Effects of 4-Chloro-2-methylphenoxypropionate (an Auxin Analogue) on Plasma Membrane ATPase Activity in Herbicide-Resistant and Herbicide-Susceptible Biotypes of Stellaria media L.

ATPase activity was examined in plasma membrane (PM) fractionsprepared from mecoprop-resistant and -susceptible biotypes ofStellaria media L. (chickweed). Treatment with the herbicidecaused an 18% increase in ATP hydrolysis, but this was not significantlydifferent from control plants and was similar for both biotypes.However, there was an overall significant biotype effect, herbicide-resistantplants having greater enzyme activity than susceptible ones.Proton-pumping was readily demonstrated in PM fractions obtainedfrom both biotypes using the fluorescent probe amino-chloro-methoxyacridine(ACMA), indicating a relatively large proportion of 'inside-out'vesicles. Proton-pumping was significantly greater in PM preparationsobtained from the resistant compared with susceptible plants.The differences in ATPase activity between the two biotypescould not be attributed to differences in the main sterol orphospholipid components of the PM. There were no effects ofthe herbicide on ATP hydrolysis in vitro, but proton-pumpingwas affected in a herbicide concentration-dependent manner.At 1·0 mol m mecoprop caused an increase in the rateof proton-pumping, whereas at 10 and 100 mol m^–6, an inhibitionin this rate was observed. Both biotypes behaved similarly,irrespective of mecoprop concentration. These data indicatethat mecoprop resistance in chickweed is unlikely to be dueto a direct effect of the herbicide on PM H+-ATPase activity. Key words: Stellaria media, mecoprop, ATPase, plasma membrane, herbicide resistance     

Ethylene, seed germination, and epinasty

Ethylene activity in lettuce seed (Lactuca satina) germination and tomato (Lycopersicon esculentum) petiole epinasty has been characterized by using heat to inhibit ethylene synthesis. This procedure enabled a separation of the production of ethylene from the effect of ethylene. Ethylene was required in tomato petioles to produce the epinastic response and auxin was found to be active in producing epinasty through a stimulation of ethylene synthesis with the resulting ethylene being responsible for the epinasty. In the same manner, it was shown that gibberellic acid stimulated ethylene synthesis in lettuce seeds. The ethylene produced then in turn stimulated the seeds to germinate. It was hypothesized that ethylene was the intermediate which caused epinasty or seed germination. Auxin and gibberellin primarily induced their response by stimulating ethylene production.     

Xylem Transport of 1-Aminocyclopropane-1-carboxylic Acid, an Ethylene Precursor, in Waterlogged Tomato Plants

Waterlogging is known to cause an increase in ethylene synthesis in the shoot which results in petiole epinasty. Evidence has suggested that a signal is synthesized in the anaerobic roots and transported to the shoot where it stimulates ethylene synthesis. Experimental data are presented showing that 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, serves as the signal. Xylem sap was collected from detopped tomato plants (Lycopersicon esculentum Mill. cv. VFN8). ACC in the sap was quantitated by a sensitive and specific assay, and its tentative chemical identity verified by paper chromatography. ACC levels in both roots and xylem sap increased markedly in response to waterlogging or root anaerobiosis. The appearance of ACC in the xylem sap of flooded plants preceded both the increase in ethylene production and epinastic growth, which were closely correlated. Plants flooded and then drained showed a rapid, simultaneous drop in ACC flux and ethylene synthesis rate. ACC supplied through the cut stem of tomato shoots at concentrations comparable to those found in xylem sap caused epinasty and increased ethylene production. These data indicate that ACC is synthesized in the anaerobic root and transported to the shoot where it is readily converted to ethylene.     

Prevention of salt-induced epinasty by α-aminooxyacetic acid and cobalt

Ethylene production in leaf petiole and laminae tissues was stimulated in tomato (Lycopersicon esculentum Mill. cv. UCT5) plants exposed to salinity-stress. At the highest salinity level (250 mM NaCl), rates of ethylene production more than doubled over those observed in non-stressed plants. Correspondingly, petiolar epinasty increased with increasing levels of stress impositions. Both responses were suppressed when either 1 mM -aminooxyacetic acid (AOA), or 100 M Co²⁺ was simultaneously applied. Co²⁺, but not AOA, had a pronounced effect on ethylene production resulting from the application of a saturating dose (2 mM) of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene. This result suggests that ethylene production is dependent upon the activity of ethylene forming enzyme (EFE). The magnitude of ethylene stimulation in leaf petioles was related to the salinity level imposed and to the induction of petiole epinasty. In the absence of stress impositions, epinastic responsiveness to ethylene or its precursor, ACC, might provide a simple, indirect criteria to adjudge salt-sensitivity among plants.Research supported by AID contract II, NEB-1070-A-00-2074-00.     

