Independent control of organogenesis and shoot tip abortion are key factors to developmental plasticity in kiwifruit (Actinidia)

BACKGROUND AND AIMS: In kiwifruit (Actinidia), the number of nodes per shoot is highly variable and is influenced by genotype and environmental conditions. To understand this developmental plasticity, three key processes were studied: organogenesis by the shoot apical meristem during shoot growth; expansion of phytomers; and shoot tip abortion. METHODS: Studies were made of organogenesis and shoot tip abortion using light and scanning electron microscopy. The effect of temperature on shoot growth cessation was investigated using temperature indices over the budbreak period, and patterns of shoot tip abortion were quantified using stochastic modelling. KEY RESULTS: All growing buds began organogenesis before budbreak. During shoot development, the number of phytomers initiated by the shoot apical meristem is correlated with the number of expanding phytomers and the mean internode length. Shoot tip abortion is preceded by growth cessation and is not brought about by the death of the shoot apical meristem, but occurs by tissue necrosis in the sub-apical zone. For most genotypes studied, the probability of shoot tip abortion is higher during expansion of the preformed part of the shoot. Lower temperatures during early growth result in a higher probability of shoot tip abortion. CONCLUSIONS: Organogenesis and shoot tip abortion are controlled independently. All buds have the potential to become long shoots. Conditions that increase early growth rate postpone shoot tip abortion.     

In vitro propagation ofSalix tarraconensis Pau ex Font Quer,an endemic and threatened plant

Summary Salix tarraconensis Pau ex Font Quer, an endemic willow species from northeast Spain, was micropropagated with nodal segments. Shoot multiplication was obtained with different cytokinins, either on Murashige and Skoog medium or woody plant medium. Best results for shoot formation were obtained on Murashige and Skoog medium containing 4.9 μM of 6-γ-γ-dimethylallylaminopurine. Shoots showed strong apical dominance, and some cultures displayed apical necrosis. Benzyladenine gave the worst results; shoots displayed very slow growth, deformed leaves, and hyperhydrity. Good rooting of shoots was obtained with different auxins or without plant growth regulators on woody plant medium. The best results (90-100%) were obtained within 20 d. On rooting media with indole-3-butyric acid or indoleacetic acid, shoot elongation was good (35-40 mm length). Apical necrosis was observed in elongating shoots on rooting medium, but this disturbance favored axillary bud sprouting and formation of new shoots. Shoot length and quality of roots decreased gradually as the concentration of naphthaleneacetic acid increased. Plant survival was 90% 4 weeks after removal fromin vitro conditions.     

Opuntia micropropagation by axillary proliferation

Cladode explants of Opuntia amyclaea were cultivated in Murashige and Skoog medium with different supplements. Benzyladenine was necessary for shoot development from pre-existing buds. Axillary proliferation was also stimulated in subsequent subcultures in the presence of benzyladenine and when the apical meristem was not present in the explant. The number of shoots and the total dry weight were maximum with 5% of sucrose in the medium. It was found that satisfactory rooting occurred when 5×10^-5 M indole butyric acid was added to the medium. Vascular contact between roots and shoots was clearly shown by histological observations. The micropropagation system developed here allows the production in 100 days of 25 000 rooted plantlets from a single cladode, by the stimulation of axillary proliferation in the absence of apical dominance.     

Regeneration of different cultivars of common bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.) via indirect organogenesis

A protocol for in vitro regeneration via indirect organogenesis for Phaseolus vulgaris cv. Negro Jamapa was established. The explants used were apical meristems and cotyledonary nodes dissected from the embryonic axes of germinating seeds. Several auxin/cytokinin combinations were tested for callus induction. The best callus production was obtained with medium containing 1.5 μM 2,4-dichlorophenoxyacetic acid. After 2 weeks of growth calli were transferred to shooting medium containing 22.2 μM 6-benzylaminopurine. Shoots regenerated with a frequency of approximately 0.5 shoots per callus, and upon transfer to rooting medium these shoots produced roots with 100% efficiency. Histological analyses of the regeneration process confirmed the indirect organogenesis pattern. Greenhouse grown regenerated plants showed normal development and were fertile. The protocol was reproducible for other nine P. vulgaris cultivars tested, suggesting a genotype independent procedure.     

