Role of extensin peroxidase in tomato (Lycopersicon esculentum Mill.) seedling growth

 It is proposed that inhibition of extensin peroxidase activity leads to a less rigid cell wall and thus promotes cell expansion and plant growth. A low-molecular-weight inhibitor derived from the cell walls of suspension-cultured tomato cells was found to completely inhibit extensin peroxidase-mediated extensin cross-linking in vitro at a concentration of 260 μg/ml. The inhibitor had no effect upon guaiacol oxidation catalyzed by extensin peroxidase or horseradish peroxidase. We have demonstrated that the light-irradiated inhibition of plant growth may be partially offset by inhibition of endogenous extensin peroxidase activity. Overall plant growth was enhanced by up to 15% in the presence of inhibitor relative to control plants. Inhibitor-treated and illuminated tomato hypocotyls grew up to 15% taller than untreated controls. The inhibitor had no effect upon etiolated plants over a 15-d period, suggesting that only low levels of peroxidase-mediated cross-linking can be found in the cell walls of etiolated plants. SDS-PAGE/Western blots of ionically bound protein from both etiolated and illuminated hypocotyls identified a doublet at 57/58.5 kDa which is immuno-reactive with antibodies raised to tomato extensin peroxidase. Levels of the 58.5-kDa protein, determined by SDS-PAGE, were at least threefold higher in illuminated tomato hypocotyls than in etiolated hypocotyls. Three fold higher levels of extensin peroxidase, elevated in-vitro extensin cross-linking activity and 15% higher levels of cross-linked, non-extractable extensin were observed in illuminated tomato hypocotyls compared with etiolated tomato hypocotyls. This suggests that white-light inhibition of tomato hypocotyl growth appears to be mediated, at least partially, by deposition of cell wall extensin, a process regulated by M_r-58,500 extensin peroxidase. Our results indicate that the contribution of peroxidase-mediated extensin deposition to plant cell wall architecture may have an important role in plant growth. Received: 22 July 1999 / Accepted: 11 October 1999     

Exogenous Ethylene Inhibits Nodulation of Pisum sativum L. cv Sparkle

Exogenous ethylene inhibited nodulation on the primary and lateral roots of pea, Pisum sativum L. cv Sparkle. Ethylene was more inhibitory to nodule formation than to root growth; nodule number was reduced by half with only 0.07 μL/L ethylene applied continually to the roots for 3 weeks. The inhibition was overcome by treating roots with 1 μm Ag⁺, an inhibitor of ethylene action. Exogenous ethylene also inhibited nodulation on sweet clover (Melilotus alba) and on pea mutants that are hypernodulating or have ineffective nodules. Exogenous ethylene did not decrease the number of infections per centimeter of lateral pea root, but nearly all of the infections were blocked when the infection thread was in the basal epidermal cell or in the outer cortical cells.     

In vitro cross-linking of extensin precursors by mustard extracellular isoforms of peroxidase that respond either to phytochrome or to wounding

The activities of extracellular peroxidase isoforms A3 and A4 from mustard (Sinapis alba) are known to respond to wounding treatments and to phytochrome status, respectively. To investigate the affinity of A3 for extensin precursors in vitro, these acidic isoforms were extracted by low-speed centrifugation of intact mustard internodes infiltrated with CaCl2 and isolated by chromatofocusing. Extensin precursors from carrot (Daucus carota) roots or mustard stems and leaves were isolated by saline extraction followed by purification on carboxymethyl-cellulose ion exchange and gel-filtration chromatography. Cross-linking of extensin precursors in vitro in the presence of peroxidase isoforms and exogenous H2 O2, was quantified following Sephacryl S-400 gel-filtration, as a shift of extensins to higher molecular mass values. Isoforms A3 A4 had similar affinities for natural extensin precursors. A cationic isoform (previously not characterized) was unable to cross-link extensin precursors. Tissue prints of mustard stems indicate that extensin precursors are present in the cell wall of all the tissues, with maximum staining in vascular bundles and epidermis. Isoforms A3 and A4 were detected in extracts from vascular bundles, cortex and pith. Only A3 was detected in epidermal extracts. The observations are consistent with a role of isoforms A3 and A4 in cross-linking of extensin in muro.Keywords: Peroxidase isoforms, mustard, extensin, carrot, cell wall proteins.      

