Cryopreservation of shoot tips from in vitro plants of sweet potato [Ipomoea batatas (L.) Lam.] by vitrification

 Routine cryopreservation of shoot tips from sweet potato [Ipomoea batatas (L.) Lam] has been hampered by their survival variability after cryogenic exposure. We examined the effects of light conditions on stock plants, sucrose preculture and cryoprotectant loading on survival after vitrification using PVS2 solution. The survival of vitrified sweet potato shoot tips cooled to approximately –208  °C was increased by preculturing with 0.3 M sucrose for 24 h at 22  °C. Survival was also enhanced by excising shoot tips immediately after the 8-h dark photoperiod. The best survival after cryogenic exposure was obtained using 2 M glycerol +0.4 M sucrose for 1 h at 22  °C followed by dehydration with PVS2 for 16 min at 22  °C. Rapid cooling was used and achieved by the immersion of foil strips into partially solidified nitrogen. Successfully vitrified and warmed shoot tips directly developed shoots on a medium containing 1 μM NAA, 0.5 μM BA and 0.1 μM kinetin with only minimum callus formation. Shoot formation occurred in all surviving shoot tips. This procedure shows promise for cryopreserving sweet potato shoot tips. Received: 2 March 1999 / Revision received: 21 September 1999 / Accepted: 29 September 1999     

Expression of CKS1At in Arabidopsis thaliana indicates a role for the protein in both the mitotic and the endoreduplication cycle

Although endoreduplication is common in plants, little is known about the mechanisms regulating this process. Here, we report the patterns of endoreduplication at the cellular level in the shoot apex of Arabidopsis thaliana L. Heynh. plants grown under short-day conditions. We show that polyploidy is developmentally established in the pith, maturing leaves, and stipules. To investigate the role of the cell cycle genes CDC2aAt, CDC2bAt, CYCB1;1, and CKS1At in the process of endoreduplication, in-situ hybridizations were performed on the vegetative shoot apices. Expression of CDC2aAt, CDC2bAt, and CYCB1;1 was restricted to mitotically dividing cells. In contrast, CKS1At expression was present in both mitotic and endoreduplicating tissues. Our data indicate that CDC2aAt, CDC2bAt, and CYCB1;1 only operate during mitotic divisions, whereas CKS1At may play a role in both the mitotic and endoreduplication cycle. Received: 11 May 1998 / Accepted: 29 September 1998     

The role of ethylene and darkness in accelerated shoot elongation of Ammophila breviligulata upon sand burial

The impact of elevated ethylene concentrations and darkness on the growth and development of shoot organs of Ammophila breviligulata was investigated under experimental conditions in a complete two-way design. The results were compared with data of partially sand buried plants. Enhanced ethylene concentrations and sand burial stimulated the formation of new stem nodes, a prerequisite for burial-induced shoot elongation. However, internode elongation itself could not be promoted by the phytohormone ethylene, by darkness, or by their interaction. Sand burial inhibited the formation of rhizomes and tillers and the investment in root and rhizome biomass. Darkness mimicked this effect for the number of rhizomes and the biomass allocated to roots and rhizomes, indicating that the change in light regime upon sand burial may play an important role in the signal transduction chain that leads to a different allocation pattern in A. breviligulata. The results are discussed within the context of alternative signals that might initiate the internode elongation response in sand-buried A. breviligulata plants. Received: 5 July 1997 / Accepted: 28 February 1998     

Cryopreservation of white poplar (Populus alba L.) by vitrification of in vitro-grown shoot tips

 Shoot tips from in vitro-grown, cold-hardened stock plants of white poplar (Populus alba L.) were successfully cryopreserved at –196  °C by one-step vitrification. After preculturing at 5  °C for 2 days on hormone-free MS medium containing different sucrose concentrations, and loading for 20 min with 2 m glycerol and 0.4 m sucrose, shoot tips were treated with the PVS2 vitrification solution and plunged directly into liquid nitrogen. Best survival rate (90%) was obtained when shoot tips were precultured on 0.09 m sucrose, hormone-free MS medium, vitrified by exposure to PVS2 solution for 60 min at 0  °C and, following cryopreservation, rewarmed at 40  °C and washed in 1.2 m sucrose solution for 20 min. Regrowth was improved by plating shoot tips on a gelled MS medium containing 1.5 μm N⁶-benzyladenine plus 0.5 μm gibberellic acid, while shoot rooting was achieved on MS medium containing 3 μm indole-3-butyric acid. Following this procedure, almost 60% rooted shoots were obtained from cryopreserved shoot tips. Received: 1 February 1999 / Revision received: 3 May 1999 · Accepted: 21 May 1999     