Developmental anatomy and branching of roots of four Zeylanidium species (podostemaceae), with implications for evolution of foliose roots

Podostemaceae have markedly specialized and diverse roots that are adapted to extreme habitats, such as seasonally submerged or exposed rocks in waterfalls and rapids. This paper describes the developmental anatomy of roots of four species of Zeylanidium, with emphasis on the unusual association between root branching and root-borne adventitious shoots. In Z. subulatum and Z. lichenoides with subcylindrical or ribbon-like roots, the apical meristem distal (exterior) to a shoot that is initiated within the meristem area reduces and loses meristematic activity. This results in a splitting into two meristems that separate the parental root and lateral root (anisotomous dichotomy). In Z. olivaceum with lobed foliose roots, shoots are initiated in the innermost zone of the marginal meristem, and similar, but delayed, meristem reduction usually occurs, producing a parenchyma exterior to shoots located between root lobes. In some extreme cases, due to meristem recovery, root lobing does not occur, so the margin is entire. In Z. maheshwarii with foliose roots, shoots are initiated proximal to the marginal meristem and there is no shoot-root lobe association. Results suggest that during evolution from subcylindrical or ribbon-like roots to foliose roots, reduction of meristem exterior to a shoot was delayed and then arrested as a result of inward shifting of the sites of shoot initiation. The evolutionary reappearance of a protective tissue or root cap in Z. olivaceum and Z. maheshwarii in the Zeylanidium clade is implied, taking into account the reported molecular phylogeny and root-cap development in Hydrobryum.     

Glutathione and glutathione disulfide affect adventitious root formation and growth in tomato seedling cuttings

To study the relationship between glutathione and rooting, tomato seedling cuttings, grown on basal- or on auxin-supplemented media, were treated with the reduced (GSH) or oxidized (GSSG) form of this antioxidant. In turn, the consequences of the depletion of GSH pool on rooting were tested using l-buthionine sulfoximine (BSO), a specific inhibitor of GSH biosynthesis. Effects of the aforementioned treatments on rooting response were assessed. GSH treatment promoted root formation on cuttings grown on both basal- and auxin-supplemented media. Whereas GSSG did not affect the number of roots formed by cuttings grown on basal medium, it strongly enhanced the rooting stimulatory effect of auxin treatment. GSH depletion resulting from BSO application did not change the number of roots formed. All the tested compounds, namely GSH, GSSG, BSO and auxin, had a strong inhibitory effect on the elongation of regenerated roots. Supplementing the rooting medium with glutathione efficiently increased the GSH level in the rooting zones, while addition of BSO led to a strong decrease in endogenous GSH level. Neither of the treatments affected the level of GSSG. Exogenous auxin affect neither GSH nor GSSG levels in rooting zones; however, in the regenerated roots, GSH level was significantly higher when the organs were formed on auxin-supplemented medium. Patterns of GSH distribution in the roots regenerated on basal- and auxin-enriched media were studied using the GSH-specific dye monochlorobimane and confocal laser scanning microscopy. GSH was found in the root apical meristem and in the elongation zone. Auxin did not change the GSH distribution; however, the number of fluorescent cells was higher when roots were regenerated on auxin-supplemented medium.     

Developmental Morphology of Roots and root-borne shoots ofPodostemum subulatum as compared withZeylanidium olivaceum (Podostemaceae—Podostemoideae) Part VII of the series ‘Morphology of Podostemaceae’

The root structure ofPodostemum subulatum is investigated and compared with that ofZeylanidium olivaceum. Podostemum has thread- or ribbon-like roots. The root tip consists of an inner apical meristem and a single-layered root cap. From roots arise numerous shoots of endogeneous origin. Their vascular bundle isab initio connected with the root bundle.By the simple (reduced) apical zonation, the roots ofPodostemum subulatum appear more advanced than the crustose roots ofZeylanidium olivaceum, which bear an ordinary (though asymmetrical) root cap. With regard to the endogeneous root-borne shoots, however,Zeylanidium appears more advanced because of the shoot dimorphism. The floriferous shoots have a short axis that grows plagiotropously above the crust surface, whereas the axes of the vegetative shoots are extremely short and remain, together with the apical meristem, within the crust. Only the leaves protrude from the crust surface.     

Architectural analysis of 1-year-old apple seedlings according to main shoot growth and sylleptic branching characteristics

Main shoot and sylleptic shoot growth characteristics were measured during and after the first year of growth of 255 Telamon x Braeburn apple seedlings. Although mean main shoot growth characteristics between branched and non-branched trees were significantly different, many non-branched trees expressed similar main shoot growth to branched trees. The variables describing length, number and position of the sylleptic shoots were used to classify branched trees into architecturally different groups. A continuum from trees with few and short shoots to trees with many long shoots is observed. The release of axillary buds from apical dominance is not under complete control by the apical meristem. Genetic seedling difference at the level of roots presumably plays an important role in sylleptic branching. Genetic variation in terms of number, position, and subsequent elongation of sylleptic shoots is clearly observed.     