Auxin and Ethylene Regulation of Petiole Epinasty in Two Developmental Mutants of Tomato, diageotropica and Epinastic

The epinastic growth responses of petioles to auxin and ethylene were quantified in two developmental mutants of tomato (Lycopersicon esculentum Mill.). In the wild type parent line, cultivar VFN8, the epinastic response of excised petiole sections was approximately log-linear between 0.1 and 100 micromolar indole-3-acetic acid (IAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) concentrations, with a greater response to 2,4-D at any concentration. When ethylene synthesis was inhibited by aminoethoxyvinylglycine (AVG), epinasty was no longer induced by auxin, but could be restored by the addition of ethylene gas. In the auxin-insensitive mutant, diageotropica (dgt), no epinastic response to IAA was observed at IAA concentrations that effectively induced epinasty in VFN8. In the absence of added IAA, epinastic growth of dgt petioles in 1.3 microliters per liter exogenous ethylene gas was more than double that of VFN8 petioles. IAA had little additional effect in dgt, but promoted epinasty in VFN8. These results confirm that tomato petiole cells respond directly to ethylene and make it unlikely that the differential growth responsible for epinasty results from lateral auxin redistribution. The second mutant, Epinastic (Epi), exhibits constitutively epinasty, cortical swelling, and root branching symptomatic of possible alternation in auxin or ethylene regulation of growth. Only minor quantitative differences were observed between the epinastic responses to auxin and ethylene of VFN8 and Epi. However, in contrast to VFN8, when ethylene synthesis or action was inhibited in Epi, auxin still induced 40 to 50% of the epinastic response observed in the absence of inhibitors. This indicates that the target cells for epinastic growth in Epi are qualitatively different from those of VFN8, having gained the ability to grow differentially in response to auxin alone. The dgt and Epi mutants provide useful systems in which to study the genetic determination of target cell specificity for hormone action.     

Brassinosteroid-induced epinasty in tomato plants

The effects of root treatments of brassinosteroid (BR) on the growth and development of hydroponically grown tomato plants (Lycopersicon esculentum Mill cv Heinz 1350) were evaluated. There was a dramatic increase in petiole bending when the plants were treated with 0.5 to 1.0 micromolar BR. The leaf angle of the treated plants was almost three times that of untreated controls. BR-induced epinasty appeared to be due to stimulation of ethylene production. Excised petioles from BR-treated plants produced more than twice as much ethylene as did untreated controls. As ethylene production increased, the degree of petiole bending also increased, and inhibition of ethylene production by AOA or CoCl₂ also inhibited epinasty. BR-treated plants had increased levels of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) in the leaf tissue. ACC appeared to accumulate primarily in the petioles with the greatest amount of ACC accumulating in the youngest petioles. Time course evaluations revealed that BR treatment stimulated ACC production. As ACC accumulated, ethylene increased, resulting in epinasty. Little or no ACC was found in the xylem sap, indicating that there was a signal transported from the roots which stimulated ACC synthesis in the leaf tissue.     

Inhibition of ethylene synthesis in tomato plants subjected to anaerobic root stress