Multiple shoot regeneration of kenaf (Hibiscus cannabinus L.) from a shoot apex culture system

 In this research, a medium was developed that would stimulate multiple shoot initiation from shoot apex explants of Hibiscus cannabinus L. (kenaf). Adventitious shoot formation on a shoot induction media supplemented with combinations of 2,4-dichlorophenoxyacetic acid (2,4-D) (0, 0.5, 2.3 μmol·l^–1) and thidiazuron (N-phenyl-N′-1,2,3-thiadiazol-5-ylurea; TDZ) (0, 1, 5, 20 μmol·l^–1) was evaluated. Multiple shoot induction medium with 1 μmol·TDZ l^–1 resulted in the highest number of regenerated shoots per explant for all three kenaf cultivars tested (Tainung 1, Tainung 2, and Everglades 71). The 2,4-D did not enhance multiple shoot formation. Additionally, kenaf cultivars 7N and SF459 also produced multiple shoots on the induction medium with 1 μmol·TDZ l^–1. Multiple shoot clumps formed after 2 weeks in culture without callus formation. Shoots elongated and rooted within 2 weeks after subculture on a plant growth regulator-free medium. A histological study demonstrated the de novo regeneration of shoots from the shoot apex. Received: 2 February 2000 / Revision received: 30 March 2000 / Accepted: 22 June 2000     

The frequency of plasmodesmata increases early in the whole shoot apical meristem of Sinapis alba L. during floral transition

 The frequency of plasmodesmata increases in the shoot apical meristem of plants of Sinapis alba L. induced to flower by exposure to a single long day. This increase is observed within all cell layers (L1, L2, L3) as well as at the interfaces between these layers, and it occurs in both the central and peripheral zones of the shoot apical meristem. The extra plasmodesmata are formed only transiently, from 28 to 48 h after the start of the long day, and acropetally since they are detectable in L3 4 h before they are seen in L1 and L2. These observations indicate that (i) in the Sinapis shoot apical meristem at floral transition, there is an unfolding of a single field with increased plasmodesmatal connectivity, and (ii) this event is an early effect of the arrival at this meristem of the floral stimulus of leaf origin. Since (i) the wave of increased frequency of plasmodesmata is 12 h later than the wave of increased mitotic frequency (A. Jacqmard et al. 1998, Plant cell proliferation and its regulation in growth and development, pp. 67–78; Wiley), and (ii) the increase in frequency of plasmodesmata is observed in all cell walls, including in walls not deriving from recent divisions (periclinal walls separating the cell layers), it is concluded that the extra plasmodesmata seen at floral transition do not arise in the forming cell plate during mitosis and are thus of secondary origin. Received: 4 October 1999 / Accepted: 23 December 1999     

Cryopreservation of in vitro-grown axillary shoot-tip meristems of mint (Mentha spicata L.) by encapsulation vitrification

 Alginate-coated meristems from in vitro-grown axillary buds of mint (Mentha spicata L.) were successfully cryopreserved by vitrification. Excised meristems from nodal segments cold hardened at 4  °C for 3 weeks were encapsulated and osmoprotected by a mixture of 2 M glycerol plus 0.4 M sucrose. These meristems were dehydrated with a highly concentrated vitrification solution (PVS2 solution) for 3 h at 0  °C prior to a plunge into liquid nitrogen. Successfully encapsulated vitrified meristems developed shoots within a week after plating without intermediary callus formation. The average rate of shoot formation amounted to nearly 90%. This procedure was successfully applied to other Mentha species. It was also confirmed that encapsulated vitrified meristems produced a much higher rate of shoot formation than the encapsulated dried meristems. Thus, this revised encapsulation vitrification method appears promising for the cryopreservation of mint and other germplasm. Received: 24 November 1998 / Revision received: 8 February 1999 / Accepted: 26 February 1999     

Tillers Do Not Influence Phytotoxicity of Imazamethabenz in Wild Oat (Avena fatua)

