Journal of Experimental Botany, Vol. 53, No. 368, pp. 505-512,
March 1, 2002
© 2002 Oxford University Press
Original Papers |
Induction of early bolting in Arabidopsis thaliana by triacontanol, cerium and lanthanum is correlated with increased endogenous concentration of isopentenyl adenosine (iPAdos)
Department of Biological Sciences, 14 Science Drive 4, National University of Singapore, Singapore 117543
Received 17 July 2001; Accepted 1 November 2001
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Abstract |
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The effects of triacontanol (TRIA), applied singly or in combination with cerium nitrate and lanthanum nitrate, on bolting of Arabidopsis thaliana were studied. Triacontanol (0.1 to 0.6 µM) added to the culture medium induced early bolting. TRIA (0.3 µM) applied with low concentrations of cerium and lanthanum caused a synergistic stimulation of bolting. In medium containing 0.3 µM TRIA, 0.1 µM cerium nitrate and 0.1 mM lanthanum nitrate, 82% of the plants bolted 20 d after seed sowing compared to only 8.6% in basal medium and 47.8% in medium with TRIA only. The changes in the endogenous concentrations of total cytokinins of the isopentenyl adenine (IP) subfamily in the leaf and root tissues were correlated with TRIA-induced early bolting. The combined treatment of TRIA (0.3 µM), cerium nitrate (0.1 µM) and lanthanum nitrate (0.1 mM) resulted in a significant increase in the endogenous concentrations of total cytokinins of the IP subfamily in the root and leaf tissues compared to plants growing in the basal medium and medium with TRIA. The exogenous application of six natural cytokinins to the plants revealed that only isopentenyl adenosine (iPAdos) was as effective as TRIA on floral bud formation. iPAdos was also found to have similar effects as TRIA on root growth and reproductive growth. These results suggest a correlation between the early bolting induced by TRIA, cerium and lanthanum and the production of higher concentrations of endogenous iPAdos.
Key words: Arabidopsis thaliana, cerium, cytokinins, early bolting, lanthanum, triacontanol.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Triacontanol (TRIA) is a 30-carbon, straight chain primary alcohol. It is a natural constituent of wax in the cuticle of plants and Ries et al. reported that it has plant growth regulator properties (Ries et al., 1977
). Subsequently, a number of papers on the effects of TRIA on plant growth and development have been published (Ries, 1985). TRIA, in nanomolar quantities, increased dry weight, CO2-fixation, reducing sugars, soluble proteins, free amino acids, and yield in many crop plants (Ries, 1985). Among these reports, there were clues that TRIA might have beneficial effects on flowering. For example, TRIA was reported to increase the quality and yield of flowers in Chrysanthemum morifolium and orchids (Skogen et al., 1982; Yee, 1983). Application of TRIA (10-7 M) increased the yield of certain seed crops (Ries, 1985, 1991) and seed production is closely related to the production of flowers.
Welebir reported that TRIA, when applied together with lanthanum, caused a synergistic stimulation in the growth of wheat, barley and rye (Welebir, 1982). Lanthanum is a member of the rare-earth elements (REEs) which comprise of a group of 15 trivalent metallic elements with similar properties. They are widely used in agriculture in China (Guo, 1985; Tang and Xiao, 1996). Applications of cerium and lanthanum, two of the REEs, were reported to increase the root growth of corn (Diatloff et al., 1995a, b), increase spike production in wheat (Meehan et al., 1993) and enhance root and shoot growth in Phaseolus radiatus and Brassica pekinensis (Velasco et al., 1979). Recently, the effects of REEs on growth and development of Arabidopsis thaliana were studied. The results showed that lanthanum and cerium promoted reproductive growth and flowering of A. thaliana, but had little effect on its vegetative growth (He and Loh, 2000). Changes in the endogenous cytokinin concentrations in plants growing in medium with or without the REE were, however, not significant. Hence it is suggested that REEs such as lanthanum might affect the sensitivity of cells and the membrane binding of plant hormones which, in turn, affect flowering (He and Loh, 2000).