Enhanced ethylene production and leaf epinasty are characteristic responses of tomato (Lycopersicon esculentum Mill.) to waterlogging. It has been proposed (Bradford, Yang 1980 Plant Physiol 65: 322-326) that this results from the synthesis of the immediate precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), in the waterlogged roots, its export in the transpiration stream to the shoot, and its rapid conversion to ethylene. Inhibitors of the ethylene biosynthetic pathway are available for further testing of this ACC transport hypothesis: aminooxyacetic acid (AOA) or aminoethoxyvinylglycine (AVG) block the synthesis of ACC, whereas CO²⁺ prevents its conversion to ethylene. AOA and AVG, supplied in the nutrient solution, were found to inhibit the synthesis and export of ACC from anaerobic roots, whereas Co²⁺ had no effect, as predicted from their respective sites of action. Transport of the inhibitors to the shoot was demonstrated by their ability to block wound ethylene synthesis in excised petioles. All three inhibitors reduced petiolar ethylene production and epinasty in anaerobically stressed tomato plants. With AOA and AVG, this was due to the prevention of ACC import from the roots as well as inhibition of ACC synthesis in the petioles. With Co²⁺, conversion of both root- and petiole-synthesized ACC to ethylene was blocked. Collectively, these data support the hypothesis that the export of ACC from low O₂ roots to the shoot is an important factor in the ethylene physiology of waterlogged tomato plants.     

The effects of the herbicide quinclorac on shoot growth in tomato is alleviated by inhibitors of ethylene biosynthesis and by the presence of an antisense construct to the 1-aminocyclopropane-1-carboxylic acid (ACC) synthase gene in transgenic plants

Reduction of shoot growth, leaf epinasty and chlorosis in young tomato plants (Lycopersicon esculentum Mill. cv. Hellfrucht/Frühstamm) treated hydroponically with 10^-7 M of the herbicide quinclorac were partially compensated when the plants were simultaneously sprayed with salicyclic acid or the oxime ether derivative PACME. Since salicyclic acid and PACME are known inhibitors of ethylene biosynthesis, it is suggested that this pathway is implicated in quinclorac action. Further support for this hypothesis was obtained in experiments with transgenic tomato plants containing an antisense gene to 1-aminocyclopropane-1-carboxylic acid (ACC) synthase in ethylene biosynthesis. When quinclorac was applied via the root antisense plants showed reduced phenotypical alterations compared to those of wild-type plants.     

Effects of root anaerobiosis on ethylene production, epinasty, and growth of tomato plants

Experiments were performed to determine the source(s) of ethylene-causing epinasty in flooded tomato plants (Lycopersicon esculentum Mill.). Simultaneous measurements were made of ethylene synthesized by the roots and shoots of tomato plants exposed to either aerobic or anaerobic atmospheres in the root zone. When the root zone was made anaerobic by a flowing stream of N₂ gas, petiole epinasty and accelerated ethylene synthesis by the shoots were observed. In soil-grown plants, ethylene synthesis by the root-soil complex increased under anaerobic conditions; but when grown in inert media under the same conditions, ethylene synthesis by roots remained constant or declined during the period of rapid epinastic growth by the petioles. Other characteristic symptoms of flooding, e.g. reduced growth and chlorosis, were also observed in plants with anaerobic roots. Pretreatment of plants with AgNO₃, an inhibitor of ethylene action, completely prevented epinasty, demonstrating that ethylene is the agent responsible for waterlogging symptoms. These results indicate that deprivation of O₂ to the roots is the primary effect of soil flooding, and that this is sufficient to cause increased ethylene synthesis in the shoot. The basis of the observed root-shoot communication is unknown, but root-synthesized hormones or specific ethylene-promoting factors may be involved.     

Effects of virus genotype and temperature on seed transmission of nepoviruses

Transmission of different nepoviruses through chickweed (Stellaria media) seed was differently affected by ambient temperature during seed production. Raspberry ringspot and tomato black ring (Scottish isolate) viruses were similarly and frequently transmitted at 14 , 18 and 22 ^oC, whereas arabis mosaic virus was transmitted most frequently at 14 ^oC, and strawberry latent ringspot and tomato black ring (German isolate) viruses at 22 ^oC. When infected by seed-borne nepoviruses, seedlings of S. media and other species were symptomless at 15–25 ^oC, and the viruses were therefore detected by inoculating sap to Chenopodium quinoa indicator plants. However, typical symptoms of arabis mosaic and tomato black ring viruses were induced by growing Nicotiana tabacum, N. clevelandii and C. quinoa seedlings infected with seed-borne virus at 33–37 ^oC during the third and fourth weeks after sowing, preceded and followed by periods at 15–25 ^oC. The proportion of N. tabacum seedlings developing symptoms was the same as that of untreated seedlings yielding sap-transmissible virus. Seed transmissibility of pseudo-recombinant isolates of raspberry ringspot and tomato black ring viruses, containing RNA-i from one virus strain and RNA-2 from another strain, depended greatly on the transmissibility of the strain contributing RNA-i. The source of RNA-2 had an additional but smaller influence. The satellite RNA (RNA-3) of tomato black ring virus was seed-transmitted in S. media and its occurrence in cultures did not affect the frequency of transmission of the virus. Results of testing the infectivity of extracts of seed from infected mother plants suggested that failure of seed transmission reflected failure to become established in the seed, not subsequent inactivation. Whereas seed transmissibility of raspberry ringspot virus is primarily dependent on information carried in RNA-i, transmissibility by nematode vectors, another property of major ecological importance, is determined by RNA-2. In the field, selection pressures presumably can act independently on the two parts of the genome but evidence was also obtained of selection for mutual compatibility of RNA-i and RNA-2.     