The response of wild oat to imazamethabenz varies with the growth stage, but the role of tillers in this regard is unclear. Removal of tillers at the three-leaf stage before spraying with imazamethabenz did not significantly affect the total shoot fresh weight measured 3 weeks later. The leaf area and dry weight of intact plants at the three-leaf stage were 17–21% greater than for plants with coleoptilar and first leaf main shoot tillers (T0 and T1) removed. The greater leaf area may have increased herbicide interception per plant. Similar fresh weight reductions in main shoot, total tillers, and total shoots were found whether imazamethabenz was applied to the plant at the two-leaf without tillers or the three-leaf with two tillers stage. Imazamethabenz applied only to the main shoot reduced total shoot dry weight more than an equivalent amount of imazamethabenz applied only to tiller T1 or applied over the whole shoot. Imazamethabenz had the least inhibitory effect on whole plant growth when applied only to T1. When ¹⁴C-herbicide was applied to the first main shoot leaf of plants at the three-leaf stage with two tillers, the ¹⁴C translocated 38% to roots, 33% to the main shoot, and nearly 30% to all tillers. When ¹⁴C-herbicide was applied to the first leaf of T1 then the ¹⁴C translocated 50% to T1, 25% to the main shoot, 20% to roots, and 5% to all other tillers. The translocation pattern and fresh weight values suggested that the presence of early tillers during herbicide application neither increased nor decreased imazamethabenz efficacy in wild oat. Received June 4, 1997; accepted June 5, 1997     

Effect of primary leaves on 59Fe uptake by roots and 59Fe distribution in the shoot of iron sufficient and iron deficient bean (Phaseolus vulgaris L.) plants

A study has been made on the effect of primary leaves on iron (Fe) distribution in the shoot. Bean (Phaseolus vulgaris L.) seedlings were precultured in nutrient solution with 8×10^-5 M FeEDTA for 4 days, and then grown further with either 8×10^-5 M FeEDTA (+Fe) or without Fe supply (-Fe) for another 5 days. Thereafter, both +Fe and -Fe plants were treated in three different ways: undisturbed; one primary leaf removed; or one primary leaf shaded, starting two hours before supply ⁵⁹FeEDTA to the roots. The +Fe plants were supplied with 8×10^-5 M ⁵⁹FeEDTA, and the -Fe plants with only 1×10^-6 M ⁵⁹FeEDTA. After 1 to 8 hour uptake periods, plants were harvested and ⁵⁹Fe in different organs was determined. Removal or shading of one primary leaf did not affect ⁵⁹Fe uptake by roots and ⁵⁹Fe translocation to the shoot in +Fe plants. In the -Fe plants, however, removal of one primary leaf decreased ⁵⁹Fe uptake by roots, whereas shading of one primary leaf had no effect on ⁵⁹Fe uptake but slightly enhanced ⁵⁹Fe translocation from roots to the shoot. The quantity of ⁵⁹Fe in primary leaves was positively correlated with quantity of ⁵⁹Fe in the stem in the -Fepplants, but not in the +Fe plants. In both, the +Fe and -Fe plants, the quantity of ⁵⁹Fe in the shoot apex was positively correlated with ⁵⁹Fe in primary leaves. The results suggest that irrespective of the Fe nutritional status of plants, the source of Fe for the shoot apex is Fe retranslocated from primary leaves.     

Direct shoot organogenesis from <Emphasis Type="Italic">in vitro</Emphasis>-derived mature leaf explants of pistachio

An efficient system was developed for direct plant regeneration from in vitro-derived leaf explants of Pistacia vera L. cv. Siirt. The in vitro procedure involved four steps that included (1) induction of shoot initials from the regenerated mature leaf tissue, (2) regeneration and elongation of shoots from the shoot initials, (3) rooting of the shoots, and (4) acclimatization of the plantlets. The induction of shoot initials was achieved on an agarified Murashige and Skoog (MS) medium with Gamborg vitamins supplemented in different concentrations of benzylaminopurine (BA) and indole-3-acetic acid (IAA). The best medium for shoot induction was a MS medium with 1 mgl⁻¹ IAA and 2 mgl⁻¹ BA. Numerous shoot primordia developed within 2–3 wk on the leaf margin and the midrib region, without any callus phase. In the second step, the shoot clumps were separated from the leaf explants and transferred to a MS medium supplemented with 1 mgl⁻¹ BA, resulting in a differentiation of the shoot initials into well-developed shoots. The elongated shoots (>3 cm long) were rooted on a full-strength MS basal medium supplemented with 2 mgl⁻¹ of indole-3-butyric acid in the third stage. Finally, the rooted plants were transferred to soil with an 80% success rate. This protocol was utilized for the in vitro clonal propagation of this important recalcitrant plant species.     