In this paper, the effects of TRIA on A. thaliana are described with the intention of clarifying whether there is a synergistic effect of TRIA and REE. TRIA is known to act by eliciting a second messenger, 9-ß-L(+)adenosine, which is structurally similar to cytokinins (Ries, 1991). Cytokinins are regarded as important components of floral stimuli in plants (Bernier et al., 1993). In this paper, it is also demonstrated that there is a correlation between the early bolting caused by TRIA and REE and an increased concentration of endogenous isopentenyl adenosine (iPAdos), a cytokinin.
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Materials and methods |
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Plant materials, culture media and culture conditions
Seeds of Arabidopsis thaliana L. Heynh. ecotype Columbia (LEHLE Seeds, USA) were surface-sterilized by soaking in 75% alcohol for 30 s followed by 15% Clorox® for 20 min. The seeds were then rinsed five times with autoclaved water prior to culture. Murashige and Skoog medium (quarter-strength) was used for seed germination and seedling growth (Murashige and Skoog, 1962
). The pH of the medium was adjusted to 5.8 before agar (Difco, 0.8%) was added. Triacontanol (Sigma, TRIA) was formulated according to the method of McKeown (cited in Ries, 1985). Cerium nitrate hexahydrate (Sigma) and lanthanum nitrate hexahydrate (Sigma) were dissolved in Mili-Q water. Membrane-sterilized stock solutions of TRIA, cerium nitrate and lanthanum nitrate were added to the autoclaved media prior to dispensing into Magenta GA7® vessels (Mangenta Corp., USA). Seeds were germinated at room temperature in the dark. Unless otherwise stated, 2-d-old seedlings were transferred to 16 h photoperiod of 54 µmol-1 m-2 s-1 provided by Cool White fluorescent lamps at 25±2 °C for growth. Bolting was scored as the appearance of a 1 cm-long inflorescence stalk.
Extraction and determination of plant growth regulators
Roots and leaves were obtained from seedlings 12, 14, 16, and 18 d after seed sowing. Plant tissues (leaf: about 1 g, root: about 0.3 g) was homogenized in 4 ml of 80% ethanol followed by 2 h incubation at 4 °C. After centrifugation at 1670 g for 3 min, the supernatant was transferred to another centrifuge tube. The tissues were re-extracted with 2 ml of 80% ethanol, centrifuged and the supernatant was pooled together. The total extracts were vacuum evaporated at 4 °C (Eppendorf Concentrator 5301) to 1/3 of the original volume. The aqueous residues were then filter-sterilized (Milipore, 0.2 µm) and the samples were stored at -20 °C. Immunoassay detection kits (Sigma Chemical Company) were used for quantitative determination of indole-acetic acid (IAA), abscisic acid (ABA), zeatin riboside (ZR), dihydrozeatin riboside (DHZR), and iPAdos. The analysis and determination of hormones were performed according to the protocols provided by the manufacturer. Briefly, the competitive antibody binding method was used to measure concentrations of hormones in plant extracts. Hormones were labelled with alkaline phosphatase and then added along with the plant extract to the antibody coated microwells. The unbound tracer was washed away before adding the substrate. The colour intensity of the mixture was read by an Ultra Microplate Reader (Bio-TEK Instrumets, Inc). All hormonal concentrations were expressed in terms of picomol g-1 fresh weight (pmol g-1 FW). All experiments were repeated at least twice. The results of cytokinin determinations are presented as concentrations of each subfamily, not as concentrations of individual compounds. This reflects the specificity of immunological assays and does not alter the interpretation of the results.
Exogenous application of iPAdos and other cytokinins
Seedlings at the 2-leaf stage were transplanted to media supplemented with iPAdos or other natural cytokinins. Roots of 50 plants were collected 21 d after seed sowing. Bolting and the appearance of flower buds were scored daily. The term ‘co-florescences’ is used to describe the lateral branches with flowers from the main inflorescence stalk. Plant height and the number of flowers produced per plant were scored 35 d after seed sowing. Each treatment was repeated at least twice, Tukey's pairwise comparisons (P=0.05) were used to compare the response of the different treatments.
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Results |
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Effects of TRIA on bolting
In basal medium, the first appearance of bolting was observed in about 13% of the plants 20 d after seed sowing (Fig. 1
). In the medium with 0.3 µM TRIA, only 18 d were required for 13% of the plants to bolt. TRIA at 0.3 µM was found to be most effective on bolting; about 93% of the plants bolted 23 d after seed sowing compared to only 39% of the plants bolted in TRIA-free medium (Fig. 1). Media supplemented with TRIA higher than 0.6 µM were less effective on induction of bolting (data not shown).