Effects of Poa trivialis, Stellaria media and Rumex obtusifolius on the growth of Lolium perenne in the glasshouse

In glasshouse experiments shoot dry weight, leaf area and tiller number of Lolium perenne, cv. S. 24, were compared when grown in monoculture and in competition with Poa trivialis, Stellaria media or Rumex obtusifolius at a range of densities and for different time intervals. The results showed that S. media and R. obtusifolius were more competitive than P. trivialis. Competition effects with S. media and R. obtusifolius appeared between the 4 and 6 wk stage, coinciding with the production of the first S. 24 tillers. Spraying with 2,4-D (0·84 kg acid equivalent/ha) for control of R. obtusifolius did not decrease grass yield whereas mecoprop (2·80 kg a.e./ha) for control of S. media damaged young L. perenne. It appeared that the optimum time for mecoprop treatment was between 4 and 6 wk after sowing when tillering had commenced. At this stage weed competition was only beginning and crop tolerance to the herbicide was adequate.     

Epinasty of Poinsettias-the Role of Auxin and Ethylene

Upward physical restraint of the normally horizontal bracts of poinsettia (Euphorbia pulcherrima Willd.) resulted in increased ethylene production and epinastic curvature of the petioles after 5 days. Downward restraint caused little change in ethylene production or epinasty, indicating that the enhanced ethylene production observed in petioles bent upwards is not due to the bending stress alone. Epinasty, measured upon removal of upward physical restraint, was not affected by spraying plants with aminoxyacetic acid to reduce ethylene production or with silver thiosulfate to prevent ethylene action. Removal of the bract blades prevented the epinastic response of the petiole, and the response was restored by applying indoleacetic acid to the cut petiole end. Redistribution of auxin appears to be responsible for both the epinasty and the increased ethylene production of reoriented poinsettia bracts.     

Rate of Change in Dormancy Level and Light Requirement in Weed Seeds During Stratification

The rate of change in dormancy level and light requirement,induced during stratification at 3.2 °C, was investigatedin seeds ofStellaria media, Cerastium fontanum, Veronica agrestisandTaraxacumofficinale.Two stratification environments, soil and wet filterpaper in petri-dishes, were used. On nine occasions during a6 week stratification period, germination was tested under threelight conditions at 3.5/18.5 °C: (1) darkness; (2) light;and (3) short-term light exposure followed by darkness. Priorto stratification, germination in all species was 89% in lightand 53% in darkness. Within 2 weeks of stratification, germinabilityinV. agrestisandS. mediaseeds decreased in all treatments. InC.fontanumandT. officinaleseeds, germinability also decreasedafter 2 weeks but only in the dark treatment, indicating inductionof a light requirement. After two more weeks of stratification,the induced dormancy inS. mediaseeds stratified in soil becameweaker and the light requirement inT. officinaleseeds was lost.Differences between the two stratification environments and/orinteractions between light conditions, stratification environmentsand time were found for all species. These results suggest that:(1) the dormancy level and the light requirement of seeds maychange dramatically over relatively short-time periods duringstratification; and (2) germinability depends on the stratificationenvironment experienced by the seeds. Predicting the dormancylevel or light requirement in a seed batch is difficult andrequires a thorough knowledge of the effect of the stratificationconditions used.Copyright 1997 Annals of Botany Company Burial; Cerastium fontanumBaumg.; chickweed; cold stratification; common mouse-ear; dandelion; dormancy; light requirement; seed; Stellaria media(L.) Vill.; speedwell; Taraxacum officinalegroup; Veronica agrestisL.; weed     