伊乐藻和黑藻断枝根和芽的发生及生长研究

为了解外来种伊乐藻的无性繁殖力、评价其生态安全性,采用插植方式比较研究了伊乐藻(Elodea nuttallii)和本土种黑藻(Hydrilla verticillata)两种沉水植物不同节数(1至4节)和不同节位断枝的不定根和新芽的发生及生长情况。通过室内4周的3次平行实验,结果表明:两者顶芽段均有形成不定根继而形成新植株的能力,而顶芽以下茎段只有本身具有腋芽的断枝才有形成新芽和不定根的能力。两者具相同节数的断枝形成不定根的百分率及根、芽长度,以具顶芽断枝的均明显高于不具顶芽断枝的,具顶芽四节断枝的不定根生成率最高达到90%以上。不具顶芽断枝形成新芽和不定根的百分率及长度随着断枝节数的增加均呈显著递增趋势,每类断枝的发芽率显著大于其生根率;伊乐藻和黑藻枝条一般分别每7节和5节具有一个腋芽,只有具腋芽断枝才能存活,因此,对不具顶芽断枝,7节和5节分别是其形成新苗所需的最短断枝长度。根和芽的长度随节位的下降大致呈递增的趋势。但是节数对形成根、芽的影响显著大于节位的影响。具顶芽断枝的顶芽的增长量和具顶芽4节断枝的生物量增量伊乐藻的高于黑藻,其余指标伊乐藻均显著低于黑藻。伊乐藻断枝的繁殖力总体上低于黑藻。       

Virus elimination and pathogen-free plantlets regeneration in Cucurbita pepo L.

An efficient in vitro protocol was established for developing pathogen-free plantlets in Cucurbita pepo through meristem culture. Meristems of about 0.3–0.5 mm in size were isolated from shoot tips of 25–30 day old in vitro grown plants. For primary establishment of isolated apical meristem, MS liquid medium supplemented with 2.0 mgl KIN and 0.5 mg/l GA₃ was found to be most effective in both cultivars. MS semisolid medium containing 2.0 mg/l BAP were found to be most effective for shoot development from primarily established meristem in both cultivars. A good number of shoots were not concomitant with good rooting. The best root induction was found in media having 1.0 mg/l IBA in cv. Bulum. It was found that cv. Bulum was better than cv. Rumbo in all stages of meristem culture. The presence of virus in plantlets was achieved by DAS-ELISA test, where 68–81% plantlets have been proved to be virus free among the studied viruses. Healthy growth and vigour was observed in meristem derived plants over their source plants after cultivation under natural conditions.     

Features of pine seedling growth under the effects of radiation]

The growth of Pinus sylvestris seedlings was delayed and polystem occurrence increased with chronic irradiation (a cumulative dose of external gamma-radiation was 6-8 Gy in 1986 and 0.7 Gy in 1990). The apical meristem activity was inhibited and central stem and main root died off under the effect of the radiation doses applied resulting in the development of plants of a specific floating form.     

Effect of benzylaminopurine on in vitro and in vivo root development in lentil, Lens culinaris Medik

The effect of benzylaminopurine (BAP) on the formation of roots from lentil shoots regenerated on media containing BAP was studied. Seedling shoot tips, first nodes and bractlets, and immature seeds cultured on the initiation media containing 2.25 or 0.225 mg/l of BAP regenerated multiple bud shoots. The regenerated shoots formed roots in percentages ranging from 4.6 to 39.9% on a rooting medium (R medium) containing 2 mg/l of indoleacetic acid. Rooting success on R medium depended upon the cytokinin used in the initiation media, its concentration, and the time elapsed during shoot formation on these media prior to transplanting regenerated shoots to R medium. In vivo study of root growth of lentil seedlings demonstrated the strong inhibitory effect of BAP on root growth reflected in a drastic reduction of the mitotic index of the root meristem. Received: 27 August 1996 / Revision received: 12 December 1996 / Accepted: 15 January 1997     

Evaluation of parameters describing the root system architecture of field grown maize plants (Zea mays L.)

The objective of this work was to study elongation curves of maize axile roots throughout their elongation period under field conditions. Relationships between their elongation rate and the extension rate of their branched region were also studied. Maize, early-maturing cultivar Dea, was grown on a deep, barrier-free clay loam (depth 1.80m). Trenches were dug during four periods until after silking and axile roots were excavated. Parameters measured were total length and the lengths of basal and apical unbranched zones. The rank of the bearing phytomer and general data about the carrying plant were also recorded. Results showed that axile roots from lower phytomers had similar elongation rates irrespective of the rank of the carrying phytomer. This elongation rate declined with root age. A monomolecular elongation model was fitted to the experimental data. Elongation was much slower in roots from upper phytomers. A rough linear relationship was found between the elongation rate of axile roots and the length of the apical unbranched zone. This result suggests that laterals appeared on a root segment a constant time after it was formed. Possible mechanisms with may account for the declining elongation rate with root age (increasing distance from aerial parts or adverse environmental conditions in deep soil layers) and variability between individual roots are also discussed.     

