Degradation of exogenous indole-3-butyric acid and riboflavin and their influence on rooting response of papaya in vitro

Varying concentrations of riboflavin were added to a De Fossard et al. (1974) basal medium containing 10 µM IBA and the effect on adventitious root initiation on shoots of Carica papaya L. was studied. Ninety percent root initiation occurred in 11 days when 1 µM riboflavin was added to the culture medium. Smaller rooting percentages were observed and roots emerged more slowly with riboflavin concentrations greater and less than 1 µM. Tissue culture media were maintained at 27°±1°C in either darkness or 12-h photoperiods for 28 days, and concentrations of riboflavin and IBA were measured at regular intervals using HPLC analysis. In a De Fossard et al. (1974) basal medium, riboflavin concentrations (0.1, 1.0, 10.0 µM) decreased rapidly in light and were independent of the presence of IBA. IBA concentration steadily decreased when media was placed in light, and increasing riboflavin concentrations accelerated the reduction of IBA levels. Concentrations of IBA and riboflavin were stable with dark incubation.Abbreviations IAA indole-3-acetic acid - IBA indole-3-butyric acid     

In vitro rooting of the apple rootstock M 26 in adult and juvenile growth phases and acclimatization of the plantlets

In order to obtain optimum conditions for in vitro propagation of the apple rootstock M 26 ( Malus pumila Mill.) in adult and juvenile growth phases, several rooting experiments were performed. Supraoptimal concentrations of indole-3-butyric acid (IBA) added to the rooting media resulted in profuse callus formation. Since extensive callus production is detrimental to the survival of the plantlets, modified culture conditions were established to reduce callus formation. A reduction of the time of exposure to IBA to 5 days and, thereafter, transfer to a hormone-free medium did not eliminate callus production. Exposure to darkness during the root initiation phase increased rooting. When the rooting medium was based on the Lepoivre formula instead of the Murashige and Skoog formula, callus formation was reduced. Optimum conditions for rooting were obtained at much lower concentration than earlier reported, being 1.25 μM for the juvenile and 0.5 μM for the adult growth phase in the range of IBA concentrations tested. Anatomical studies revealed that root initials are formed after 5 days of IBA-treatment. Therefore, we transferred shoots directly to non-sterile conditions after the root-inducing phase. This resulted in a 90% survival of the plantlets. Subculture on hormone-free medium can thus be eliminated when the optimum auxin concentration is known.     

Micropropagation of Two Australian Native Fruit Species,Davidsonia pruriens and Davidsonia jerseyana G. Harden & J.B. Williams

Nodal explants from in vitro grown seedlings of Davidsonia pruriens and D. jerseyana, established on MS media were treated with various concentrations of three cytokinins. D. pruriens developed optimum shoot growth in terms of shoot height and number of leaves per shoot when 1.0 µM BA was added to basal MS medium while optimum shoot growth for D. jerseyana was obtained when 0.01 µM 2iP was added to the medium. Optimum root initiation and development was obtained when actively growing axillary shoots were cultured on 1/2MS medium plus 32.2 µM IBA for 3–5 days for D. pruriens and 2–3 days for D. jerseyana before transfer to PGR-free medium containing 10 µM riboflavin. Root initiation of more than 80% was achieved with multiple genotypes of D. pruriens and three genotypes of D. jerseyana using juvenile material. The plantlets were transferred to pots and grown in the greenhouse with a success rate of 60% for D. pruriens and 75% for D. jerseyana. Adult D. jerseyana stem explants produced 2–5 shoots per nodal explant upon treatment with 0.1 µM BA. Side shoots from adult D. jerseyana produced similar results for shoot multiplication as for juvenile material. Protocol for multiplication of adult D. pruriens was achieved with much greater difficulty by using material from the green house. Axillary shoots were initiated when 100 µM TDZ was applied to the stem of an adult pot plant and the resultant side shoots were cultured on MS medium containing 1.0 µM BA and 1.0 µM GA₃.     

