists of the mesocotyl, the node, and the coleoptile (with enclosed primary leaf). Since the 1930s, it is known that auxin (indole-3-acetic acid, IAA), produced in the tip of the coleoptile, is the central regulator of turgor-driven organ growth. Fifty years ago, it was discovered that antibiotics that suppress protein biosynthesis, such as cycloheximide, inhibit auxin (IAA)-induced cell elongation in excised sections of coleoptiles and stems. Based on such inhibitor studies, the concept of "growth-limiting proteins (GLPs)" emerged that was subsequently elaborated and modified. Here, we summarize the history of this idea with reference to IAA-mediated shoot elongation in maize (Zea mays) seedlings and recent studies on the molecular mechanism underlying auxin action in Arabidopsis thaliana. In addition, the analysis of light-induced inhibition of shoot elongation in intact corn seedlings is discussed. We propose a concept to account for the GLP-mediated epidermal wall-loosening process in coleoptile segments and present a more general model of growth regulation in intact maize seedlings. Quantitative proteomic and genomic studies led to a refinement of the classic "GLP concept" to explain phytohormone-mediated cell elongation at the molecular level (i.e., the recently proposed theory of a "central growth regulation network," CGRN). Novel data show that mesocotyl elongation not only depends on auxin but also on brassinosteroids (BRs). However, the biochemical key processes that regulate the IAA/BR-mediated loosening of the expansion-limiting epidermal wall(s) have not yet been elucidated.

Wound-induced green leaf volatiles cause the release of acetylated derivatives and a terpenoid in maize

Green leaf volatiles (GLVs), generally occurring C 6 alcohols, aldehydes and acetates from plants, play an important role in plant–plant communication. These compounds induce intact plants to produce jasmonic acid, and induce defense-related gene expression and the release of volatile compounds. Here, we address wound-induced GLVs cause the release of acetylated derivatives and a terpenoid, ( E)-4,8-dimethylnona-1,3,7-triene (DMNT) in intact maize, which may be a type of plant–plant interaction mediated by airborne GLVs. Upon exposure of intact maize seedlings to wound-induced GLVs, ( Z)-3-hexenyl acetate was consistently the most abundant compound released. Exogenous application of individual alcohols and aldehydes mostly resulted in the release of corresponding acetate esters. C 6-alcohols with a double bond between the second and third, or the third and fourth carbon atoms, C 5- or C 6-aldehydes, and ( Z)-3-hexenyl acetate triggered the release of DMNT. When ( Z)-3-hexenyl acetate and hexyl acetate were used to treat maize seedlings, they were recovered from the plants. These data demonstrated that: (1) apart from direct adsorption and re-release of acetate esters, absorption and conversion of exogenous alcohols and aldehydes into acetate esters occurred, and (2) DMNT was induced by a range of aldehydes and unsaturated alcohols.

Bicarbonate-induced alkalinization of the xylem sap in intact maize seedlings as measured in situ with a novel xylem pH probe.

In higher plants the pH of the xylem sap plays an important role in drought signaling, growth regulation, and plant nutrition. However, the interpretation of the data is very controversial. The main reason for this is that the xylem pH in intact plants was not directly accessible hitherto. We present here a novel, minimally-invasive probe based on the xylem pressure-potential probe (used for measuring directly xylem pressure and the electrical potential between root xylem sap and medium). Single-tipped, double-barreled capillaries were used, one barrel served as H(+)-selective electrode, whereas pressure and electrical potential were recorded by the other one. Upon insertion of the probe into the root xylem of maize (Zea mays) seedlings, pH values ranging between about 4.2 and 4.9 were monitored when the roots were immersed in standard nutrient solution. The pH did not respond to changes in light irradiation (up to 300 micromol m(-2) s(-1)), but increased upon exposure of the root to 5 or 20 mm bicarbonate in the bath solution. Changes in pH could also be recorded in transpiring plants when the root was cut below the insertion point of the probe and placed in media with different pH. The data support the hypothesis of Mengel ([1994] Plant Soil 165: 275-283) that upon external supply with bicarbonate Fe is immobilized in the leaf apoplast via changes in xylem pH.