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Effects of TRIA in the presence of cerium nitrate and lanthanum nitrate
Addition of 0.1–0.5 µM cerium nitrate and lanthanum nitrate to the medium containing 0.3 µM TRIA was found to be more effective on bolting when compared to the medium with only 0.3 µM TRIA added (Fig. 2). The effects of 18 combinations were investigated and some of the results are presented in Fig. 2. The combination of 0.3 µM TRIA, 0.1 µM cerium nitrate and 0.1 µM lanthanum nitrate was found to be most effective (Fig. 2). For example, on day 20, 82% of the plants bolted in this medium compared to 8.6% in basal medium and 49% in medium with only 0.3 µM TRIA added. In TRIA-free medium with 0.1 µM cerium nitrate and 0.1 µM lanthanum nitrate, only about 13% of the plants bolted on day 20. These results indicate a synergistic effect of TRIA (0.3 µM), cerium nitrate (0.1 µM) and lanthanum nitrate (0.1 µM) on bolting.
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Changes in endogenous concentrations of cytokinins and other plant growth regulators during bolting
IP subfamily:
In the leaf tissues of the plants growing in basal medium, the endogenous concentrations of the IP subfamily from days 12–16 was about 100 pmol g-1 FW (Fig. 3a). On day 18, the concentration was about doubled. Addition of 0.1 µM cerium nitrate and 0.1 µM lanthanum nitrate had no significant effect on the IP subfamily concentration compared to the controls (Fig. 3a). However, with the addition of 0.3 µM TRIA, the IP subfamily concentration was found to increase to about 270 pmol g-1 FW on day 14. In the presence of TRIA (0.3 µM), cerium nitrate (0.1 µM) and lanthanum nitrate (0.1 µM), the concentration of the IP subfamily increased to 464 pmol g-1 FW, but decreased to about 174 pmol g-1 FW on day 18 (Fig. 3a).
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The concentration of the IP subfamily cytokinins in the root tissues was about 10 times higher than the concentration in the leaf tissues (Fig. 3
). In the control plants, the IP subfamily concentration in the root tissues increased from 1041.6 pmol g-1 FW on day 12 to 4373.5 pmol g-1 FW on day 16 and then decreased to about 1521 pmol g-1 FW on day 18 (Fig. 3b). Addition of 0.1 µM cerium nitrate and 0.1 µM lanthanum nitrate had no significant effects on the concentration of the IP subfamily in root tissues. Addition of 0.3 µM TRIA, however, greatly increased the concentration to 5062.8 pmol g-1 FW on day 14. The incorporation of 0.1 µM cerium nitrate and 0.1 µM lanthanum nitrate in medium containing TRIA (0.3 µM) resulted in an even higher (7822.6 pmol g-1 FW) concentration of the IP subfamily in the roots on day 14. These results indicated that the combined treatment of TRIA, cerium nitrate and lanthanum nitrate resulted in a synergistic production of the IP subfamily cytokinins. The concentration of the IP subfamily in the root tissues decreased after day 14 (Fig. 3b).
Zeatin (Z) subfamily:
In leaf tissues of plants growing in the basal medium, the concentration of Z subfamily cytokinins remained relatively stable between days 12 and 18 (Fig. 4a). The combined treatment of TRIA (0.3 µM), cerium nitrate (0.1 µM) and lanthanum nitrate (0.1 µM) increased the concentration of Z subfamily members on days 12 and 14, but subsequently it declined to the concentration of the control plants (Fig. 4a).
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In the root tissues of the control plants, the endogenous concentrations of Z subfamily decreased from day 12 to day 14 (Fig. 4b
). The application of 0.1 µM cerium and 0.1 µM lanthanum had no significant effects on the concentrations of Z subfamily members in root tissues. The addition of 0.3 µM TRIA or the combination of 0.3 µM TRIA, 0.1 µM cerium nitrate and 0.1 µM lanthanum nitrate significantly decreased the concentrations of the Z subfamily on days 12 and 14 in the root tissues (Fig. 4b).