Factors affecting induction and development of in vitro rooting in apple rootstocks

Shoots of apple rootstocks raised in vitro were transferred to various rooting media to study the effect of different factors on root initiation and development. Various concentrations of indole-3-butyric acid (IBA) initiated rooting but maximum rooting percentage was found with 2.0 and 2.5 mg l(-1) of IBA in M7 and with 1.0 mg l(-1) of IBA in MM106. The drawback was that the roots were thick, short and with profuse callus. The presence of activated charcoal (AC) in the rooting medium improved the rooting quality but reduced the rooting percentage in both the rootstocks. In high auxin dip of 70, 80 and 90 mg l(-1) IBA for 2, 2 and 1 hr showed 75-85 per cent rooting in M7, but lacked reproducibility of the results. Whereas in MM106, 66 - 70 % rooting was achieved with 70 mg l(-1) of IBA dip for 3 h. Root induction in shoots in IBA containing liquid medium (LM) in dark for few days and root elongation in IBA--free medium in light proved most effective. On the other hand, continuous light treatment showed reduced rooting. Reduction of MS salts and sucrose in root elongation medium showed decreased rooting. Plantlets from two--stage rooting procedure showed more rapid growth and satisfactory survival during hardening of plants and on transfer to field.     

Effect of light and riboflavin on indolebutyric acid-induced root formation on apple in vitro

Maximum root formation on apple ( Malus ) shoots cultured in vitro occurred after an incubation in the dark on medium containing 3.2 or 10 μ M indolebutyric acid (IBA) plus riboflavin. Omission of riboflavin or culture in light resulted in a significant decrease in the number of roots formed. About 95% of the absorbed IBA was inactivated by conjugation, ca 4% was extracted as the free IBA acid (IBAH) and only 1% as IAAH. It was investigated whether the decrease in root formation caused by exposure to light or omission of riboflavin during culture was parallelled by a shift in the concentrations of the physiologically active auxin compounds (IBAH and IAAH) in the stem base, i.e. the location where the roots emerge. At least 90% of the absorbed ³H-IBA was located in the stem base. Omission of riboflavin, either in the dark or in the light, had no effect on the IBAH and IAAH concentrations, whereas root formation decreased significantly. Incubation in the light on medium containing 10 μ M IBA with or without riboflavin and culture in the dark on medium containing 3.2μ M IBA plus riboflavin resulted in similar IBAH and IAAH concentrations. However, the number of roots was significantly lower after culture in the light. Therefore, we conclude that the synergistic effect of riboflavin and the antagonistic effect of light on IBA-induced root formation are not solely based on changes in the concentrations of the active auxin components resulting from IBA uptake.     

Improved plant regeneration in Capsicum annuum L. from nodal segments

Multiple shoots were induced by culturing nodal explants excised from 1-month-old aseptic seedlings of red pepper (Capsicum annuum L. cv. Pusa Jwala) on Murashige and Skoog (MS) medium supplemented with (0.1–10 μM) thidiazuron (TDZ). The rate of multiple shoot induction per explant was maximum (14.4 ± 0.06) on MS medium supplemented with 1.0 μM TDZ. Regenerated shoots were elongated well on growth regulator free MS medium. Adventitious roots were induced two weeks after transfer of elongated shoots to MS medium supplemented with auxins (IAA, IBA or NAA) in different concentrations. Optimum root formation frequency was obtained in medium containing 1.0 μM IBA. Ex-vitro rooting was also achieved by pulse treatment with 300 μM IBA for 10 min. Rooted shoots were transplanted in plastic pots containing garden soil (with 90 % survival rate), where they grew well and attained maturity. Regenerated plants were phenotypically and cytologically normal.     

Stimulation of adventitious root formation in non-woody stem cuttings by uridine

Uridine strongly stimulated adventitious root formation in stem cuttings of sunflower (Helianthus annuus L.), mung bean (Vigna radiata L.) and common bean (Phaseolus vulgaris L.). A dose response curve of uridine induced rooting showed that the optimum concentration of uridine was 0.1 µM. At all concentrations employed, uridine had no significant effect on root elongation. The rooting response of stem cuttings to the optimal concentration of indole-3-butyric acid (10 µM) in combination with 0.1 µM uridine did not significantly differ from their response to either of these compounds when applied alone. However, the rooting response of the cuttings to sub-optimal IBA (0.01 µM) was significantly stimulated by uridine. These findings suggested that uridine may have stimulated rooting by increasing the sensitivity of the rooting tissue to auxin.     