Ethylene Production and Activation of Hydrolytic Enzymes during Acclimation of Maize Seedlings to Partial Flooding

The effects of partial flooding on the partial pressure of oxygen and carbon dioxide in water around the roots, ethylene production by intact maize (Zea mays L.) seedlings, the activities of hydrolytic enzymes (pectinase, xylanase, and cellulase) in adventitious roots, and the growth of adventitious and main roots were studied. Aggravated hypoxia resulted in the accelerated ethylene production and the activation of enzymes destroying cell walls in the adventitious roots; as a result, the latter changed their growth pattern. The conclusion is that the interrelated responses are adaptive ones, and the adventitious roots play a key role in plant adaptation.

Exploring the Radial and Longitudinal Aeration of Primary Maize Roots by Means of Clark-Type Oxygen Microelectrodes

Clark-type oxygen microelectrodes were used to measure the radial and longitudinal oxygen distribution in aerenchymatous and nonaerenchymatous primary roots of intact maize seedlings. A radial intake of oxygen from the rooting medium was restricted by embedding the roots in 1% agar causing aeration to be largely dependent upon longitudinal internal transport from the shoot. In both root types, oxygen concentrations declined with distance from the base, and were lower in the stele than in the cortex. Also, the bulk of the oxygen demand was met internally by transport from the shoots, but a little oxygen was received by radial inward diffusion from the surrounding agar, and in some positions the hypodermal layers received oxygen from both the agar and the cortex. Near to the base, the oxygen partial pressure difference between the cortex and the center of the stele could be as much as 6–8 kPa. Nearer to the tip, the differences were smaller but equally significant. In the nonaerenchymatous roots, cortical oxygen partial pressures near the apex were becoming very low (< 1 kPa) as root lengths approached 100 mm, and towards the center of the stele values reached 0.1 kPa or lower. However, the data indicated that respiratory activity did not decline until the cortical oxygen pressure was less than 2 kPa. Mathematical modeling based on Michaelis–Menten kinetics supported this and suggested that the respiratory decline would be mostly restricted to the stele until cortical oxygen pressures approached very low values. At a cortical oxygen pressure of 0.75 kPa, it was shown that respiratory activity in the pericycle and phloem might remain as high as 80–100% of maximum even though in the center of the stele it could be less than 1% of maximum. Aerenchyma production resulted in increases in oxygen concentration throughout the roots with cortical partial pressures of ca. 5–6 kPa and stelar values of ca. 3–4 kPa near the tips of 100 mm long roots. In aerenchymatous roots, there was some evidence of a decline in the oxygen permeability of the epidermal–hypodermal cylinder close to the apex; a decline in stelar oxygen permeability near the base was indicated for both root types. There was some evidence that the mesocotyl and coleoptile represented a very significant resistance to oxygen transport to the root.

Light Induction of 1-Aminocyclopropane-1-carboxylic Acid Synthase Activity in Quinclorac-Treated Maize Seedlings

The effect of light on quinclorac (3,7-dichloro-8-quinolinecarboxylic acid)-induced phytotoxicity and on 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity in intact maize (Zea mays L. cv. Honey Bantam) seedlings was investigated. The root-treatment of intact seedlings with quinclorac significantly reduced the FW of the plants, and water and chlorophyll contents of the first leaves under illumination, but not in the dark. Quinclorac (50 μM)-treated seedlings produced approximately two-fold larger amounts of ethylene in the light than in the dark. 2,5-Norbornadiene (NBD), a competitive inhibitor of ethylene, significantly recovered the quinclorac-induced decrease in water and chlorophyll. For determining ACC synthase activity, the seedlings were treated with quinclorac (50 μM) for 12 hr in the dark, and then transferred to light or kept in darkness. Quinclorac significantly enhanced ACC synthase activity in the shoot 6 hr after exposure to light, while no significant activation was observed in the dark. These results suggest that light is involved in the phytotoxic action of quinclorac in intact maize seedlings through an enhancement of ethylene biosynthesis, and that the generation of unknown light-regulated factor(s) might be responsible for the induction of ACC synthase activity following quinclorac treatment.

Interaction between electron transport at the plasma membrane and nitrate uptake by maize (Zea mays L.) roots.

