The role of endogenous cytokinins in correlation between cotyledon and its axillary bud and in hypocotyl regeneration of flax

When flax seedlings are decapitated above cotyledons and three days later one of the two cotyledons is removed then the remaining cotyledon stimulates in four to five days growth of its axillary bud. It has been found that content of endogenous cytokinins was higher in the stimulated bud as compared with the other one already 12 h after the cotyledon removal. Flax seedlings decapitated under cotyledons regenerate adventitious buds on thy hypocotyl stump during 5–6 days. The endogenous fytohormonal preparation of this regeneration was investigated in the 20 mm apical part of the hypocotyl stump. Decrease in auxin and increase in gibberellins was already found during the first day after decapitation while the level of cytokinins increased as late as three days after the apex removal.     

A mathematical model for storage and recall functions in plants

In plantlets of Bidens pilosa L., under severely limiting environmental conditions the growth of the buds at the axil of the cotyledons (cotyledonary buds) is asymmetric (i.e. one of the buds starts growing before the other one), this asymmetry being oriented by the pricking of one of the cotyledons (i.e. pricking one cotyledon increases the probability that the bud at the axil of the other cotyledon be the first to start to grow). As long as the plant apex (i.e. the terminal bud) is present, the growth of the cotyledonary buds is inhibited (apical dominance), but the souvenir of the asymmetric message caused by sub-optimal environmental conditions and the orientation given by the cotyledon pricking is always present in the plant and can be revealed by removing the apex. Depending on the conditions for removing the plant apex and/or on the application of a variety of symmetrical treatments (e.g. thermal treatment, symmetrical pricking treatments, etc.) the stored asymmetry will either take effect (the bud at the axil of the non-pricked cotyledon will be the first to start to grow more often than the other one) or not (both buds will have equal chance to be the first to start to grow). This has been termed 'recalling' the stored asymmetry. By combining several successive symmetrical treatments, it is possible to reversibly switch on and off the recall function several times. This recall of the stored plant-asymmetry is analogous to the evocation function of a memory system. In this paper, we will present first a discrete logical version of the observed interaction structure between the main components of the bud growth system, then a continuous differential version, taking into account the main features of the observed experimental reality and trying to explain this phenomenology. The interaction structure of both the discrete and the continuous models presents similar positive and negative feedback circuits, necessary condition for observing multistationarity and stability.     

Storage and recall of environmental signals in a plant: modelling by use of a differential (continuous) formulation

The breaking of the symmetry of bud growth in Bidens seedlings involves a sort of plant 'memory'. An asymmetrical stimulus (e.g., the pricking of one of the seedling cotyledons) stores a 'symmetry-breaking' signal within the plants (function STO). Depending on other stimuli received by the seedlings, the stored signal may remain silent or be recalled (RCL function) and take effect in the seedling morphogenesis (asymmetry of the growth of the cotyledonary buds, with a statistical advantage to the bud at the axil of the non-stimulated cotyledon). We show that this memory mechanism can be interpreted by a model taking into account a genetic control exerted on a non-linear enzymatic system that is able to choose trajectories going to different attractors, depending on the stimulation intensity.     

Inhibition of photoperiodic floral induction in Pharbitis nil by ethylene

Application of ethylene at 100 ppm or higher completely inhibitedflowering in Pharbitis nil when made during an inductive darkperiod. Exposing plants to ethylene before or after the inductivedark period produced only slight or almost no inhibition. Ethylenewas effective when it was applied only to a cotyledon, but wasineffective when applied only to a receptor bud. Ethylene hadno effect on translocation of the floral stimulus. Ethylene-treatedcotyledon did not transfer any flower inhibiting entity. Thus,ethylene is considered to inhibit the induction process(es)in cotyledons. Except for an initial temporary cotyledon epinasty, ethylenetreatment had no effect on the subsequent growth and vigor ofplants. This temporary cotyledon epinasty disappeared withinthe next 24 hr. (Received May 4, 1972; )     

Control of a symmetry-breaking process in the course of the morphogenesis of plantlets of Bidens pilosa L.