Efficient rooting for establishment of papaya plantlets by micropropagation

A low cost micropropagation protocol to produce high quality root systems which are easy and economical to acclimatize is essential for large-scale micropropagation of papaya (Carica papaya L.). In this study, individual shoots (>0.5 cm) with 2_∼3 leaves from in vitro papaya multiple shoots were cultured on MS agar medium containing 2.5 μM IBA under dark conditions for 1 week for root induction. They were then transferred to agar or vermiculite media, containing half strength MS medium, under aerated or non-aerated conditions, for root development. Rooting percentage of shoots cultured for 2 weeks in aerated vermiculite was 94.5%, compared with 90.0% in non-aerated vermiculite, 71.1% in aerated agar, and 62.2% in non-aerated agar. Shoots with roots were acclimated in vermiculite under 100% RH for 1 week and then under ambient conditions for 2 weeks in a temperature-controlled growth chamber (28 °C). The survival rates of the plantlets were 94.5% from aerated vermiculite, 87.8% from non-aerated vermiculite, 42.2% from aerated agar, and 35.6% from non-aerated agar. Thus, root induction in low-concentration IBA agar medium followed by root development in vermiculite containing half strength MS medium under aerated conditions results in efficient rooting of in vitro papaya shoots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Adventitious root formation 'in vitro' in apple rootstocks (Malus pumila) I. Factors affecting the length of the auxin-sensitive phase in M.9

The length of the auxin-sensitive phase of root initiation 'in vitro' in the apple rootstock M.9 ( Malus pumila Mill.) has been determined using the auxins indol-3yl-acettc acid (IAA) at 2.8 × 10⁻⁵ M and indol-3yl-butyric acid (IBA) at 1.5 × 10⁻⁵ M in the presence and absence of 10⁻³ M phloroglucinol (PG). PG synergised IBA-induced rooting after 4 days exposure, but contact times exceeding 8 days decreased root number. In contrast, PG consistently synergised IAA-induced rooting in the dark for contact periods up to 13 days with the highest rooting being recorded at 9 days. An irradiance of 20 W m⁻² from fluorescent lamps halved IAA-induced rooting irrespective of the presence or absence of PG. The culture of shoots at temperatures of 22,25 and 29°C during the root initiation phase (auxin present) and the root emergence phase (auxin absent) produced no difference in rooting response. In the presence of PG the use of liquid culture in place of agar-solidified culture during the auxin-sensitive phase reduced root number but not rooting percentage.     

The Effect of Indole-3-Butyric Acid and Riboflavin on the Morphogenesis of Adventitious Roots of Eucalyptus ficifolia F. Muell. Grown In Vitro

Shoot explants from seedling-derived culture of Eucalyptus ficifoliaF. Muell. cultured on a rooting medium free from indole-3-butyricacid (IBA) develop a root system (Type I) consisting of a fewcomparatively long roots and only small amounts of callus. IBAat 5.0 µM in a rooting medium free from riboflavin inducesthe development, on the shoot explants, of a compact root system(Type II) consisting of callus and many short roots. Riboflavinwhen exposed to light, is able to photo-oxidize IBA; the degreeof photo-oxidation depends on the photon fluence of the lightreceived. The rooting response of the cultures reflects thedegree of photo-oxidation of IBA: concentrations of IBA fromabout 10^–4M to 10^–6M in the rooting medium induceformation of the Type II root system whilst photo-oxidationof the auxin to concentrations of about 10^–8M or lowerinduces the formation of the Type I root system. Thus, exogenousriboflavin and exogenous IBA are linked in a distinct light-induced,riboflavin-mediated change in root morphogenesis. The anatomyof root development in the Type I and Type II root systems wasstudied and factors affecting the development were defined.Characteristics of riboflavin and IBA breakdown in various lightregimes were determined and related to root morphogenesis. Theresults and their implications are discussed. Key words: Auxin photo-oxidation, Riboflavin, Root morphogenesis, Tissue culture     

Effect of Agar, MS Medium Strength, Sucrose and Polyamines on in vitro Rooting of Syzygium Alternifolium

This paper describes the effect of agar, MS basal medium strength, sucrose and polyamines on the in vitro rooting of Syzygium alternifolium realized by a two step procedure involving root initiation (RI) and root elongation (RE). RI was carried out on solidified MS medium supplemented with 1.0 mg dm⁻³ indole-3-butyric acid (IBA) for 3 weeks, and RE following transfer to half-strength MS medium devoid of growth regulators for another 3 weeks. Agar and MS basal medium concentrations played important role on rooting response as well as on health of rooted shoots. Sucrose concentration was positively correlated with the rooting percentage, root number per shoot and root length. The combination of polyamines and 1.0 mg dm⁻³ IBA increases rooting percentage compared to media containing only 1.0 mg dm⁻³ IBA. Optimum rooting was attained with half-strength MS medium containing 1.0 mg dm⁻³ IBA, 2 % sucrose, 10 μM spermine and 0.8 % agar. This revised version was published online in June 2006 with corrections to the Cover Date.     