In the present study nitrate uptake by maize (Zea mays L.) roots was investigated in the presence or absence of ferricyanide (hexacyanoferrate III) or dicumarol. Nitrate uptake caused an alkalization of the medium. Nitrate uptake of intact maize seedlings was inhibited by ferricyanide while the effect of dicumarol was not very pronounced. Nitrite was not detected in the incubation medium, neither with dicumarol-treated nor with control plants after application of 100 microM nitrate to the incubation solution. In a second set of experiments interactions between nitrate and ferricyanide were investigated in vivo and in vitro. Nitrate (1 or 3 mM) did neither influence ferricyanide reductase activity of intact maize roots nor NADH-ferricyanide oxidoreductase activity of isolated plasma membranes. Nitrate reductase activity of plasma-membrane-enriched fractions was slightly stimulated by 25 microM dicumarol but was not altered by 100 microM dicumarol, while NADH-ferricyanide oxidoreductase activity was inhibited in the presence of dicumarol. These data suggest that plasma-membrane-bound standard-ferricyanide reductase and nitrate reductase activities of maize roots may be different. A possible regulation of nitrate uptake by plasmalemma redox activity, as proposed by other groups, is discussed.

Physical strain-mediated microtubule reorientation in the epidermis of gravitropically or phototropically stimulated maize coleoptiles.

During gravitropic and phototropic curvature of the maize coleoptile, the cortical microtubules (MTs) adjacent to the outer epidermal cell wall assume opposite orientations at the two sides of the organ. Starting from a uniformly random pattern during straight growth in darkness, the MTs reorientate perpendicularly to the organ axis at the outer (faster growing) side and parallel to the organ axis at the inner (slower growing) side. As similar reorientations can be induced during straight growth by increasing or decreasing the effective auxin concentration, it has been proposed that these reorientations may be used as a diagnostic test for assessing the auxin status of the epidermal cells during tropic curvature. This idea was tested by determining the MT orientations in the coleoptile of intact maize seedlings in which the gravitropic or phototropic curvature was prevented or inversed by an appropriate mechanical counterforce. Forces that just prevented the coleoptile from curving in a gravity or light field prevented reorientations of the MTs. Forces strong enough to overcompensate the tropic stimuli by enforcing curvature in the opposite direction induced reorientations of the MTs opposite to those produced by tropic stimulation. These results show that the MTs at the outer surface of the coleoptile respond to changes in mechanical tissue strain rather than to gravitropic or phototropic stimuli and associated changes at the level of auxin or any other element in the signal transduction chain between perception of tropic stimuli and asymmetric growth response. It is proposed that cortical MTs can act as strain gauges in a positive feed-back regulatory circle utilized for amplification and stabilization of environmentally induced changes in the direction of elongation growth.

Mineral nutrient supply, cell wall adjustment and the control of leaf growth

ABSTRACTThe possibility that changes in the plasticity of expanding cell walls are involved in regulating early leaf growth responses to nutrient deficiencies in monocot plants was investigated. Intact maize seedlings (Zea mays L.) which were hydroponically grown with their roots in low‐nutrient solution (1 mol m−3 CaCl2) showed early inhibition of first‐leaf growth, as compared with seedlings on complete nutrient solution. This early inhibition of leaf growth was not associated with reduced cell production. However, segmental elongation along the cell expansion zone at the base of the leaf and the lengths of mature epidermal cells were reduced by the low‐nutrient treatment. Solute (osmotic) potentials in the expanding leaf tissues were unchanged. In contrast, low‐nutrient treatments significantly altered leaf plasticity, i.e. the irreversible extension caused by applying a small force in the direction of leaf growth. For example, in vivo plasticity decreased, along with leaf growth, after transfer of seedlings from complete nutrient solution to low‐nutrient solution for 15 h. Conversely, in vivo plasticity increased, along with leaf growth, after transfer of plants previously grown on low‐nutrient solution to complete nutrient solution for 15 h. The nutrient treatments also induced similar changes in the in vitro plasticity of the expanding leaf cell walls. There were no consistent changes in elasticity. Thus, reductions in the plasticity of expanding leaf cell walls appear to be involved in controlling the early inhibition of maize leaf growth by root imposition of nutrient stress.

The effects of electric field on the growth of intact seedlings and coleoptile segments ofZea mays L.