A mechanism involving transport, storage and retrieval of a symmetry-breaking message controls the relative growth rate of the cotyledonary buds of plantlets of Bidens pilosa L. The asymmetry was induced by administering a few needle pricks to one cotyledon of each plant. The storage of the symmetry-breaking message was independent of the number of pricks (all or nothing process) and irreversible. However, various treatments could render the plants either able to retrieve the stored symmetry-breaking message (in which case, the bud opposite to the pricked cotyledon began to elongate statistically sooner than the one associated with the stimulated cotyledon) or not (both buds then had an equal chance to be the first to start to grow). The retrieval process was also associated with a temporal oscillation. At the level of the whole plants, bud growth was observed only after the removal of apical dominance, and its degree of asymmetry was expressed by use of a parameter g ranging from zero (symmetrical case) to ± 1 (full asymmetry in favor of one of the cotyledonary buds). The highest g-values observed in the present contribution were of the order of 0.5. At the cellular level, the pricking of one cotyledon caused a number of cells, which were within the meristem of the bud associated with the pricked cotyledon and were in cell-cycle phases S or G2, to undergo cellular division and then be blocked in phase G1, whereas the cells of the opposite bud were practically unchanged.We thank Drs. Jean Guern (Laboratoire Hormones végétales, Gif-sur-Yvette, France) Grégoire Nicolis (Service de Chimie Physique II, Bruxelles, Belgique) and Erasmo Marrè (Dipartimento di Biologia, Università di Milano, Milano, Italy) for stimulating discussions, and Mrs. Chantal Eraud, Monique Loiseau and Monique d'Alleizette for their technical assistance.     

The role of gibberellins and cytokinins in the growth correlative effect of cotyledons in flax and pea

It is wellknown that following the amputation, or darkening of one cotyledon in decapitated flax seedlings, the opposite remaining, or illuminated, cotyledon exerts a stimulatory effect on the growth of its axillary bud. For the induction of this stimulating effect a 21–72 h continuous darkening of the cotyledon is sufficient. Endogenous gibberellins take part in the stimulation effect of the illuminated cotyledon, since their level in the illuminated cotyledon increases as early as 12–48 h following the darkening of the opposite cotyledon. The apical part of the cotyledon has a higher growth stimulatory effect on the growth of the cotyledonary axillary bud than the basal half. This again is associated with endogenous gibberellins the level of which is higher in the apical half of the cotyledon than in the basal one. Upon removal of the root and hypocotyl base in decapitated flax seedlings deprived of one cotyledon, the remaining cotyledon loses its stimulatory influence, so that the bud of the amputated cotyledon grows more vigorously (Dostál 1955). In this growth correlative phenomenon the root may be substituted by cytokinin BA applied in the form of a 0.1–1.0 per cent paste onto the remaining cotyledon, for again in this case the bud of the preserved cotyledon grows more vigorously. Following the decapitation of the axillary of the amputated cotyledon in decapitated pea seedlings with an intact root and deprived of one cotyledon, the axillary of the remaining cotyledon grows more intensively than the serial of the removed one. If the plants operated on in the same way are deprived of the root, the serial of the removed cotyledon gains a correlative growth predominance. If the plants deprived of root are cultivated at the same time in a solution of BA (10–20 mg 1⁻¹), the correlative predominance is acquired by the axillary of the remaining cotyledon. In growth correlations between cotyledons and their axillary buds in pea seedlings the root may thus be substituted by exogenous cytokinin, as well.     

The Influence of Cotyledons in Axillary and Adventitious Shoot Production from Cotyledonary Nodes of Cucumis sativus L. (Cucumber)

Cucumber explants including at least part of the cotyledon,a short section of hypocotyl, and the apical bud, are capableof producing multiple axillary buds from the seedling apex andadventitious shoots from the hypocotyl base in a medium whichcontains 2·0 mg dm^–3 of kinetin. Removal of theapical bud triples the number of shoots produced from the apexof explants with two intact cotyledons but does not affect shootproduction from explants with some or all of their cotyledonsremoved. The area of intact cotyledon also influences morphogenesis,as explants with both cotyledons removed, failed to produceadventitious shoots from the hypocotyl base. Culture in continuousdarkness entirely prevents shoot development from the explantbase, but has little influence on shoot production from theapex. The influence of endogenous growth regulators and apicaldominance on the morphogenesis of shoots in cucumber seedlingsare discussed. Key words: Cucumber, cotyledons, apical dominance, morphogenesis, adventitious shoots, Cucumis sativus     

Developmental and structural aspects of somatic embryos formed on medium containing 2,3,5-triiodobenzoic acid