Micropropagation ofPenstemon serrulatus and iridoid formation in regenerated plants

A method for the micropropagation ofPenstemon serrulatus Menz. from shoot tips or nodal segments was developed. Multiple microshoot cultures (up to 20 shoots from a single explant) were obtained by maintenance of shoot tip explants on Schenk & Hildebrandt medium (SH) supplemented with 4.4 µM benzyladenine (BA) or 8.9 µM BA and 0.57 µM indole-3-acetic acid (IAA). Microshoots developed into numerous, normal shoots when explants were transferred to SH medium containing 2.9 µM IAA or 2.5 µM indole-3-butyric acid (IBA). Shoot cultures were also established from nodal segments (max. 6.8 shoots per segment) when they were placed on SH medium with 0.49 µM IBA and 2.2 µM BA. Rooting of shoots was better on SH medium containing auxin (IBA, NAA or IAA) than on SH medium without growth regulators. The plantlets were then transferred to pots and grown in the greenhouse. Four-month-old regenerated plants demonstrated similar iridoid content (leaves contained 3.83% dry wt. penstemide and 1.8% dry wt. serrulatoloside) as the original plants.     

Micropropagation of photinia employing rhizobacteria to promote root development

An alternative protocol was developed for in vitro propagation of photinia (Photinia × fraseri Dress), an ornamental shrub, using the plant growth-promoting rhizobacteria (PGPR) Azospirillum brasilense and Azotobacter chroococcum during rhizogenesis. Shoot tips from four-year-old mature plants, cut in spring and summer, were used as initial explants. They were cultured on Murashige–Skoog (MS) medium with Gamborg’s vitamins, N⁶-benzyladenine (BA: 11.1 μM) and gibberellic acid (GA₃: 1.3 μM), obtaining 63% of established explants. The highest shoot length (22.9 mm) and multiplication rate (4.3) was achieved by cultivating for four weeks in the same basal medium supplemented with 4.4 μM BA. Both auxin induction and bacterial inoculation were used for rooting. Elongated shoots were treated with two concentrations of indole-3-butyric acid (IBA: 4.9 or 49.2 μM) during 6 days for auxin induction. Then, the shoots were transferred to an auxin-free medium and inoculated with A. brasilense Cd, Sp7 or A. chroococcum (local strain). Bacterial inoculation induced earlier rooting of photinia shoots. A. brasilense Cd with 49.2 μM IBA pulse showed a significant increase (P ≤ 0.05) in root fresh and dry weight (105%, 137%), root surface area (65%) and shoot fresh and dry weight (32%, 62%). A. brasilense Sp7 enhanced the root fresh weight (34%) and root surface area (41%) while no significant differences with A. chroococcum inoculation were detected. The PGPR inoculated micro-cuttings in combination with auxin induction pulses may play a useful role in root organogenesis of micropropagated plants.     

In vitro plantlet formation from adventitious buds on juvenile seedlings of Hinoki cypress (Chamaecyparis obtusa)

Adventitious buds were induced on one-week-old juvenile seedlings of Hinoki cypress on Campbell and Durzan's basic medium containing 10 µM (2.25 mg/l) benzyladenine (BA) and 0.027 µM (0.005 mg/l) naphthaleneacetic acid (NAA) and they elongated into shoots on the same medium lacking benzyladenine. About 70% of shoots could be rooted on the same basic medium containing 14.8 µM (3 mg/l) indole-3-butyric acid (IBA), 0.54 µM (0.1 mg/l) naphthaleneacetic acid and 2.66 µM (1 mg/l) riboflavin. These plantlets were potted in perlite. They were micropropagated about 100 times per seedling within 18 months.Present address (until 3rd, October 1986): Katsuaki Ishii, CSIRO Division of Forest Research, PO Box 4008, Canberra, A.C.T. 2600 Australia     

Effect of N6-isopentenyladenine concentration on growth initiation in vitro and rooting of bilberry and lingonberry microshoots