Mechanism of Auxin-induced Ethylene Production

Indoleacetic acid-induced ethylene production and growth in excised segments of etiolated pea shoots (Pisum sativum L. var. Alaska) parallels the free indoleacetic acid level in the tissue which in turn depends upon the rate of indoleacetic acid conjugation and decarboxylation. Both ethylene synthesis and growth require the presence of more than a threshold level of free endogenous indoleacetic acid, but in etiolated tissue the rate of ethylene production saturates at a high concentration and the rate of growth at a lower concentration of indoleacetic acid. Auxin stimulation of ethylene synthesis is not mediated by induction of peroxidase; to the contrary, the products of the auxin action which induce growth and ethylene synthesis are highly labile.     

The mobilization of defence mechanisms in the early stages of pea seed germination against Ascochyta pisi

Ascochyta pisi is a necrotrophic pathogenic fungus, which mainly survives between seasons through infected seeds. Defence responses of pea embryo axes to A. pisi were investigated in the heterotrophic phase of seed germination and during the transition from the heterotrophic to the autotrophic phase. Germinated pea seeds, both non-inoculated and inoculated with A. pisi, were cultured in perlite for 96 h. Polarographic studies performed on intact embryo axes of germinating pea seeds infected with A. pisi showed a high respiratory intensity in time from 48 to 96 h after inoculation. Forty-eight-hour embryo axes of germinating pea seeds exhibited the highest respiration rate, which in infected axes was maintained at the following time points after inoculation. Moreover, at 72 and 96 h after inoculation, respiratory intensity was by 64% and 73% higher than in the control. Electron paramagnetic resonance analysis revealed a higher concentration of semiquinone free radicals with g values of g _||?=?2.0031?±?0.0004 and g _⊥?=?2.0048?±?0.0004 in infected axes than in the control. Generation of superoxide anion radical was also higher in infected axes than in the control but stronger at 72 and 96 h after inoculation. Starting from 72 h after infection, the level of Mn²⁺ ions in infected axes decreased in relation to the control. At the same time, the highest activity of superoxide dismutase (EC 1.15.1.1) and catalase (EC 1.11.1.6) was observed in 72-h infected axes. In turn, the activity of peroxidase (EC 1.11.1.7) up to 72 h after infection was lower than in the control. In 48-h infected embryo axes, a very high level of pterocarpan pisatin was observed. Infection of germinating pea seeds with A. pisi restricted mainly the growth of the epicotyl, but did not inhibit the increase in length and fresh weight of root embryo axes versus cultivation time. These results indicate that in pea during the stages of seed germination and early seedling growth, protective mechanisms are induced in embryo axes against A. pisi.     

Effect of salinity on growth, ion content, and cell wall chemistry in Atriplex prostrata (Chenopodiaceae)

Atriplex prostrata Boucher, a facultative halophyte, exhibits significant reduction in height and biomass and in the width of the cortex and vascular tissue under saline conditions. Therefore, the goal of this investigation was to determine the effect of salinity on plant growth as well as on the patterns of lignification, peroxidase activity, and extensin deposition. Biomass, leaf area, internode length, water potential, photosynthesis, transpiration, and ion content were measured. In addition, lignin, peroxidase, and extensin were, respectively, examined via phloroglucinol staining, peroxidase staining, and immunostaining with extensin antibody on tissue prints of free-hand stem sections. Length of internodes and leaf area significantly decreased with increased salinity, and net photosynthesis declined dramatically as well. There was a significant accumulation of Na+ in organs when plants were grown in saline solutions, while the concentration of K+, Ca2+, and Mg2+ decreased. The signals in tissue prints showed that soluble peroxidase and extensin accumulated in the first three internodes of A. prostrata grown under saline conditions. In contrast, lignification was reduced under saline growth conditions in the third and fourth internodes. These results indicate that extensin may replace lignin in providing mechanical support for cells, while stems remain in a juvenile stage because of growth retardation caused by salinity.     

Immunocytolocalization of extensin in developing soybean seed coats by immunogold-silver staining and by tissue printing on nitrocellulose paper

In soybean seed coats the accumulation of the hydroxyproline-rich glycoprotein extensin is regulated in a developmental and tissue- specific manner. The time course of appearance of extensin during seed development was studied by Western blot analysis and by immunogold- silver localization. Using these techniques extensin was first detected at 16-18 d after anthesis, increasing during development to high levels at 24 d after anthesis. Immunogold-silver localization of extensin in the seed coat showed marked deposition of the glycoprotein in the walls of palisade epidermal cells and hourglass cells. The immunolocalization of extensin in developing soybean seeds was also made by a new technique--tissue printing on nitrocellulose paper. It was found that extensin is primarily localized in the seed coat, hilum, and vascular elements of the seed.     