Cotyledon explants of ginseng (Panax ginseng C.A. Meyer) zygotic embryos produced somatic embryos at a high rate (68%) on medium without any growth regulators. Under this culture condition, apparent polar somatic embryogenesis occurred near the basal-excised portion of the cotyledons. When the cotyledon explants were cultured on medium containing 2,3,5-triiodobenzoic acid (TIBA), an auxin polar-transport inhibitor, the frequency of somatic embryo formation markedly decreased and was completely inhibited on medium containing 20 μM TIBA. On medium containing 5–10 μM, somatic embryos developed sporadically on the surface of the cotyledons and had a normal embryo axis but jar-shaped cotyledons. Embryos with jar-shaped cotyledons were also observed to occur at a high frequency when the early globular embryos formed on hormone-free medium were transferred to medium containing 20 μM TIBA. From these results, it was deduced that endogenous auxin in the cotyledon explants plays an important role in the induction of somatic embryos and that the cotyledon development in somatic embryos is also related to the polar transport of endogenous auxin. Received: 11 October 1996 / Revised version received: 8 January 1997 / Accepted: 26 January 1997     

COTYLEDON PHOTOSYNTHESIS DURING SEEDLING GROWTH OF COTTON,GOSSYPIUM HIRSUTUM L.

Respiration and net photosynthetic O₂ production by cotton cotyledons were determined from an early age through the senescent stage. Various treatments were applied to cotyledons to assess the importance of current photosynthesis as compared to translocation of reserves to seedling development. Rates of respiration and net photosynthesis per cm² were high on 1-day-old cotyledons, but the rates decreased sharply with rapid expansion to reach a fairly stable rate. Respiration per cotyledon decreased linearly with age until the onset of senescence, then exhibited a distinct climacteric rise followed by a sharp decrease. Net photosynthesis per cotyledon increased until expansion was completed and then decreased linearly and steeply with age. Excision of cotyledons, inhibition of photosynthesis either chemically or by covering, and removal of the terminal bud indicated that current photosynthesis is a potent force behind early epicotyl growth.     

Ontogenetic Anatomy of Streptocarpus grandis(Gesneriaceae) with Implications for Evolution of Monophylly

The monophylly of Streptocarpus grandis was examined ontogeneticallyand anatomically. When the seed is shed, the embryo is composedof a hypocotyl and two equal-sized cotyledons, lacking rootand shoot apices. During germination, cell division and subsequentcell enlargement occur in the hypocotyl and cotyledons. Thehypocotyl soon produces a primary root from its distal tip;this involves surface and subsurface cells at the point of attachmentof the suspensor remnant. In the cotyledons, cell enlargementand differentiation occur basipetally, leaving small meristematiccells at the bases. These small cells give rise to the basalmeristem in one of the two cotyledons, which contributes toan accrescent cotyledon. The groove meristem, which later differentiatesinto an inflorescence, arises in place of shoot apices whenthe cotyledons become visibly unequal in size. It later exhibitsa tunica-corpus like configuration and differentiates directlyinto an inflorescence meristem. The evolution of this uniquegrowth of one-leaved Streptocarpus is discussed with regardto morphogenetic data.Copyright 2000 Annals of Botany Company Anisocotyly, developmental anatomy, evolution, Gesneriaceae, one-leaf plant, ontogeny, Streptocarpus grandis     

Morphology of carrot somatic embryos formed in medium with abscisic acid

Somatic embryogenesis was induced from embryogenie cells derived from cotyledon expiants cultured on MS medium supplemented with 1 mg/L 2,4-D. In order to clarify the effect of abscisic acid (ABA) on the morphology of somatic embryos, embryogénie cell clumps or developing somatic embryos were treated continuously, or briefly, with ABA during culture. When embryogenie cells in MS medium without 2,4-D were treated with 0.04 mg/L ABA for the first week, normal embryos with two cotyledons increased slightly and embryos with anomalous cotyledons decreased. However when cell clumps in 2,4-D-free medium were treated with ABA in the second week normal embryos with two cotyledons decreased prominently and this decrease of normal embryos also occurred in the continuous ABA treatment during culture. Thus the morphological abnormalities in somatic embryogenesis occurred by exogenous ABA treatment beyond globular stage or by continuous treatment. The length of somatic embryos with anomalous cotyledons was larger than that of normal embryos with two cotyledons in control but both the normal and anomalous somatic embryos treated with ABA were almost similar in length. Somatic embryos formed in medium with ABA were larger in size than those in control due mainly to enlarged cotyledons. The enlarged cotyledons were composed of a greater number of cells than those of control. Therefore the enlargement of cotyledon by exogenous ABA seems to be not due to the enlargement of cells in cotyledons.     