Buds and shoot tips of wild bilberry (Vaccinium myrtillus L.) and lingonberry (V. vitis-idaea L.) plants were cultured on a modified MS medium containing N⁶-isopentenyladenine (2iP), 9.8–78.4 μM, in order to study the effect of the 2iP-concentration on the initiation of growth. The experiment was first performed in the autumn and repeated in the spring to determine the influence of season on growth initiation. To optimise rooting, three different rooting treatments were tested for the bilberry and lingonberry microshoots. Shoots were rooted either in vitro with 0.49 μM IBA (indole-3-butyric acid) or ex vitro, incubating microshoots in 2.07 mM KIBA-solution (potassium salt of IBA) before planting, or microshoots were planted directly on peat without exogenous auxin. The best 2iP concentration for the initiation of the growth for bilberry was 49.2 μM and for lingonberry 24.6 μM. It was observed that increasing the 2iP concentration at the growth initiation stage increased the number of brownish explants both in bilberry and in lingonberry microcultures. Spring was a considerably better time than autumn for the initiation of new growth, for both species. The results of the rooting test showed that the KIBA-treatment before planting on peat increases rooting efficiency in both bilberry and lingonberry. This revised version was published online in June 2006 with corrections to the Cover Date.     

Micropropagation of <Emphasis Type="BoldItalic">Embelia ribes</Emphasis> Burm f. through proliferation of adult plant axillary shoots

Micropropagation of Embelia ribes was achieved through proliferation of axillary shoots obtained from mature plants. Nodal shoot segments, collected March–May, exhibited high-frequency (75%) shoot initiation when cultured on Murashige and Skoog (MS) basal medium supplemented with thidiazuron (TDZ) at 1.13 μM and indole-3-butyric acid (IBA) at 0.49 μM. Subculture of sprouted shoots from the original explants on medium containing TDZ (1.13 and 0.45 μM) during the first and second subcultures was found essential for further shoot proliferation, while inhibition of shoot elongation by TDZ could be overcome by transferring shoot cultures onto MS medium containing 6-benzylaminopurine (BAP; 11.10 μM) for the third subculture. Treating the explants with an antioxidant mixture of 568 μM ascorbic acid, 119 μM citric acid, and 307 μM glutathione prior to inoculation, coupled with subculture at 2-wk intervals onto fresh medium, both helped to reduce browning of the explants and facilitated production of five to six shoots/explant. MS medium supplemented with BAP (4.44 μM) and IBA (0.49 μM) induced shoot multiplication, producing five to six shoots/explant with a shoot length of 3 to 4 cm over a 4-wk culture period. Shoots of 3 to 4 cm in length exhibited 100% rooting within 4 wk after transfer to media containing half the nutrient salt concentration of MS medium with 3.69 μM IBA. Ex vitro rooting in the greenhouse from the in vitro shoots treated with 4.93 μM IBA for 30 min exhibited 95% rooting in soilrite™ medium in a 4-wk period. About 85% of micropropagated plants were established successfully in root trainers. Three-month-old, hardened plants could further be successfully established in the field. In 1 yr, by using the above protocol, 3,200 plants could be produced from a single shoot and 2,700 could be established in the field.     

Responses of the photosynthetic system and peroxidase activity to the rooting conditions of oak micropropagation

The effects of indole butyric acid (IBA) concentration and the duration of the plant growth regulator (PGR) treatment on the rooting ability, peroxidase level and photosynthetic activity of young Quercus robur L. plants were studied. Four-week-old oak shoots were transplanted to rooting media supplemented with 10 or 20 μM IBA. After 2, 3, 4, 5 or 7 days the shoots were transplanted again to a fresh, PGR-free medium. On the tenth day after transfer to rooting medium, the CO₂ fixation capacity, pigment content and peroxidase activity were measured. The photosynthetic parameters varied as a function of the time spent on medium containing PGR, showing maximum values in plants transplanted on the third to fourth day to PGR-free medium. The rooting percentage of these plants reached its maximum within two weeks. However, peroxidase activity was the highest in plants transferred later to PGR-free medium. The most pronounced stimulating effect on rooting was achieved with the higher initial IBA concentration followed by a transfer to PGR-free medium on the third to fourth day. These plants showed the highest vitality and the best rooting ability. This revised version was published online in June 2006 with corrections to the Cover Date.     