Increased 1-Aminocyclopropane-1-Carboxylic Acid Oxidase Activity in Shoots of Flooded Tomato Plants Raises Ethylene Production to Physiologically Active Levels

Soil flooding increased 1-aminocyclopropane-1-carboxylic (ACC) acid oxidase activity in petioles of wild-type tomato (Lycopersicon esculentum L.) plants within 6 to 12 h in association with faster rates of ethylene production. Petioles of flooded plants transformed with an antisense construct to one isoform of an ACC oxidase gene (ACO1) produced less ethylene and had lower ACC oxidase activity than those of the wild type. Flooding promoted epinastic curvature but did so less strongly in plants transformed with the antisense construct than in the wild type. Exogenous ethylene, supplied to well-drained plants, also promoted epinastic curvature, but transformed and wild-type plants responded similarly. Flooding increased the specific delivery (flux) of ACC to the shoots (picomoles per second per square meter of leaf) in xylem sap flowing from the roots. The amounts were similar in both transformed and wild-type plants. These observations demonstrate that changes in ACC oxidase activity in shoot tissue resulting from either soil flooding or introducing ACC oxidase antisense constructs can influence rates of ethylene production to a physiologically significant extent. They also implicate systemic root to shoot signals in regulating the activity of ACC oxidase in the shoot.     

An Evaluation of the Effects of Ethylene on Carbon Assimilation in Lycopersicon esculentum Mill

Leaf and whole plant gas exchange rates of Lycopersicon esculentumMill, were studied during several days of continuous exposureto ethylene. Steady-state photosynthesis and transpiration ratesof control and ethylene-treated individual leaves were equivalent.However, the photosynthesis and transpiration rates of treatedleaves required at least five times longer to reach 50% of thesteady-state rate. This induction lag was attributed to ethylene—inducedleaf epinasty and temporary acclimation to lower incident lightlevels immediately prior to measurement of gas exchange. Thewhole plant net carbon exchange rate (NCER) of a representativetreated plant was also reduced by 51% after 24 h exposure toethylene relative to both its pre-treatment rate and that ofthe control. Ethylene exposure reduced the growth rate of thetreated plant by 50% when expressed as carbon (C) gain. Theinhibition of NCER and growth rate associated with epinastywas completely reversed when the epinastic leaves were returnedto their original positions and light interception was re-established.The results demonstrate that the inhibition of whole plant CO₂assimilation is indirect and due to reduced light interceptionby epinastic leaves. Morphological changes caused by environmentalethylene are thus shown to reduce plant C accumulation withoutinhibiting leaf photosynthesis processes per se. Key words: Ethylene, carbon assimilation, growth     

Interpreting Plant Responses to Clinostating: I. MECHANICAL STRESSES AND ETHYLENE

The severe epinasty and other symptoms developed by clinostated leafy plants could be responses to gravity compensation and/or the mechanical stresses of leaf flopping. Epinasty in cocklebur (Xanthium strumarium L.), tomato (Lycopersicon esculentum Mill.), and castor bean (Ricinus communis L.) is delayed by inhibitors of ethylene synthesis and action (aminoethoxyvinylglycine and Ag⁺), confirming the role of ethylene in clinostat epinasty. To test the possibility that clinostat mechanical stresses (leaf flopping) cause ethylene production and, thus, epinasty, vertical plants were stressed with constant, gentle, horizontal, or vertical shaking or with a quick, back-and-forth rotation (twisting). Clinostat leaf flopping was closely approximated but with a minimum of gravity compensation, by turning plants so their stems were horizontal, rotating them quickly about the stem axis, and then returning them to the vertical, repeating the treatment every four minutes (clinostat rotation time). None of these mechanical stresses produced significant epinasties, but vigorous hand-shaking (120 seconds per day) generated minor epinasties, as did Ag⁺ applied daily (concentrations high enough to cause leaf browning). Plants gently inverted every 20 minutes developed epinasty at about the same rate and to about the same extent as clinostated plants, but plants inverted every 20 minutes and immediately returned to the upright position did not become epinastic. It is concluded that clinostat epinasty is probably caused by disturbances in the gravity perception mechanism, rather than by leaf flopping.     