1,8-Cineole inhibits root growth and DNA synthesis in the root apical meristem ofBrassica campestris L.

1,8-cineole is a volatile growth inhibitor produced bySalvia species. We examined the effect of this allelopathic compound on the growth of other plants usingBrassica campestris as the test plant. Cineole inhibited germination and growth ofB. campestris in a dosedependent manner. WhenB. campestris was grown for 5 days with various concentrations of cineole, the length of the roots was found to be shorter as the concentration of cineole increased, whereas the length of the hypocotyl remained constant up to 400 μM cineole, indicating that cineole specifically inhibited growth of the root. The mitotic index in the root apical meristem of 3-day-old seedlings decreased from 5.6% to 1.6% when exposed to 400 μM cineole, showing that cineole inhibits the proliferation of root cells. We then examined the effect of cineole on DNA synthesis by indirect immunofluorescence microscopy using antibody raised against 5-bromo-2′-deoxyuridine (BrdU, an analogue of thymidine) in thin sections of samples embedded in Technovit 7100 resin. The results clearly demonstrated that cineole inhibits DNA synthesis in both cell nuclei and organelles in root apical meristem, suggesting that cineole may interfere with the growth of other plant species by inhibiting DNA synthesis in the root apical meristem.     

Shoots from MM. 106 apple rootstocks grown in a polythene tunnel do not contain a higher IAA concentration than shoots from field-grown plants

MM. 106 apple rootstock plants grown in a polythene tunnel show greater apical dominance and a higher propensity to root as cuttings than plants grown in the field. Experiments were conducted to test the hypothesis that the growth habit and rooting behaviour of polythene tunnel plants were caused by increased concentrations of idole-3yl-acetic acid. Cuttings taken from field-grown plants which had been sprayed with IAA showed increased rooting. In shoots of both field-grown and polythene tunnel-grown plants endogenous IAA levels were highest in the upper shoot region and declined progressively with distance from the apex. Plants grown in the polythene tunnel, however, did not contain significantly higher IAA levels than field plants. The analytical data do not support the hypothesis that the growth and rooting behaviour of plants grown in a polythene tunnel were caused by increased concentrations of IAA.Abbreviations IAA indol-3yl-acetic acid     

Influence of root restriction and ethylene exposure on apicaldominance of petunia (Petunia xhybrida Hort. Vilm.-Andr.)

Reducing rooting volume restricted root growth during theproduction of Petunia x hybrida'Orchid and resulted in an unfavorable increase in apicaldominance. Exposing young petunia seedlings to ethylene counteracted theeffects of root restriction. Rooting volumes of 9, 28, 58, or 160mL restricted the development of lateral shoots, therebyincreasing apical dominance compared to plants grown in 162 mLrooting volumes. Ethephon, an ethylene-producing compound, increased thedevelopment of lateral shoots of seedlings grown in rooting volumes rangingfrom 28 mL to 160 mL. At a rooting volume of 9mL, ethylene exposure was not capable of reducing the growth ofthe main shoot; apical dominance remained strong in both the control andethephon-treated plants. Because lateral shoot development was not restrictedby rooting volumes greater than 160 mL, exposing these plants toethylene did not result in supplementary lateral shoot development. Levels ofindole-3-acetic acid (IAA), isopentenyladenosine (iPA), and zeatin riboside(ZR) decreased on a whole shoot basis as rooting volume decreased from 162 to58 mL. Indoleacetic acid levels in ethephon-exposed plantsdecreased 20% compared to the control. The cytokinins iPA and ZR showedno response to ethylene exposure; however, the ratio of auxin/cytokinindecreased 24% compared to the control. The decrease in theauxin/cytokinin ratio was associated with an increase in the number and lengthof lateral shoots.     