Quantitation of endogenous levels of IAA,IAAsp and IBA in micro-propagated shoots of hybrid chestnut pre-treated with IBA

Endogenous levels of indole-3-acetic acid (IAA), indole-3-acetylaspartic acid (IAAsp) and indole-3-butyric acid (IBA) were measured during the first 8 d of in vitro rooting of rootstock from the chestnut ‘M3’ hybrid by high performance liquid chromatography (HPLC). Rooting was induced either by dipping the basal ends of the shoots into a 4.92-mM IBA solution for 1 min or by sub-culturing the shoots on solid rooting medium supplemented with 14.8-μM IBA for 5 d. For root development, the induced shoots were transferred to auxin-free solid medium. Auxins were measured in the apical and basal parts of the shoots by means of HPLC. Endogenous levels of IAA and IAAsp were found to be greater in IBA-treated shoots than in control shoots. In extracts of the basal parts of the shoots, the concentration of free IAA showed a significant peak 2 d after either root inductive method and a subsequent gradual decrease for the remainder of the time course. The concentration of IAAsp peaked at day 6 in extracts of the basal parts of shoots induced with 14.8-μM IBA for 5 d, whereas shoots induced by dipping showed an initial increase until day 2 and then remained stable. In extracts from basal shoot portions induced by dipping, IBA concentration showed a transient peak at day 1 and a plateau between day 2 and 4, in contrast to the profile of shoots induced on auxin-containing medium, which showed a significant reduction between 4 and 6 d after transferred to auxin-free medium. All quantified auxins remained at a relatively low level, virtually constant, in extracts from apical shoot portions, as well as in extracts from control non-rooting shoots. In conclusion, the natural auxin IAA is the signal responsible for root induction, although it is driven by exogenous IBA independently of the adding conditions.     

An in vitro microplant bioassay using clonal white ash to test for tall fescue allelopathy

Axillary shoots from three selected white ash (Fraxinus americana L.) clones were harvested from in vitro shoot cultures. Roots were initiated by pulsing excised shoots for eight days in the dark in MS medium supplemented with 2% sucrose, 0.7% agar, 5 M NAA, and 1 M IBA. Pulsed shoots were transferred to a root elongation medium consisting of 25% MS macrosalts, full-strength microsalts and organics, 1% sucrose, 0.7% agar and no auxins. When roots were visible (6–10 days after transfer to root elongation medium), microplants were transferred to vessels containing the same minimal medium and tall fescue (Festuca elatior var. arundinacea (Schreb.) Wimm.) leaf extracts, leaf leachates, or soil leachates from plant boxes with and without tall fescue sod. After four weeks in vitro, primary adventitious and secondary root growth was reduced by extracts obtained from 5 and 10 g ground leaves per 100 ml of medium. Leachates obtained from 5 g soaked leaves per 100 ml of medium stimulated primary root growth. Soil leachates from bare soil also stimulated primary root growth. Variation was observed among the clones for root growth when plantlets were grown in extracts or leachates from tall fescue.     

Rooting blockage in the tobacco rac mutant occurs at the initiation phase,and induces diversion to xylem differentiation

ABSTRACT

The rac mutant of Nicotiana tabacum L. cv. Xanthii is impaired in adventitious root formation. The objective of the present study was to determine whether or not the root induction phase occurs in the rac mutant, and if so, to determine what causes the induced cells to become incapable of organising root primordia. To this end, rac and wild-type shoots were cultured in vitro for 7 days under conditions suitable for obtaining roots in the wild-type (i.e., exposure to 5 µM indole-3-butyric acid for 4 h, and then transfer to hormone-free medium), and then histologically and biochemically analysed during culture. The variations in peroxidase activity, and in cellular levels of auxins and polyamines revealed that the induction phase occurs in rac shoots, although it lasts longer than in the wild-type ones. Furthermore, both auxin and polyamines were consistently higher in rac shoots compared to the wild-type. After induction, auxin and putrescine levels abruptly decreased in the wild-type shoots, whereas they decreased much more slowly in the rac mutant. The histological analysis of the wild-type shoots showed that the abrupt decrease in auxin and polyamine levels were correlated with a normal initiation phase. In fact, wild-type shoots showed cell divisions in the procambium already at day 2, resulting in the formation of root primordia at day 4, and in root emergence between days 5 and 7. In rac shoots, despite the fact that the procambium cells were activated to undergo cell division, the initiation phase was highly perturbed, and the procambial cells developed tracheary elements instead of adventitious roots. The different morphogenic responses of the two genotypes are discussed in the light of the differences in auxin content after the induction phase.