Self-shading, carbon gain and leaf dynamics: a test of alternative optimality models

A simple model of shoot-level carbon gain is presented addressing the optimal number and life span of leaves in relation to alternative optimality criteria: (1) maximizing carbon export from the shoot, or (2) maximizing the rate of leaf production at the shoot tip. Additionally, the processes that cause declining assimilation with leaf age are considered in relation to (1) leaf position on the shoot (e.g., self-shading) versus (2) leaf age per se. Using these alternative scenarios, only a model based on position-dependent assimilation and maximization of leaf production rates resulted in quantitative predictions for all aspects of leaf dynamics on the shoot (i.e., leaf number, life span, and birth rate), while other approaches predicted that one or more parameters would be infinite. This formulation of the model also predicted that leaves should be maintained on the shoot until the diurnal carbon balance declines to zero, in contrast with other scenarios which predict that leaves should be shed while maintaining a positive carbon balance. Predictions of the model were supported by the results of a field study of carbon gain and leaf dynamics in saplings of three species of tropical pioneer trees (Carica papaya, Cecropia obtusifolia, and Hampea nutricia) which differ in the number of leaves per shoot. The results illustrate that in these fast-growing plants, leaf production and height growth may be more appropriate measures of performance than net carbon export from the shoot, and suggest that leaf senescence is primarily a function of the position of a leaf within the canopy, rather than its chronological age. Received: 13 October 1998 / Accepted: 27 January 1999     

Agrobacterium-mediated transformation of Lupinus mutabilis L. using shoot apical explants

A procedure for regenerating plants of Lupinus mutabilis from shoot apices, from which the leaf primordia and initial cell layer(s) of the apical meristem were removed, has been used to generate transgenic plants following Agrobacterium tumefaciens-mediated gene delivery. Transformation competent cells, from which buds developed, were located at the periphery of the apical meristem. Kanamycin resistant plants were obtained which expressed β-glucuronidase activity. Integration of the neomycin phosphotransferase II and β-glucuronidase genes into the genomes of transgenic plants was confirmed by non-radioactive DNA-DNA hybridisation. This is the first report of the generation of transgenic plants in L. mutabilis.     

Leaf Emergence of Spring Wheat Receiving Varying Nitrogen Supply at Different Stages of Development

We examined effects of nitrogen (N) supply on leaf emergenceof spring wheat (Triticum aestivum L.) grown in sand with nutrientsolution containing different N concentrations (9NO₃: 1NH₄).In expt 1, the cultivar 'Gamenya' received nutrient solutiontwice weekly containing a constant N supply ranging from 50to 2400 µM N. In expts 2 and 3, cultivars 'Aroona' and'Gamenya' were irrigated hourly with nutrient solution containingeither low (L = 50 µM N) or high (H = 2000 µM N)N supply. In expt 2, the N supply to half of the plants receivingL and H was changed at the double ridge stage of apical development,producing plants receiving LL, LH, HL and HH. In expt 3, N supplywas changed firstly when the main stem apex was vegetative (oneto two leaves) and secondly when the main stem apex was at doubleridge stage (four to five leaves), producing plants receivingLLL, LHL, HLH and HHH. Leaves on the main stem and primary tillerswere counted. Rate of leaf emergence was estimated from regressionof number of leaves against thermal time; the phyllochron wascalculated as 1/ rate of emergence. Severely N-deficient plants (which had at least a 60% reductionin shoot dry weight) had slower rates of leaf emergence (expt1). Fluctuating N supply sometimes, but not always, changedthe rate of leaf emergence (expts 2 and 3). The N supply beforedouble ridge stage had bigger effects on the phyllochron thanthat afterwards (expt 3). The phyllocrons of the main stemswere generally lower than those of tillers, with a greater differencebetween stems in N-deficient plants. Low N supply at the vegetativeapex stage decreased the total number of leaves on the mainstem, while low N supply after double ridge did not.Copyright1994, 1999 Academic Press Nitrogen, stress, spring wheat, Triticum aestivum, leaf emergence, phyllochron, apical development     