Morphogenetic responses from in vitro cultured seedling explants of mung bean (Vigna radiata L. Wilczek)

The morphogenetic responses of seedling explants of mung bean (Vigna radiata L. Wilczek cv ML-5) were studied in vitro. Direct induction of shoots/plants was possible from shoot tip, cotyledon and cotyledonary node explants. Dedifferentiation of the explants viz; Shoot tip, cotyledons, cotyledonary node, primordial leaves and roots was obtained on basal medium supplemented with auxin and cytokinin. Shoot regeneration was limited to primary calli while rhizogenesis was of common occurrence in established calli. In addition to differences in hormonal requirements, the various explants showed preferential growth in different basal media.     

Seed-specific overexpression of a potato sucrose transporter increases sucrose uptake and growth rates of developing pea cotyledons

During the storage phase, cotyledons of developing pea seeds are nourished by nutrients released to the seed apoplasm by their maternal seed coats. Sucrose is transported into pea cotyledons by sucrose/H+ symport mediated by PsSUT1 and possibly other sucrose symporters. PsSUT1 is principally localised to plasma membranes of cotyledon epidermal and subepidermal transfer cells abutting the seed coat. We tested the hypothesis that endogenous sucrose/H+ symporter(s) regulate sucrose import into developing pea cotyledons. This was done by supplementing their transport activity with a potato sucrose symporter (StSUT1), selectively expressed in cotyledon storage parenchyma cells under control of a vicilin promoter. In segregating transgenic lines, enhanced [(14)C]sucrose influx into cotyledons above wild-type levels was found to be dependent on StSUT1 expression. The transgene significantly increased (approximately 2-fold) transport activity of cotyledon storage parenchyma tissues where it was selectively expressed. In contrast, sucrose influx into whole cotyledons through the endogenous epidermal transfer cell pathway was increased by only 23% in cotyledons expressing the transgene. A similar response was found for rates of biomass gain by intact cotyledons and by excised cotyledons cultured on a sucrose medium. These observations demonstrate that transport activities of sucrose symporters influence cotyledon growth rates. The attenuated effect of StSUT1 overexpression on sucrose and dry matter fluxes by whole cotyledons is consistent with a large proportion of sucrose being taken up at the cotyledonary surface. This indicates that the cellular location of sucrose transporter activity plays a key role in determining rates of sucrose import into cotyledons.     

A MORPHOGENETIC STUDY OF LANCEOLATE,A LEAF-SHAPE MUTANT IN THE TOMATO

Mathan , D. S., and J. A. Jenkins . (U. California, Berkeley.) A morphogenetic study of lanceolate, a leaf-shape mutant in the tomato . Amer. Jour. Bot. 49(5): 504–514. Illus. 1962.—The single-gene mutant, lanceolate (La/La⁺), which has simple, entire leaves rather than the oddpinnately compound leaves of the normal tomato (Lycopersicon esculentum), differs from normal (La⁺/La⁺) in many characters which can be related to fewer and larger cells in meristematic regions. The homozygous lanceolate (La/La) is sometimes lethal, but is usually expressed as reduced, which consists of a hypocotyl without cotyledons, or occasionally either by modified, in which there is a single fused cotyledon and a bud, or by narrow, in which the bud produces a shoot with simple leaves even smaller than those of lanceolate and a tendril-like inflorescence without flowers. In an attempt to overcome the effect produced by the lanceolate gene, the mutant was treated with a number of substances. Embryos without cotyledons cultured in White's medium plus adenine or tyrosine or both gave a higher frequency of narrow plants. With added gibberellic acid, no narrow plants developed from embryos without cotyledons. When tyrosine was sprayed on young narrow plants, they developed larger leaves and morphologically normal but sterile flowers. On the other hand, gibberellic acid sprayed on young lanceolate plants altered their development in the direction of narrow.     

Inhibitory effect of polyamines on the activity of endopeptidase in mung bean cotyledons

The activity of cysteine endopeptidase (EP) in the cotyledons of mung bean seeds increased with time after germination. When cotyledons were excised from the embryonic axis in the course of seedling growth, the activity of EP in the excised cotyledon markedly dropped during the following incubation of 1 d. However, the level of EP protein in excised cotyledons, as examined by immunoblotting, was similar to that in axis-attached cotyledons at the corresponding stage. Thus, it seems that the low activity of EP in excised cotyledons is not due to a decrease in the content of EP protein, but due to a loss of the activity of existing EP. Treatment of attached cotyledons with polyamines (PAs; putrescine and spermidine [Spd]) resulted in a decrease in EP activity, while the same PA-treatment brought about little alteration in the level of EP protein. This indicates that PAs somehow produce an inhibitory effect on the activity of EP. Axis-removal resulted in an accumulation of Spd in the cotyledon. The possibility is suggested that PA, especially Spd, is involved in the inhibition of EP activity in excised mung bean cotyledons.     