Dihydrozeatin (DHZ) subfamily, IAA and ABA:
In root tissues of the control plants, the endogenous concentrations of DHZ subfamily cytokinins declined from 231.4 pmol g-1 FW on day 12 to 54.6 pmol g-1 FW on day 18. Addition of cerium nitrate (0.1 µM) and lanthanum nitrate (0.1 µM) or TRIA (0.3 µM) or their combinations had no major significant effects on the concentrations of the DHZ subfamily, IAA and ABA in leaf tissues and root tissues (data not shown).
Effects of exogenous iPAdos and other cytokinins on floral bud formation
As the change in the concentration of the IP subfamily was correlated with TRIA-induced early bolting (Fig. 3), further experiments were conducted to see whether iPAdos added to the culture medium could also induce early bolting. Preliminary experiments revealed that plants at the 2-leaf stage responded most effectively to exogenous iPAdos (data not shown) and hence they were used in this set of experiments.
In medium with 0.1 and 1.0 µM iPAdos, the earliest observation of plants with floral buds was on day 19, 2 d earlier than plants growing in the basal medium (Fig. 5). On day 21, about 69% of plants in the medium with 1.0 µM iPAdos were observed to have floral buds whereas in the basal medium there were only 20% of plants with floral buds (Fig. 5). The effects of zeatin (Z), ZR, dihydrozeatin (DHZ), DHZR, and isopentenyladenine (iPAde) were also investigated. Except that 3% of the plants growing in the medium with 0.1 µM iPAde were observed to form floral buds on day 20, no flower bud was observed in the plants growing in all media supplemented with these cytokinins on days 19 and 20. This indicates that, among the six natural cytokinins tested, iPAdos was the only one that effectively induced early floral bud formation.
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Effects of exogenous TRIA, iPAdos, iPAde, and Z on root growth and reproductive growth
Application of TRIA (0.3 µM) not only induced early bolting, but also significantly increased root growth and flower production (Table 1). The increased flower production was attributed mainly to the development of more co-florescences (the lateral branches with flowers from the main inflorescence stalk). To confirm the correlation between TRIA and the increase in the concentration of endogenous iPAdos further, the effects of TRIA, iPAdos, iPAde, and Z on root growth and the number of main inflorescence stalks and co-florescences were compared (Table 1). The addition of 0.1 µM iPAdos or 0.3 M TRIA to the basal medium both resulted in increased root growth and co-florescences (Table 1). The addition of Z significantly reduced root growth and the effect of iPAdos and TRIA on root growth is significantly different from that of iPAde. The addition of iPAde or Z was observed to produce multi-inflorescence stalks without co-florescences (Table 1). These results suggest that only iPAdos had similar effects to TRIA on root growth and reproductive growth.
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Discussion |
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The timing of the transition from vegetative growth to flowering is of importance in agriculture, horticulture, and plant breeding. The nature of the flowering-inducing signals, however, remains unclear (O'Neill, 1992
). The transition to flowering is probably under ‘multifactorial control’ and several chemicals (assimilates and phytohormones) are likely to participate in floral induction (Bernier, 1988; Bernier et al., 1993). Among these factors, cytokinins are considered to be one of the most important physiological signals that induce flowering (Bernier, 1988; Machackova et al., 1993; Lejeune et al., 1994; Bonhomme et al., 2000). However, research findings on the roles of cytokinins on flowering were not consistent. This is mainly because the major forms of cytokinins varied from species to species (Lejeune et al., 1988) and different techniques were used for cytokinin determination. For example, in Nicotiana tabacum, no free cytokinin bases (Z, DHZ, iPAde) were detected in prefloral transition apices (Dewitte et al., 1999). Instead, a 3-fold decrease in the concentration of cytokinin ribosides (ZR, DHZR, iPAdos) was observed (Dewitte et al., 1999). In the root exudates of Sinapis alba, the activity of iPAdos increased in the induced plants and the activity of Z and ZR in induced plants was a little higher than in the control (Lejeune et al., 1988). A transient increase in ZR and iPAdos concentrations occurred in root and stem tissues of Boronia megastigma within days of transferring the plant to cool (17/9 °C day/night) conditions (Day et al., 1995). According to Bernier et al., iPAdos synthesized in the roots of Sinapis alba were first transported through the xylem to mature leaves, and then converted into iPAde prior to be transported into apical meristems to participate in floral initiation (Bernier et al., 1993). In the present study, the first appearance of bolting was observed on day 20 after seed sowing in basal medium (Fig. 1). The concentrations of Z subfamily and DHZ subfamily cytokinins in leaf tissues remained relatively constant from days 12–18, however, a substantial increase in IP subfamily cytokinins was observed on day 16 in root tissues and on day 18 in leaf tissues (Figs 3, 4). Since actual transition was likely to occur several days prior to the visible bolting, the significant increase of the IP subfamily on day 18 in leaf tissues and its implication in subsequent reproductive growth of the plant deserves further investigation. Further results showed that exogenous application of iPAdos to plants at the 2-leaf stage could induce early floral initiation (Fig. 5).