Correlative Inhibition in the Shoot of Agropyron repens ( L.) Beauv

Correlative inhibition was investigated in plants of Agropyronrepens at two temperatures. Reciprocal inhibition ocrurred betweenthe main shoot apex and the outgrowing axillary shoots, withthe balance of inhibition varying with temperature. Apical dominancewas stronger at 10 °C than at 20 °C , but even at 10°C release of apical dominance by decapitation had onlyminor effects on the timing of outgrowth, growth pattern andrate of dry weight aocumulation of the axillary shoots. Dominanceof the main shoot apex by the axillary shoots was stronger at20 °C than at 10 °C. Removal of axillary buds preventeddecline in size and activity of the main shoot apex ard resultedin increased rates of primordium initiation, leaf emergenceand dry weight accumulation in the main shoot. It is suggestedthat a system of reciprocal dominance provides a mechanism formaintaining the characteristic habit of the grass plant andlimits growth in height of vegetative shoots. Agropyron repens (L.) Beauv, couch grass, correlative inhibition, apical dominance, shoot, apex     

Light intensity, gibberellin content and the resolution of shoot growth in Brassica

The role of gibberellins (GAs) in the regulation of shoot elongation is well established but the phytohormonal control of dry-matter production is poorly understood. In the present study, shoot elongation and dry-matter production were resolved by growing Brassica napus L. seedlings under five light intensities (photon flux densities) ranging from 25 to 500 μmol m⁻² s⁻¹. Under low light, plants were tall but produced little dry weight; as light intensity was increased, plants were progressively shorter but had increasing dry weights. Endogenous GAs in stems of 16- and 17-d-old plants were analyzed by gas chromatography-selected ion monitoring with [²H₂] internal standards. The contents of GAs increased dramatically with decreasing light intensity: GA₁, GA₃, GA₈ and GA₂₀ were 62, 15, 16 and 32 times higher, respectively, under the lowest versus highest light intensities. Gibberellin A₁₉ was not measured at 25 μmol m⁻² s⁻¹ but was 9␣times greater in the 75 compared to 500 μmol m⁻² s⁻¹ treatment. Shoot and hypocotyl lengths were closely positively correlated with (log) GA concentration (for example: r ² = 0.93 for GA₁ and hypocotyl length) but shoot dry matter was negatively correlated with GA concentration. The application of gibberellic acid (GA₃) produced elongation of plants grown under high light, indication that their low level of endogenous GA was limiting shoot elongation. Although endogenous GA₂₀ showed the greatest influence of light treatment, metabolism of [³H]GA₂₀ and of [³H]GA₁ was only slightly influenced by light intensity, suggesting that neither 2β- nor 3β-hydroxylation were points of metabolic regulation. The results of this study indicate that GAs control shoot elongation but are not directly involved in the regulation of shoot dry weight in Brassica. The study also suggests a role of GAs in photomorphogenesis, serving as an intermediate between light condition and shoot elongation response. Received: 18 June 1998 / Accepted: 29 July 1998     

Characterization of transgenic rice plants that express rgp1, the gene for a small GTP-binding protein from rice

 The rgp1 gene, which encodes a small GTP-binding protein from rice, was introduced into rice protoplasts by electroporation. Transformed protoplasts were cultured on liquid protoplast-culture medium for 1 month, and then cells that had proliferated were transferred to a selection medium that contained 50 mg/l hygromycin B. Among 50 colonies that were selected and transferred to regeneration medium, 3 colonies generated shoots. However, two of the three shoots failed to form roots and ceased growing. A single regenerated shoot that formed roots was planted in soil and transferred to a greenhouse. Southern hybridization showed that the regenerated plant harbored a single copy of the introduced gene. The transformant (T₀) plant was shorter than the controls, it developed three times as many tillers as controls, it developed three times as many tillers as control plants but it produced mostly sterile seeds. In a test of hygromycin resistances, viable seeds segregated into resistant and sensitive seedings at a ratio of approximately 1 : 3. The progeny (T₁) plants were short with many tillers, and some produced seeds normally. The T₂ seedlings grew more rapidly than control seedlings for the first 28 days after germination, but control plants subsequently outgrew the T₂ plants. Northern blotting analysis revealed that the rgp1 gene in T₂ plants was expressed consitutively throughout all developmental stages. The results suggest that the observed phenotypic changes were due to expression of the exogenous rgp1 gene. Received: 21 September 1997/Accepted: 31 March 1998     

