AOBPreview originally published online on August 15, 2003
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Annals of Botany 92: 601-612, 2003
© 2003 Annals of Botany Company
Day and Night Temperature Responses in Arabidopsis: Effects on Gibberellin and Auxin Content, Cell Size, Morphology and Flowering Time
1 Department of Horticulture and Crop Sciences, Agricultural University of Norway, N-1432 Ås, Norway and 2 Department of Biology, Faculty of Science, University of Tromsø, N-9037 Tromsø, Norway
* For correspondence. Fax +47 6494 7802, e-mail elin.thingnas{at}ipf.nlh.no
Received: 14 April 2003;; Returned for revision: 30 May 2003. Accepted: 30 June 2003; Published electronically: 15 August 2003
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
The effect of 16 different day (DT) and night (NT) temperature combinations (DT and NT 12, 17, 22 and 27 °C) on rosette leaf growth, flower stem elongation and flowering time in Arabidopsis thaliana Ler was investigated. Final leaf length decreased with increasing NT due to a combination of reduced elongation period and reduced elongation rate. Final stem length increased with increasing DT due to increased elongation rate, and decreased with increasing NT due to a decrease in elongation period. Under NT 27 °C, however, stem elongation rate increased greatly, resulting in the same final stem length as under NT 12 °C. The transition to flowering was accelerated by increasing NT. A linear regression analysis was performed to clarify the relationship between final leaf length, final stem length and flowering time with DIF (DT minus NT) and/or ADT (average daily temperature). For all three variables, the effect of DIF depended on ADT and vice versa. The relationship of final stem length with DIF also depended on the temperature range. Increased cell volume in flower stems developing at DT/NT 22/12 °C gave rise to longer and thicker stems compared with stems developing at DT/NT 12/22 °C. GC–MS analysis (gas chromatography–mass spectrometry) showed that the endogenous level of IAA was 56 % higher in stems grown under DT/NT 22/12 °C compared with DT/NT 12/22 °C. Of the 12 gibberellins analysed, however, only the level of non-bioactive GA29 was affected by the temperature treatment.
Key words: Arabidopsis thaliana, auxin, day temperature, DIF, elongation, flowering time, GC–MS analysis, gibberellins, leaf length, light microscope, night temperature, stem length.
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Plants given the same average daily temperature (ADT) can show different growth patterns due to differences in day and night temperatures (DT and NT). Went (1944) introduced the term ‘thermoperiodism’ to describe the effect of alternating DT and NT on various plant responses. This diurnal variation in temperature influences internode length, plant height, petiole and flower stem length, chlorophyll content, leaf and shoot orientation and flowering (Myster and Moe, 1995). Erwin et al. (1989) studied the effects of 25 different DT and NT combinations on the growth of Lilium longiflorum. They concluded that stem elongation and leaf orientation were influenced more by the difference (DIF) between DT and NT than by absolute DT and NT. Many subsequent studies on a wide range of plant species, most of them performed out of horticultural interest, also concluded that DIF is a significant factor in determining stem extension responses. DIF has become a valuable concept in providing growers with a simple and effective way of calculating elongation responses to temperature (Erwin and Heins, 1995). Generally, plants elongate more when DT is warmer than NT (positive DIF) compared with the opposite regime where DT is cooler than NT (negative DIF). However, DT and NT affect the flowering time of plant species differently (Myster and Moe, 1995).
The physiology behind thermoperiodic elongation responses in plants is still unclear. Past studies on the cellular anatomy of fully grown internodes have shown that DIF exerts its growth effect via changes in cell length (Erwin et al., 1994), or both cell length and number of cells (Strøm and Moe, 1997). In general, enhanced cell elongation and division are related to hormonal factors. Gibberellins (GAs) are a class of essential growth hormones that regulate cell elongation and cell division, but auxin (IAA) also regulates cell size (Yang et al., 1996).
Genetic approaches, using GA mutants, have provided insight into the involvement of GA in thermoperiodic control of stem elongation (Grindal et al., 1998b). Exogenously applied bioactive GAs have also been found to reduce or eliminate the inhibition of stem elongation by negative DIF (Grindal et al., 1998a; Myster et al., 1997b). Furthermore, from analysis of endogenous GA levels, it has been suggested that alternating NT and DT affects certain steps in the early 13-hydroxylation pathway for the biosynthesis of GA1 (Jensen et al., 1996) and the 2ß-hydroxylation pathway that inactivates GA1 to GA8 (Grindal et al., 1998b). In Arabidopsis thaliana and other long-day (LD) rosette plants, it has been shown that photoperiodic control of stem elongation is mediated by GAs (Talon and Zeevaart, 1990; Zeevaart et al., 1993; Xu et al., 1997).