The experiments were carried out with 96-h-old intact maize seedlings and 10 mm long coleoptile segments cut 4 mm below the tip. The electric fields were applied longitudinally along the seedlings. The electric field (15 V) caused inhibition of the elongation growth of intact seedlings which was dependent on both the polarity and the duration of the applied voltage. The growth inhibition was greater when the tip of the shoot was positive relative to the roots. The electric field also caused inhibition of indole-3-acetic acid (IAA) and fusicoccin (FC) induced growth of maize coleoptile segments excised from electrically treated seedlings. IAA-induced growth of coleoptile segments was greater when the tip of the shoot was negative to the roots (not in the case of FC-treated segments and intact seedlings). It was suggested that apart from the changes induced by electric field in transport system of auxin the electric field affected also the activity of plasmalemma proton pump.

Inhibition of maize primary root elongation by spermidine: Effect on cell shape and mitotic index

Spermidine applied for 18 h to intact maize seedlings through their roots reduces root growth 70%, and the effect is reversible. Histological observations of longitudinal sections of 0.4-cm root apical segments from 2-day-old maize seedlings grown for 18 h in 0.5 m CaSO4 solution with or without 1 mm spermidine contribute to the explanation of spermidine-dependent slow root growth. In the meristematic zone a strong reduction of the mitotic index and in the elongation zone an inhibition of cell elongation occur simultaneously. Cell shape analysis along the growth axis of the maize root apex expressed in terms of form factor (FCircle) values substantiates the dual effect of spermidine on mitotic activity and cell elongation.

Potassium inhibition of calcium and magnesium accumulation in roots of intact maize seedlings

Abstract The effects of solution potassium (K) on the accumulation of magnesium (Mg) and calcium (Ca) in roots was assessed with 7‐day‐old corn (Zea mays L.) seedlings. Root accumulation of Ca and Mg was studied in replacement type experiments, i.e. the ratio of Ca to Mg was altered but the concentration of Ca plus Mg was kept constant at 2 mM. Concentrations of Ca in the tissue after 12 h exceeded that of Mg, except when Mg comprised over 85% of the divalent cations in the uptake solution. The addition of 0.1 mM K to the Uptake solutions significantly altered the accumulation of Ca and Mg. The concentration of Mg was affected more than that of Ca. The relative effects of solution K on Mg and Ca accumulation in roots could be explained by comparing the relative acid‐base characteristics of the metals.

Rapid and reversible modifications of extension capacity of cell walls in elongating maize leaf tissues responding to root addition and removal of NaCl

ABSTRACTA creep extensiometer technique was used to provide direct evidence that short (20 min) and long‐term (3d) exposures of roots to growth inhibitory levels of salinity (100mol m‐3 NaCl) induce reductions in the irreversible extension capacity of cell walls in the leaf elongation zone of intact maize seedlings (Zea mays L.). The long‐term inhibition of cell wall extension capacity was reversed within 20 min of salt withdrawal from the root medium. Inhibited elongation of leaf epidermal tissues was also reversed after salt removal. The salt‐induced changes in wall extension capacity were detected using in vivo and in vitro assays (shortly after localized freeze/thaw treatment of the basal elongation zone). The rapid reversal of the inhibition of wall extensibility and leaf growth after salt removal from root medium of long‐term salinized plants, suggested that neither deficiencies in growth essential mineral nutrients nor toxic effects of NaCl on plasmamembrane viability were directly involved in the inhibition of leaf growth. There was consistent agreement between the scale, direction and timing of salinity‐induced changes in leaf elongation growth and wall extension capacity. Rapid metabolically regulated changes in the physical properties of growing cell walls, caused by osmotic (or other) effects, appear to be a factor regulating maize leaf growth responses to root salinization.

Characterization of Paraquat Transport in Protoplasts from Maize (Zea mays L.) Suspension Cells.