In vitro morphogenesis of Cucumis melo var. inodorus

In vitro morphogenesis of C. melo L. var. inodorus was studied by the induction of adventitious buds and somatic embryos. Organogenesis was obtained from cotyledon segments and leaf discs in culture medium supplemented with benzylaminopurine (1 mg l⁻¹) and somatic embryogenesis was induced in medium containing 2,4-dichlorophenoxyacetic acid (5 mg l⁻¹) + thidiazuron (1 mg l⁻¹). Through histological analysis it was possible to verify that in cotyledonary explants, protuberances that do not develop into well-formed shoot buds and leaf primordia are more frequently formed than complete shoot buds, resulting in a low frequency of plant recovery in the organogenic process. A high percentage of explants responded with the formation of somatic embryos; the microscopical analysis showed that the somatic embryos lacking well developed apical meristems had a low conversion rate into plants. Plant recovery was not obtained from leaf-disc explants, with high rates of contamination and formation of protuberances which did not develop into shoot buds. Histological sections showed the development of epidermis and leaf hairs, indicating those structures could be leaf primordia; however, these were not associated with a shoot apical meristem. This revised version was published online in June 2006 with corrections to the Cover Date.     

Patterns of Assimilate Distribution and Source--sink Relationships in the Young Reproductive Tomato Plant (Lycopersicon esculentum Mill.)

Patterns of distribution of ¹⁴C were determined in 47-day-oldtomato plants (Lycopersicon esculentum Mill.) 24 h after theapplication of [¹⁴C]sucrose to individual source leaves fromleaves 1–10 (leaf 1 being the first leaf produced abovethe cotyledons). The first inflorescence of these plants wasbetween the ‘buds visible’ and the ‘firstanthesis’ stages of development. The predominant sink organs in these plants were the root system,the stem, the developing first inflorescence and the shoot ‘apex’(all tissues above node 10). The contribution made by individualsource leaves to the assimilate reaching these organs dependedupon the vertical position of the leaf on the main-stem axisand upon its position with respect to the phyllotactic arrangementof the leaves about this axis. The root system received assimilateprincipally from leaf 5 and higher leaves, and the stem apexfrom the four lowest leaves. The developing first inflorescencereceived assimilates mainly from leaves in the two orthostichiesadjacent to the radial position of the inflorescence on thevertical axis of the plant; these included leaves which weremajor contributors of ¹⁴C to the root system (leaves 6 and 8)and to the shoot apex (leaves 1 and 3). This pattern of distributionof assimilate may explain why root-restriction treatments andremoval of young leaves at the shoot apex can reduce the extentof flower bud abortion in the first inflorescence under conditionsof reduced photoassimilate availability. Lycopersicon esculentum Mill, tomato, assimilate distribution, source-sink relationships     

Genetic analyses of the formation of the serrated margin of leaf blades in Arabidopsis : combination of a mutational analysis of leaf morphogenesis with the characterization of a specific marker gene expressed in hydathodes and stipules

Developmental control of the formation of the serrated margin of leaf blades was investigated. First, the expression was characterized of a marker gene encoding β-glucuronidase in strain #1-35-38, a transgenic strain of Arabidopsis thaliana (L.) Heynh, derived by the use of a previously described transposon-tagging system. In strain #1-35-38, expression of the marker gene was tissue-specific, being restricted to stipules and the toothed margins of laminae. Using this transgenic marker gene, we examined the development of leaf blade margins in Arabidopsis. We compared the pattern of expression of the marker gene in the leaves of the wild-type plant with that in plants carrying the asymmetric leaves1 (as1) mutation, which causes dramatic changes in leaf-blade morphology in Arabidopsis. The as1 mutant showed normal morphology of early leaf primordia. The mutation affected the development of leaf segmentation in Arabidopsis without any change in the number or morphology of cells in laminae. The as1 mutation affected leaf morphology independently of mutations in other genes known to affect leaf morphogenesis, such as the acaulis1 mutation and the angustifolia mutation. Based upon these results, the development of the morphology of leaf margins in Arabidopsis is discussed. Received: 9 January 1997 / Accepted: 24 June 1997