Orientation of cell growth in the etiolated pea stem

Summary Both ethylene and IAA induce swelling in the sub-apical region of etiolated pea plants. The modified cells of these two types of swellings differ both morphologically and in their enzyme composition. In ethylene the cell walls become thickened within 24 h and the level of peroxidase is enhanced; ethylene does not affect cellulase levels. IAA induced swellings are not accompanied by early thickening of cell walls or enhanced peroxidase activity, but IAA greatly increases the level of cellulase. It is proposed that the retardation of extension growth by ethylene treatment results from the deposition of longitudinal microfibrils in the walls and that cross linking bonds in the polysaccharide matrix prevent their separation. Lateral expansion can occur, however, in the presence of auxin-induced cellulase which breaks or prevents the formation of these bonds.     

Peroxidases in Tobacco Abscission Zone Tissue: II. Time Course Studies of Peroxidase Activity during Ethylene-induced Abscission

Ethylene-induced abscission in flower pedicels of Nicotiana tabacum L. cv. Little Turkish causes a progressive increase in peroxidase activity during the first 4 hours of a 5-hour time course ethylene treatment period, with decrease in peroxidase activity occurring between 4 hours and 5 hours, when the supernatant extracts of abscission zone segments are tested spectrophotometrically for peroxidase activity, using guaiacol and hydrogen peroxide. Nonethylene-treated tissue has a much lower level of peroxidase activity over the same time course period. In ethylene-treated tissue the decline in break-strength correlates with the beginning of increase in peroxidase activity (3 hours). When the abscission zone area of the pedicel is further divided into proximal, abscission zone, and distal portions, respectively, the ethylene-treated tissue has the highest peroxidase activity in the abscission zone portion, with the maximum peak occurring at 4 hours and decreasing between 4 hours and 5 hours. Acrylamide gel electrophoresis of enzyme breis from ethylene-treated aand nonethylene-treated plants reveals that no new peroxidase isozymes are formed in response to ethylene, indicating an increase in the amount of one or in both of the two already existing isozyme banding patterns. The measurement of protein in the proximal, abscission zone, and distal segments, over a 5-hour ethylene treatment period, indicates that it is being translocated in a distal to proximal direction in the abscission zone pedicel. The possible participatory role for peroxidase in ethylene-induced tobacco flower pedicel abscission are discussed.     

Stress-Induced Legume Root Nodule Senescence. Physiological, Biochemical, and Structural Alterations

Nitrate-fed and dark-stressed bean (Phaseolus vulgaris) and pea (Pisum sativum) plants were used to study nodule senescence. In bean, 1 d of nitrate treatment caused a partially reversible decline in nitrogenase activity and an increase in O₂ diffusion resistance, but minimal changes in carbon metabolites, antioxidants, and other biochemical parameters, indicating that the initial decrease in nitrogenase activity was due to O₂ limitation. In pea, 1 d of dark treatment led to a 96% decline in nitrogenase activity and sucrose, indicating sugar deprivation as the primary cause of activity loss. In later stages of senescence (4 d of nitrate or 2–4 d of dark treatment), nodules showed accumulation of oxidized proteins and general ultrastructural deterioration. The major thiol tripeptides of untreated nodules were homoglutathione (72%) in bean and glutathione (89%) in pea. These predominant thiols declined by approximately 93% after 4 d of nitrate or dark treatment, but the loss of thiol content can be only ascribed in part to limited synthesis by γ-glutamylcysteinyl, homoglutathione, and glutathione synthetases. Ascorbate peroxidase was immunolocalized primarily in the infected and parenchyma (inner cortex) nodule cells, with large decreases in senescent tissue. Ferritin was almost undetectable in untreated bean nodules, but accumulated in the plastids and amyloplasts of uninfected interstitial and parenchyma cells following 2 or 4 d of nitrate treatment, probably as a response to oxidative stress.     