No studies on the effects of diurnal temperature variations on growth and development in the widely used model plant A. thaliana have been reported. A. thaliana is a rosette annual species with separate vegetative and reproductive growth phases. The present study investigates the responses to 16 different combinations of four DTs and NTs, in respect of stem elongation, leaf growth and flowering time in A. thaliana Ler. One of the objectives was to determine the effects of absolute DTs and NTs at the plant level, and to see whether this could be related to DIF and/or ADT. Furthermore, a study of the flower stem at the cellular and hormonal levels was performed on plants grown at two opposite DT/NT combinations.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Plant material and growth conditions during pre-cultivation (Table 1)
Seeds of Arabidopsis thaliana (L.) Heynh., ecotype Landsberg erecta (Ler), were stratified in darkness at 4 °C for 3 d on filter paper saturated with water. Seeds were then transferred to 10-cm pots filled with a moistened 3 : 1 mixture (v/v) of standard fertilized peat (Floralux, Nittedal torvindustri A/S, Nittedal, Norway) and Perlite. Plants were grown in a glasshouse compartment at 22 °C and short days (SD) (10-h photoperiod), one seedling per pot for growth and flowering time studies, three seedlings per pot for cellular studies and five seedlings per pot for GA and IAA analysis. At a photon flux density (PPFD) (400–700 nm) of 100 µmol m–2 s–1 or lower, measured at rosette level, natural daylight was supplemented with high-pressure sodium (HPS) lamps giving 130 µmol m–2 s–1. Daylight was reduced to 70 % with curtains (JLS-70; AB Ludvik Svenson, Gothenburg, Sweden) at 300 µmol m–2 s–1 or higher. Water pressure deficit was 0·66 kPa, and plants were irrigated with ordinary tap water. In the study of leaf growth and flowering time, pre-cultivation ended after 14 d when the plants had two true leaves with petioles. For growth and cell studies, and quantification of GAs and IAA in the flower stem, the plants were grown for 14 d under SD, followed by natural day length (about 17-h photoperiod, May 2000, N59°) until they showed the first sign of bolting (10 d).
Growth conditions during the experimental period (Table 1)
After pre-cultivation, the plants were transferred to four controlled environment cabinets (Conviron, Controlled Environments Ltd, Winnipeg, Canada), each maintained at a constant temperature of 12, 17, 22 or 27 °C (± 1 °C). Day (12 h) and night (12 h) periods were controlled by turning the lights on and off. The 16 different day and night temperature combinations shown in Table 1 were provided by moving the plants to their respective temperature regimes twice daily, at the start and end of each photoperiod. Light was provided by white fluorescent tubes (F96T12/CW/1500; General Electric, USA) combined with incandescent lamps (60 W; Osram, Drammen, Norway). A PPFD (400–700 nm) of 196 ± 8 µmol m–2 s–1 was measured at rosette level. The plants were watered as required, and every 2 weeks fed with a complete nutrient solution (electrical conductivity = 1·5 mS cm–1). The water pressure deficit was 0·55 ± 0·04 kPa at all temperatures. For cell studies and quantification of IAA and GAs in the flower stem, plants were grown under the DT/NT combinations 22/12 °C and 12/22 °C only.
|
Growth measurements
All growth measurements were made on four to six plants from each treatment, and the experiment was repeated once. Leaf growth was recorded for true rosette leaf number five by measuring the length of the whole leaf (leaf lamina and petiole) and the maximum width of the leaf lamina, using a digital slide calliper. Leaf length was measured every second day until the leaf stopped growing. Leaf width was measured at final leaf length, in only one replicate. The transition from vegetative to reproductive growth, here referred to as ‘flowering time’, was measured in two ways. First, by counting the number of days from transfer to experimental conditions to the macroscopic appearance of flower buds in the centre of the rosette and, secondly, by counting the number of primary leaves (length
5 mm) in the rosette at the same time. The length of the primary inflorescence shoot, here called the (flower) stem, was measured each day until it stopped increasing. The length measured, using a ruler, was the distance from the base to the first flower on the stem. The elongation period was defined as the number of days from the start of the temperature treatment to achievement of the final length (±1 mm and ±0·5 mm for stem and leaf, respectively). The elongation rate for this period (mm day–1) was then calculated.