Protoplasts isolated from maize (Zea mays L.) suspension cells were used to study transport of paraquat. [14C]Paraquat uptake was measured in 400-[mu]L centrifuge tubes using silicon oil centrifugation techniques. Approximately 50% of accumulation from a 100 [mu]M paraquat solution occurred in the first 10 s, and net accumulation reached a maximum after about 10 min. Membrane binding accounted for about 30% of apparent accumulation. Concentration-dependent uptake kinetics were characterized by a non-saturating curve, which was resolved into a linear and a saturable component. The Km of the saturable component was 132 [mu]M, and the Vmax was 0.512 nmol [mu]L of protoplasts-1 min-1. In the absence of sucrose, the Vmax of the saturable component was reduced by 52%, suggesting that paraquat uptake across the plasmalemma is energy dependent. Measurement of concentration-dependent binding of paraquat to burst protoplasts showed a linear response. This suggests that the linear component from intact protoplast concentration kinetics represented paraquat binding to the plasmalemma surface. Calcium inhibited the saturable component, and this inhibition was shown by Lineweaver-Burk analysis to be noncompetitive. Putrescine, a divalent cationic polyamine with a charge distribution similar to that of paraquat, competitively inhibited paraquat uptake. These results show that paraquat transport characteristics at the plasmalemma of maize protoplasts are similar to those reported earlier for paraquat transport in roots of intact maize seedlings.

Investigations into the Cation Specificity and Metabolic Requirements for Paraquat Transport in Roots of Intact Maize Seedlings

The effect of inorganic polyvalent cations and metabolic inhibitors on the transport of paraquat into maize root epidermal and cortical cells was investigated. Increasing concentrations of Ca 2+ caused a progressive inhibition of the saturable component of paraquat uptake, which represents influx across the plasmalemma, and a more modest inhibition in the slope of the linear component, which is due to binding of paraquat within the cell wall. Lineweaver-Burk transformation of the saturable component from these experiments indicated that Ca 2+ inhibition of paraquat uptake is noncompetitive. Reciprocal interactions between paraquat and Ca 2+ were observed, in that paraquat also inhibited Ca 2+ influx into root cells. Saturable paraquat uptake was not affected when the calcium channel blocker verapamil (400 μ M) was present in the uptake medium, indicating that paraquat does not move across the plasmalemma via calcium channels. Mg 2+ and La 3+ also inhibited the saturable component in a manner similar to that of Ca 2+. Cyanide (500 μ M) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) (20 μ M) had little effect on the saturable component in intact roots, suggesting that paraquat uptake into maize root cells is not directly dependent on cytoplasmic ATP levels or H + gradients. When roots reloaded for 160 min with 100 μ M paraquat (to increase cytoplasmic paraquat levels, and in turn, reduce the inwardly directed chemical gradient for paraquat influx) were exposed to 20 μ M CCCP, a 37% reduction in the V max for saturable paraquat uptake was determined. These results suggest that the driving force for paraquat uptake is the gradient for electrochemical potential of paraquat across the plasmalemma.

Transport Interactions between Paraquat and Polyamines in Roots of Intact Maize Seedlings

Interactions between absorption of paraquat and the polyamines putrescine, cadaverine, and spermine in roots of intact maize (Zea mays L. cv 3377 Pioneer) seedlings were examined. Concentration-dependent kinetics for paraquat and putrescine influx were similar and both kinetic curves could be resolved into a linear and a saturable component. The linear component was previously shown to represent cell wall/membrane binding. The saturable components for paraquat and putrescine uptake, which represent influx across the plasmalemma, had K(m) values of 98 and 120 micromolar, respectively, and V(max) values of 445 and 456 nanomoles per gram fresh weight per hour, respectively. Lineweaver-Burk transformation of the saturable component of paraquat influx in the presence of varying concentrations of putrescine indicated that the diamine competitively inhibited the saturable component of paraquat uptake. Reciprocal experiments similarly demonstrated that paraquat competitively inhibited the saturable component of putrescine uptake. Competitive inhibition of both paraquat and putrescine influx could also be demonstrated with the diamine cadaverine, which has a charge distribution similar to that of paraquat and putrescine. In contrast, the larger, tetravalent polyamine spermine appeared to noncompetitively inhibit the influx of paraquat and putrescine. These results strongly suggest that paraquat enters maize root cells via a carrier system that normally functions in the transport of diamines with a charge distribution similar to that of paraquat.