Does osmotic regulation of amylase activity in bean cotyledons exist?

The hypothesis that the promotive effect of the embryo axis of the germinating bean seed on amylase activity in the cotyledons is mediated by an osmoregulative mechanism was examined. After 2 days of germination the action of the axis on amylolytic activity was already clearly revealed, whereas at the same time it did not have any influence on osmotic pressure in the cotyledons. When the axis was attached to one cotyledon during 4 days of incubation, osmotic pressure in the cotyledon was lower than its value in the cotyledons of the intact seedling, whereas amylolytic activity was similar in both treatments. It was concluded that the tested hypothesis is not valid in the case of the bean seedling. External osmotic agents brought about a decrease in the level of amylase in the cotyledons, but this does not prove that osmotic changes which are brought about by production of internal metabolites are involved in the regulation of amylase synthesis.     

Histological investigation of walnut cotyledon fragments for a better understanding of in vitro adventitious root initiation

Summary Rapid formation of adventitious roots by walnut cotyledon fragments in vitro was traced by light microscopy. It was shown that this plant model is characterized by two major developmental processes: a) confined elongation of the cotyledon petiole caused by a limited number of cell divisions and b) formation of a morphogenetic zone around each initially wounded vascular bundle within 36 h after detachment of the embryonic axis. During the first phase of development, granular storage protein bodies dissolved, and starch grains were deposited mainly in the distal portion of the cotyledon fragments. Rapidly, new globular protein bodies were formed, and phenolic inclusions accumulated in the vacuoles of epidermal and subepidermal cells and of individual cells close to the vascular bundles. Each adventitious root was found to be in continuity with a single vascular bundle of the cotyledon petiole. A short auxin treatment suppressed the formation of large roots and induced numerous tiny rootlets dispersed all over the surface of the cotyledons.Abbreviations IBA indole-3-butyric acid     

Senescence progression in a single darkened cotyledon depends on the light status of the other cotyledon in Cucurbita pepo (zucchini) seedlings: potential involvement of cytokinins and cytokinin oxidase/dehydrogenase activity

Darkness mediates different senescence-related responses depending on the targeting of dark treatment (whole plants or individual leaves) and on the organs that perceive the signal (leaves or cotyledons). As no data are available on the potential role of darkness to promote senescence when applied to individual cotyledons, we have investigated how darkness affects the progression of senescence in either a single or both individually darkened cotyledons of young 10-day-old Cucurbita pepo (zucchini) seedlings. Strong acceleration of senescence was observed when both cotyledons were darkened as judged by the damage in their anatomical structure, deterioration of chloroplast ultrastructure in parallel with decreased photosynthetic rate and photochemical quantum efficiency of PSII. In addition, the endogenous levels of cytokinins (CKs) and IAA were strongly reduced. In a single individually darkened cotyledon, the structure and function of the photosynthetic apparatus as well as the contents of endogenous CKs and IAA were much less affected by darkness, thus suggesting inhibitory effect of the illuminated cotyledon on the senescence of the darkened one. Apparently, the effect of darkness to accelerate/delay senescence in a single darkened cotyledon depends on the light status of the other cotyledon from the pair. The close positive correlation between CK content and the activity of CK oxidase/dehydrogenase (CKX; EC 1.4.3.18/1.5.99.12) suggested that CKX was essentially involved in the mechanisms of downregulation of endogenous CK levels. Our results indicated that CKX-regulated CK signaling could be a possible regulatory mechanism controlling senescence in individually darkened cotyledons.     

The Roles of the Cotyledons and Auxin in the Adventitious Root Formation of Hypocotyl Cuttings of Light-Grown Cucumber Seedlings

Adventitious root formation in encumber hypocotyl cuttings was studied. Root formation was quantitatively related to the amount of the cotyledons attached to the hypocotyl. Complete removal of the cotyledons diminished root formation entirely. Hut the removal of the apical bud had no effect. Treatment of the hypocotyl with triiodobenzoic acid resulted in the inhibition of root formation. On t he other hand, IAA promoted root formation. Promoting concentration of IAA was 1 mg/1 for the cuttings with intact cotyledons and 10 mg/1 for those with l/8th of the cotyledons. The first two or three days of treatment was most effective. The presence of auxin (IAA-like. substance) in cucumber seedlings was demonstrated by paper chromatography and the pea straight test. It is concluded that the cotyledon is necessary for root formation in cucumber hypocotyl cuttings and that auxin is at least one of the factors supplied from the cotyledons.