Purification and Partial Characterization of Tomato Extensin Peroxidase

Early plant defense response is characterized by elevation of activity of peroxidases and enhanced insolubilization of hydroxyproline-rich glycoproteins, such as extensin, in the cell wall. The insolubilization process (cross-linking between soluble extensin precursor molecules) is catalyzed by extensin peroxidases. We have ionically eluted extensin peroxidases from intact water-washed suspension-cultured tomato (hybrid of Lycopersicon esculentum Mill. and Lycopersicon peruvianum L. [Mill.]) cells and purified them to homogeneity by molecular sieve and cation-exchange chromatography. Four ionic forms of peroxidase (PI,PII,EPIII, and EPIV) were resolved; only the latter two cross-linked tomato soluble extensin. The molecular weight (34,000-37,000), amino acid composition, and isoelectric point (9.0) of the extensin peroxidases were determined. Substrate specificities of the enzymes were investigated: soluble extensin and potato lectin (a hydroxyproline-rich glycoprotein with a domain that strongly resembles extensin) were cross-linked by only two forms of the enzyme, whereas bovine serum albumin, aldolase, insulin, a number of other marker proteins, and proteins eluted from tomato cells (except extensin) could not be cross-linked. We have also isolated a yeast elicitor that enhances total peroxidase activity and extensin insolubilization within 1 h of challenge in cultured cells of tomato. A highly sensitive enzyme-linked immunosorbent assay technique using polyclonal antiserum raised against soluble tomato extensin was used to demonstrate extensin insolubilization in vivo. A tomato cell-wall peroxidase that cross-links extensin has been purified and may have a role in plant defense.     

Immunogold localization of callose and other cell wall components in pea nodule transfer cells

Summary Transfer cells are located adjacent to xylem and phloem elements in pea nodule vascular tissues. The composition of the labyrinthine wall intrusions was investigated by immunogold labeling using specific antibody probes. Callose antigen was found at the base of newly formed cell wall intrusions and also in adjacent plasmodesmata. Sections through developed labyrinthine intrusions revealed that wall ingrowths had an internal structure with small domains of callose suggesting the presence of channels or vents. Xyloglucan and pectin antigens were uniformly distributed within the wall, but the distribution of extensin antigens was variable, with different antigens being detected in different regions of the wall ingrowth. A lectinlike glycoprotein, PsNLEC-1, was localized in intercellular spaces associated with nodule transfer cells. Previously, expression of this component was observed in other types of cells showing complex involution of the plasma membrane, namely root cortical cells harboring arbuscular mycorrhizae and nodule cells harboring nitrogen-fixing rhizobia.     

EFFECT OF ETHYLENE ON CELLULAR DIFFERENTIATION IN ETIOLATED PEA SEEDLINGS

Ethylene (50 ppm) and 2,4-D (1 mm ) inhibit xylogenesis and completely prevent fiber lignification in the third internode of etiolated pea seedlings (Pisum sativum var. Alaska) during 96–240 hr of treatment. When ethylene is removed after 72 hr, normal differentiation resumes; thus the gas causes no irreversible damage to the cells. A combined treatment with 50 ppm ethylene and 50 μm 2,4-D or treatment with 1 mm 2,4-D alone causes inhibition similar to that observed with 50 ppm ethylene, and 50 μm 2,4-D is partially effective. The inhibition of xylogenesis and fiber lignification caused by 2,4-D is partly or wholly due to auxin-induced ethylene production.     

Effects of diclofop and diclofop-methyl on membrane potentials in roots of intact oat, maize, and pea seedlings

Growth and electrophysiological studies in roots of intact diclofop-methyl susceptible and resistant seedlings were conducted to test the hypothesis that the herbicide acts primarily as a proton ionophore. The ester formulation of diclofop, at 0.2 micromolar, completely inhibited root growth in herbicide-susceptible oat (Avena sativa L.) after a 96 hour treatment, but induced only a delayed transient depolarization of the membrane potential in oat root cortical cells. Root growth in susceptible maize (Zea mays L.) seedlings was dramatically reduced by exposure to 0.8 micromolar diclofop-methyl, while the same diclofop-methyl exposure hyperpolarized the membrane potential within 48 hours after treatment. Furthermore, exposure of maize roots to the protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP) (50 nanomolar), inhibited growth by only 31%, 96 hours after treatment, while the same CCCP exposure depolarized the resting potential by an average of 32 millivolts. Thus, the protonophore hypothesis cannot account for a differential membrane response to phytotoxic levels of diclofop-methyl in two susceptible species. From the results of others, much of the evidence to support the protonophore hypothesis was obtained using high concentrations of diclofop acid (100 micromolar). At a similar concentration, we also report a rapid (3 minute) diclofop-induced depolarization of the membrane potential in roots of susceptible oat and maize, moderately tolerant barley (Hordeum vulgare L.), and resistant pea (Pisum sativum L.) seedlings. Moreover, 100 micromolar diclofop acid inhibited growth in excised cultured pea roots. In contrast, 100 micromolar diclofop-methyl did not inhibit root growth. Since the membrane response to 100 micromolar diclofop acid does not correspond to differential herbicide sensitivity under field conditions, results obtained with very high levels of diclofop acid are probably physiologically irrelevant. The results of this study suggest that the effect of diclofop-methyl on the membrane potentials of susceptible species is probably unrelated to the primary inhibitory effect of the herbicide on plant growth.     