Characterization of the transport and cellular compartmentation of paraquat in roots of intact maize seedlings

Uptake and compartmentation of paraquat was investigated in intact roots of hydroponically grown maize ( Zea mays L.) seedlings. Because this investigation focused on the transport of a potentially phytotoxic species, electrophysiological studies were conducted to determine the effect of paraquat exposure on root-cell membrane integrity. Exposure of roots to 1 m M paraquat for up to 40 min or 0.1 m M paraquat for up to 140 min had little effect on the root-cell membrane potential, which indicates that the relatively brief paraquat exposures used for this study (up to 2 hr) had little effect on membrane integrity. The time course for [ 14C]paraquat accumulation in roots was linear over a 75-min period. Concentration-dependent kinetics for paraquat influx were nonsaturating up to 1 m M and could be resolved into a linear and a saturable component. The linear component was determined to be radiolabeled paraquat remaining in the apoplasm of root epidermal and cortical cells following a 15-min desorption period that was employed to remove cell wall [ 14C]paraquat. The saturable component displayed Michaelis-Menten kinetics with K m = 90 μ M and V max = 458 nmol g fresh wt −1 hr −1. Compartmental analysis of [ 14C]paraquat efflux from intact roots revealed the following estimated distribution of [ 14C]paraquat at the end of a 2-hr loading period: 76% in the cell wall free space, 16% in the cytoplasm, and about 8% in the vacuole. Estimated efflux half-times were 7.6 min, 29 min, and 4.7 hr from the cell wall, cytoplasm, and vacuole, respectively. These results suggest that paraquat enters the symplasm of plant roots via a carrier-mediated system similar to that proposed for animal tissues. In addition, efflux data indicate that while paraquat slowly accumulates in the vacuole, it can also move back out across the tonoplast and plasmalemma.

Effect of Inorganic Cations and Metabolic Inhibitors on Putrescine Transport in Roots of Intact Maize Seedlings

The specificity and regulation of putrescine transport was investigated in roots of intact maize (Zea mays L.) seedlings. In concentration-dependent transport studies, the kinetics for putrescine uptake could be resolved into a single saturable component that was noncompetitively inhibited by increasing concentrations of Ca(2+) (50 micromolar to 5 millimolar). Similarly, other polyvalent cations, including Mg(2+) (1.8 millimolar) and La(3+) (200 micromolar), almost completely abolished the saturable component for putrescine uptake. This suggests that putrescine does not share a common transport system with other divalent or polyvalent inorganic cations. Further characterization of the putrescine transport system indicated that 0.3 millimolar N-ethyl-maleimide had no effect on putrescine uptake, and 2 millimolar p-chloromercuribenzene sulfonic acid only partially inhibited transport of the diamine (39% inhibition). Metabolic inhibitors, including carbonylcyanide-m-chlorphenylhydrazone (20 micromolar) and KCN (0.5 millimolar), also partially inhibited the saturable component for putrescine uptake (V(max) reduced 48-60%). Increasing the time of exposure to carbonylcyanide-m-chlorphenylhydrazone from 30 minutes to 2 hours did not significantly increase the inhibition of putrescine uptake. Electrophysiological evidence indicates that the inhibitory effect on putrescine uptake by these inhibitors is correlated to a depolarization of the membrane potential, suggesting that the driving force for putrescine uptake is the transmembrane electrical potential across the plasmalemma.

Physical restraints underlying short-term inhibition by auxin of root elongation in intact maize seedlings

This report investigates physical changes associated with the short-term inhibition of root elongation in intact maize seedlings (Zea mays L. vs. Halamish) by exogenous auxin. Movement of root tips was assayed by video microscopy in control roots, roots grown for 45 min in 10−6 M indole3-acetic acid (IAA), or roots chilled for 3 min at 11°C. IAA and chilling treatments similarly reduced root elongation rates (from 29 ± 6 μm min−1 to 6 ± 2 μm min−1). Initial rates of root tip contraction induced by 300 mOsmol mannitol were used to calculate tissue contractibility values. These allowed a comparison of effects of IAA and chilling treatments on apparent rates of water transport out of the root tip tissues. Chilling treatment reduced root tip contractibility by 66%, whereas IAA had much less effect (26% reduction). Roots were also exposed to an osmotic jump treatment; the initial osmotically induced increase in elongation rate was used to determine root tip extensibility values. Both IAA and chilling treatments reduced root tip extensibilities by 57%. Inhibition of wall-yielding properties, rather than hydraulic limitations, appeared to be primarily associated with inhibition of intact root tip elongation by exogenous IAA.