Rooting of blue honeysuckle microshoots

Rooting of axillary shoots of two blue honeysuckle forms, Lonicera caerulea f. caerulea and L. caerulea f. edulis, was studied. Both in vitro and ex vitro rooting procedures were used, and the effects of mineral and auxin concentrations of the rooting media were tested. Reduced mineral nutrient concentrations of modified MS medium allowed more root elongation but did not affect the primary root number. The rooting percentage was high (≥ 90) in the form caerulea microcuttings but low (< 40) in the form edulis microcuttings when not treated with auxin. The rooting frequency and primary root number of the form edulis shoots could be increased up to 100 with 10 roots per microcutting, in the continuous presence of auxin. The continuous auxin treatments repressed the elongation and increased the diameter of primary roots and induced callus formation at the base of the shoots. Differences in root systems were related to equimolar concentrations of the auxins indole-3-butyric acid, indole-3-acetic acid and α-naphthaleneacetic acid, but the differences were diminished after one month ex vitro. After transfer ex vitro, several of the roots formed in vitro and some microcuttings died. A high rooting percentage and a good ex vitro survival and root growth of the form edulis microplants were achieved by a 7-day pulse with 4 μM indole-3-butyric acid followed by rooting ex vitro. This revised version was published online in June 2006 with corrections to the Cover Date.     

Growth delay in 9L rat brain tumor spheroids after irradiation with single and split doses of X rays

The response of 9L spheroids to irradiation with single and split doses of X rays has been investigated. Irradiation with single doses caused a dose-dependent decrease in spheroid growth rate, which eventually returned to the growth rate for unirradiated spheroids. This delay appeared to be related to cell survival. When spheroids were irradiated with two 4-Gy doses of X rays separated by various times the amount of growth delay was intermediate between that observed with single doses of 4 and 8 Gy. For relatively short times (15-90 min), recovery probably resulted from repair processes, but for longer times (up to 24 hr), recovery also appeared to depend on cellular redistribution and repopulation effects.     

Pericycle cell proliferation and lateral root initiation in Arabidopsis

In contrast with other cells generated by the root apical meristem in Arabidopsis, pericycle cells adjacent to the protoxylem poles of the vascular cylinder continue to cycle without interruption during passage through the elongation and differentiation zones. However, only some of the dividing pericycle cells are committed to the asymmetric, formative divisions that give rise to lateral root primordia (LRPs). This was demonstrated by direct observation and mapping of mitotic figures, cell-length measurements, and the histochemical analysis of a cyclin-GUS fusion protein in pericycle cells. The estimated duration of a pericycle cell cycle in the root apical meristem was similar to the interval between cell displacement from the meristem and the initiation of LRP formation. Developmentally controlled LRP initiation occurs early, 3 to 8 mm from the root tip. Thus the first growth control point in lateral root formation is defined by the initiation of primordia in stochastic patterns by cells passing through the elongation and young differentiation zones, up to where lateral roots begin to emerge from the primary root. Therefore, the first growth control point is not restricted to a narrow developmental window. We propose that late LRP initiation is developmentally unrelated to the root apical meristem and is operated by a second growth control point that can be activated by environmental cues. The observation that pericycle cells divide and lateral root primordia form without intervening mitotic quiescence suggests that lateral organ formation in roots and shoots might not be as fundamentally different as previously thought.     

In vitro regeneration and propagation of chickpea (Cicer arietinum L.) from meristem tips and cotyledonary nodes

Summary Two representative cultivars ofCicer arietinum, the desi-type cv.Annigeri and the kabuli-type cv.ICCV6, were regenerated in vitro and clonally propagated from cotyledonary nodes and meristem tips. The explants were dissected from 1-wk-old seedlings aseptically germinated on WH medium. In both cultivars, all nodes cultured on B5 medium supplemented with 4.4μM 6-benzylaminopurine developed up to seven shoots per node within 3 wk. Meristem tips were much better suited for multiple shoot formation. Cultured on DKW-C-a medium supplemented with 4.4μM 6-benzylaminopurine and 0.05μM indole-3-butyric acid, 96% of the meristem tips produced up to 10 shoots per explant. A new method in improving clonal propagation was subdividing the meristem tips. Doing so, multiple shoot formation was considerably enhanced: up to 90 shoots per original explant could be obtained with cv.Annigeri, and up to 50 with cv.ICCV6. Indole-3-butyric acid proved to be the best rooting factor. From several media tested, the best root induction and development was achieved on WH medium supplemented with 2.5μ M indole-3-butyric acid: 72% rooting with cv.Annigeri and 68% rooting with cv.ICCV6. With both cultivars there were no differences in rooting capacity between shoots of nodal origin and those derived from meristem tips. The plantlets obtained were transferred into soil and kept under greenhouse conditions. The survival frequency was 28% with cv.Annigeri and 23% with cv.ICCV6. R₀ plants remained smaller than seed-grown controls and produced only a few fertile seeds. There was no difference between R₁ plants and controls in growth, development, and seed set.