The relationship of the peroxidative indoleacetic Acid oxidase system to in vivo ethylene synthesis in cotton

Since peroxidase and manganese have been implicated in both auxin destruction and ethylene production, the effect of auxins and high tissue levels of manganese on the peroxidative indoleacetic acid oxidase system and the internal level of ethylene was determined in cotton (Gossypium hirsutum L. cv. Watson GL-7). The highest level of manganese tested produced manganese toxicity symptoms, including necrotic lesions, accompanied by an increase in internal ethylene levels at about 15 days after treatment initiation. Statistically significant increases in indoleacetic acid oxidase and peroxidase activity were first observed 2 days later and were paralleled by tissue manganese levels above 7.4 milligrams per gram dry weight and internal ethylene levels of 0.77 microliters per liter air. Eight hours after application of 2,4-dichlorophenoxyacetic acid or indoleacetic acid, the internal levels of ethylene were increased to above 6.6 microliters per liter air in cotton plants, and levels of this magnitude were maintained for a 72-hour period of observation. Modification of peroxidase and indoleacetic acid oxidase activity in auxintreated plants definitely occurred well after the elevation of internal ethylene levels. While ethylene levels and indoleacetic acid oxidase activity were increased by both experimental approaches, the earlier appearance of increased ethylene indicates that the peroxidative indoleacetic acid oxidase system in cotton is not involved in ethylene synthesis or that this enzyme is not the rate-limiting factor when ethylene synthesis is increased. Ethylene, as well as auxin destruction, may be involved in some of the long term plant responses to toxic levels of manganese. The findings also suggest that auxin-induced ethylene may play a role in the elevation of peroxidase and indoleacetic acid oxidase activity eventually seen in extracts of plants treated with auxins. The data support the assumption that the enzymatic portion of the indoleacetic acid oxidase system in cotton is a peroxidase.     

Enzymic cross-linkage of monomeric extensin precursors in vitro

Rapidly growing tomato (Lycopersicon esculentum) cell suspension cultures contain transiently high levels of cell surface, salt-elutable, monomeric precursors to the covalently cross-linked extensin network of the primary cell wall. Thus, we purified a highly soluble monomeric extensin substrate from rapidly growing cells, and devised a soluble in vitro cross-linking assay based on Superose-6 fast protein liquid chromatography separation, which resolved extensin monomers from the newly formed oligomers within 25 minutes. Salt elution of slowly growing (early stationary phase) cells yielded little or no extensin monomers but did give a highly active enzymic preparation that specifically cross-linked extensin monomers in the presence of hydrogen peroxide, judging from: (a) a decrease in the extensin monomer peak on fast protein liquid chromatography gel filtration, (b) appearance of oligomeric peaks, and (c) direct electron microscopical observation of the cross-linked oligomers. The cross-linking reaction had a broad pH optimum between 5.5 and 6.5. An approach to substrate saturation of the enzyme required extensin monomer concentrations of 20 to 40 milligrams per milliliter. Preincubation with catalase completely inhibited the cross-linking reaction, which was highly dependent on hydrogen peroxide and optimal at 15 to 50 micromolar. We therefore identified the cross-linking activity as extensin peroxidase.     