There are several reports exploring the mode of action of TRIA on plants (Ries, 1985, 1991; Morre et al., 1991). A signal transduction mechanism had been envisaged for the action of TRIA which acted by eliciting a second messenger: 9-ß-L(+)adenosine (Ries et al., 1990). 9-ß-L(+)adenosine is structurally similar to cytokinins (Ries, 1991). This study's results showed that there was correlation between iPAdos induction and TRIA treatment. Firstly, in TRIA-treated root tissues, the peak of total cytokinins of the IP subfamily was observed on day 14, which was 2 d earlier than that in root tissues of the control plants (Fig. 3). These results were well correlated with the authors' observation that in TRIA-supplemented medium, plants bolted earlier than those in basal medium (Fig. 1). Secondly, only iPAdos was found to be as effective as TRIA on early floral initiation (Fig. 5). Thirdly, the effects of iPAdos on root growth and reproductive growth were similar to those of TRIA (Table 1).
Welebir reported observing a synergistic stimulation on growth of wheat, barley and rye when TRIA was applied together with lanthanum (Welebir, 1982). In A. thaliana, a synergistic stimulation on root growth and bolting was observed when TRIA was applied in the presence of 0.1 µM cerium nitrate and 0.1 µM lanthanum nitrate (Fig. 2). A much higher concentration of iPAdos in leaf and root tissues was observed on day 14 (Fig. 3) in the presence of TRIA, cerium and lanthanum compared to the addition of TRIA alone. Previous results showed that cerium nitrate and lanthanum nitrate significantly enhanced the effects of iPAdos on root growth and flowering in A. thaliana (He and Loh, 2000). Whether such an increase in iPAdos after TRIA+La+Ce treatment resulted in the synergistic effects remains to be elucidated. In addition, despite TRIA+La+Ce are permanent components of the culture medium, the increase in iPAdos concentrations in roots and leaves are transient (Fig. 3). Such transient increase in cytokinins was also observed in Boronia megastigma within days after transferring the plant to cool conditions (Day et al., 1995). In Chenopodium rubrum, a transient increase in iPAde was noted in leaves under inductive photoperiodic treatment (Machackova et al., 1993). As cytokinins in the apical part come from roots and possibly leaves, fluxes of cytokinins might be important for floral induction (Machackova et al., 1993). Thus the changes of fluxes of cytokinins during flower induction merit further interest (Machackova et al., 1993).
The ability to manipulate endogenous cytokinin concentrations without the use of exogenous cytokinins is important in the study of flowering physiology. Cytokinins are important signals in flowering (Bernier et al., 1993; Machackova et al., 1993; Lejeune et al., 1994). They were normally applied exogenously at about 10-6 M for effective induction of flowering in culture (for examples: Chambers et al., 1991; Metha et al., 1993; Roberts et al., 1993; Bonhomme et al., 2000; He and Loh, 2000). However, the endogenous concentrations of cytokinins occurs at 10-12 mol g-1 FW (Fig. 3). This study revealed that TRIA and REE have a synergistic effect when applied together and an increase in the concentrations of endogenous iPAdos in A. thaliana was detected. Such treatment may have the potential to be used as a non-cytokinin-based method for promoting flowering and the study of flowering physiology. However, whether TRIA and REE have similar effects on other plant species has yet to be investigated.
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Notes |
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1 To whom correspondence should be addressed. Fax: +657795671. E-mail: dbslohcs{at}nus.edu.sg
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References |
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