Effect of chlorsulfuron on ethylene evolution from sunflower seedlings

The effect of the herbicide chlorsulfuron (2-chloro-N-[(4-methoxy - 6 - methyl -1, 3,5 - triazin - 2 - yl)aminocarbonyl]benzenesulfonamide) on ethylene production in light-grown sunflower (Helianthus annuus L.) seedlings was examined. Application of chlorsulfuron to the apex stimulated ethylene production in all tissues examined: cotyledons, hypocotyls, and roots. The greatest stimulation occurred in the upper portion of the hypocotyl adjacent to, and including, the cotyledonary node. Ethylene evolution from hypocotyls excised from treated seedlings was stimulated over control levels 1 day after herbicide application and reached a maximum (approx. 75 x control or 17 nl/g f wt/h) 2 to 3 days after treatment. Labeling and inhibitor studies indicated that the ethylene produced was derived primarily from methionine. Chlorsulfuron treatment stimulated the rate of accumulation of the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), as well as the ability of the tissue to convert exogenous ACC to ethylene. Chlorsulfuron had little effect on ethylene production when administered to the hypocotylsin vitro. Removal of the cotyledons from treated seedlings reduced the rate of ethylene evolution from the hypocotyls. These results suggest that stimulation of ethylene production in sunflower hypocotyls by chlorsulfuron is not a wound response but rather is dependent on factors derived from the cotyledons.     

Failure of Ethylene to Change the Distribution of Indoleacetic Acid in the Petiole of Coleus blumei X frederici during Epinasty

The effect of ethylene on the distribution of applied indoleacetic acid in the petiole of Coleus blumei Benth. X C. frederici G. Taylor has been investigated during the development of epinastic curvature. Using intact plants, ¹⁴C-IAA was applied to the distal region of the leaf lamina and the accumulation of label in the abaxial and adaxial halves of 5 mm petiole sections was determined after 1.5, 3, and 6 hours. Over this period the label was transported out of the lamina into the petiole at a rate of at least 66 mm hr⁻¹. Of the total amount of label in the petiole sections, 24 to 30% was located in the adaxial half and this distribution was not altered significantly by exposing plants to an atmosphere containing 50 μl/l ethylene. Thus when epinastic curvature is induced by ethylene there is no associated increase in the IAA content of the expanding adaxial half. The role of endogenous IAA in petiole epinasty was studied by restricting its movement with DPX 1840 (3,3a-dihydro-2-[p-methoxyphenyl]-8H-pyrozolo{5,1-a}isoindol-8-one). The leaf petioles still showed an initial epinastic response to ethylene. It is concluded that ethylene-induced epinasty is not dependent upon either any change in the transport of IAA or its redistribution within the petiole.     

Genetic dissection of the role of ethylene in regulating auxin-dependent lateral and adventitious root formation in tomato

In this study we investigated the role of ethylene in the formation of lateral and adventitious roots in tomato ( Solanum lycopersicum ) using mutants isolated for altered ethylene signaling and fruit ripening. Mutations that block ethylene responses and delay ripening – Nr ( Never ripe ), gr ( green ripe ), nor ( non ripening ), and rin ( ripening inhibitor ) – have enhanced lateral root formation. In contrast, the epi ( epinastic ) mutant, which has elevated ethylene and constitutive ethylene signaling in some tissues, or treatment with the ethylene precursor 1-aminocyclopropane carboxylic acid (ACC), reduces lateral root formation. Treatment with ACC inhibits the initiation and elongation of lateral roots, except in the Nr genotype. Root basipetal and acropetal indole-3-acetic acid (IAA) transport increase with ACC treatments or in the epi mutant, while in the Nr mutant there is less auxin transport than in the wild type and transport is insensitive to ACC. In contrast, the process of adventitious root formation shows the opposite response to ethylene, with ACC treatment and the epi mutation increasing adventitious root formation and the Nr mutation reducing the number of adventitious roots. In hypocotyls, ACC treatment negatively regulated IAA transport while the Nr mutant showed increased IAA transport in hypocotyls. Ethylene significantly reduces free IAA content in roots, but only subtly changes free IAA content in tomato hypocotyls. These results indicate a negative role for ethylene in lateral root formation and a positive role in adventitious root formation with modulation of auxin transport as a central point of ethylene–auxin crosstalk.