Phytochrome Control of Cell Wall-bound Hydroxyproline Content in Etiolated Pea Epicotyls

The red light inhibition of growth of the intact pea (Pisum sativum L. cv. Alaska) third internode was correlated with an increase in the content of cell wall-bound hydroxyproline. These changes were detected 3 hours after irradiation, and possibly at 1 hour. Far red light reversed the effects of red light. The iron chelator α,α′-dipyridyl reversed the red light effects on both growth and hydroxyproline content. Using segments incubated in vitro, no phytochrome-mediated change in hydroxyproline content could be observed, perhaps because of an overwhelming wounding response. If plants were irradiated in situ and grown for 8 hours before excision and incubation of segments, some enhancement of hydroxylation by red light was detectable both colorimetrically and radioisotopically. The red light inhibition of segment growth was reversed by α,α′-dipyridyl. These results are examined in reference to the role of extensin in normal and induced growth cessation.     

Inhibition by ethylene of polyamine biosynthetic enzymes enhanced lysine decarboxylase activity and cadaverine accumulation in pea seedlings

Exposing etiolated pea seedlings to ethylene which inhibited the activity of arginine decarboxylase and S-adenosylmethionine decarboxylase caused an increase in the level of cadaverine. The elevated level of cadaverine resulted from an increase in lysine decarboxylase activity in the tissue exposed to ethylene. The hormone did not affect the apparent K_m of the enzyme, but the apparent V_max was increased by 96%. While lysine decarboxylase activity in the ethylene-treated plants increased in both the meristematic and the elongation zone tissue, cadaverine accumulation was observed in the latter only. The enhancement by ethylene of the enzyme activity was reversed completely 24 hours after transferring the plants to an ethylene-free atmosphere. It is postulated that the increase in lysine decarboxylase activity, and the consequent accumulation of cadaverine in ethylene-treated plants, is of a compensatory nature as a response to the inhibition of arginine and S-adenosylmethionine decarboxylase activity provoked by ethylene.     

Effects of Gibberellin on the Arrangement and the Cold Stability of Cortical Microtubules in Epidermal Cells of Pea Internodes

Longitudinal microtubules are predominant in epidermal cellsof the 3rd internodes of dwarf pea (Pisum sativum L. cv. LittleMarvel) seedlings. In more than 50% of the cells, cortical microtubulesare running parallel to the cell axis. GA₃ promotes elongation of the internodes and gives rise toa predominance of transverse microtubules. In more than 60%of the GA₃-treatd cells, cortical microtubules are running transverseto the cell axis. Longitudinal microtubules in the GA₃-untreated cells are resistantto low-temperature treatment, but transverse microtubules inthe GA₃-treated cells are sensitive to it. Longitudinal microtubulesare present in GA₃-treated epidermal cells with low frequency.They are resistant to low-temperature treatment. Longitudinal, oblique and transverse microtubules are presentwith almost the same frequency in epidermal cells of the 3rdinternodes of tall pea (cv. Early Alaska) seedlings. GA₃ promoteselongation of the internodes also in tall pea seedlings, butit does not alter the direction of cortical microtubules sodistinctly as it does in dwarf pea seedlings. As in dwarf pea seedlings, longitudinal microtubules are resistantto low-temperature treatment, and transverse microtubules aresensitive to it in tall pea seedlings. (Received September 19, 1986; Accepted December 26, 1986)     

Differential effect of auxin on in vivo extensibility of cortical cylinder and epidermis in pea internodes

The effect of auxin indole-3-acetic acid (IAA) on growth and in vivo extensibility of third internode sections from red light grown pea seedlings (Pisum sativum L. cv Alaska) and the isolated tissues (cortex plus vascular tissue = cortical cylinder, and epidermis) was investigated. Living tissue was stretched at constant force (creep test) in a custom-built extensiometer. In the intact section, IAA-induced increase in total (E_tot), elastic (E_el), and plastic (E_pl) extensibility is closely related to the growth rate. The extensibility of the cortical cylinder, measured immediately after peeling of intact sections incubated for 4 hours in IAA, is not increased by IAA. Epidermal strips, peeled from growing sections incubated in IAA, show a E_pl increase, which is correlated to the growth rate of the intact segments. The isolated cortical cylinder expands in water; IAA has only a small growth-promoting effect. The extensibility of the cortical cylinder is not increased by IAA. Epidermal strips contract about 10% on isolation. When incubated in IAA, they do not elongate, but respond with an E_pl increase. The amount of expansion of the cortical cylinder and contraction of the epidermis (tissue tension), measured immediately following excision and peeling, stays constant during IAA-induced growth of intact sections. The results support the hypothesis that IAA induces growth of the intact section by causing an E_pl increase of the outer epidermal wall. The driving force comes from the expansion of the cortical cylinder which is under constant compression in the intact section.