Cellular study of the stem
Fresh, fully developed flower stems grown at DT/NT 22/12 °C and DT/NT 12/22 °C were harvested when they had stopped elongating. Sample sections (10 mm) from ten plants were taken from the middle part of the stems, between the base and the first flower, from each treatment in two independent experiments. Within each treatment, five random samples were selected for cross-sections and five for longitudinal sections, and fixed in 2 % paraformaldehyde and 1·25 % glutaraldehyde in 50 mmol L–1 L-piperazine-N-N'-bis(2-ethane sulfonic) acid buffer (pH 7·2) for 24 h at room temperature. Fixed tissue was rinsed in the same buffer and dehydrated in an ethanol series (70–80–90–96–4 x 100 %). Samples for light microscopy (LM) were infiltrated with LR white acrylic resin (TAAB Laboratories Equipment Ltd, Berkshire, UK), and polymerized at 60 °C for 24 h. Semi-thin (1–2 µm) sections were cut with a diamond knife. The sections were dried on silanated slides, stained with Stevenel’s blue (del Cerro et al., 1980), mounted with immersion oil, and imaged and analysed in a Leitz Aristoplan light microscope using video microscopy (Leica DC 100) and digital image processing (Image Pro Plus, ver. 3·0). Epidermal cells, cortical cells (the two outermost layers) and pith cells (the middle of the pith) were sampled from the longitudinal sections and the length of the cells of five random areas on each section measured (n 30). Epidermal and pith cell areas and pith cell numbers were recorded from cross-sections. Epidermal cell areas from three random areas on each section were measured. Areas and numbers of pith cells were recorded along two lines across the centre of the pith.
Quantification of GAs and IAA
Flower stems of A. thaliana grown at DT/NT 22/12 °C and DT/NT 12/22 °C were harvested after 5 d of temperature treatment, when the plants were predicted to be on the linear portion of the elongation curve and to be growing at maximum elongation rate. Mean stem height was 33 ± 4 mm (19–25 % of final length) under DT/NT 22/12 °C and 17 ± 2 mm (17–21 % of final length) under DT/NT 12/22 °C. Each sample consisted of stem tissue from 11–16 plants (1·2–2·5 g f. wt) and included the whole primary stem with leaves, side shoots and flowers. Samples were immediately frozen in liquid nitrogen, freeze-dried and stored at –70 °C. The samples were processed to a homogenous powder before cold 80 % (v/v) ethanol and internal standards, [17,17-2H2]-GAs (25–50 ng) and [13C6]-IAA (50 ng), were added. GAs and IAA were extracted overnight at –20 °C. Sample purification and GC–MS analysis (gas chromatography–mass spectrometry) were as described by Junttila et al. (1997), except that the step involving polyvinylpolypyrrolidone was omitted. Measurements were made twice for each of the two replicates.
Statistical analysis
Analysis of variance using the general linear model procedure in Minitab was performed separately for each response (e.g. final leaf length, final stem length) and the significance for each model factor evaluated. The respective effects of DT and NT were assessed by analysis of the 4 x 4 factorial treatment set given by combinations of the temperatures 12, 17, 22 and 27 °C. Responses were calculated by averaging the effects of a specific DT over the four NTs, and of a specific NT over the four DTs. The DT x repeat and NT x repeat interactions were pooled together with the higher order interaction DT x NT x repeat as residual error. Linear regression analysis was performed using the regression procedure in Minitab. In the second replicate of the cellular study of the stem, the pith was hollow which made it impossible to measure pith cell length and area. A two-sample t-test was thus used to test for significant treatment effects. To cope with the possible unequal variances and non-Gaussian distribution of the area data, a non-parametric bootstrap method (Efron and Tibshirani, 1993) was implemented in Matlab (Math Works Inc.). In the cases of two replicates, a two-way analysis of variance (ANOVA) was used to test for significance.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Leaf growth
Final leaf length decreased significantly with increasing NT, and was 20 % longer at NT 12 °C than at NT 27 °C (Table 2). DT had no significant effect on final leaf length (Table 2). There was no evident effect of either DT or NT on leaf width (data not shown).
|
Leaf elongation followed a sigmoid pattern for all 16 DT/NT combinations (Fig. 1). DT significantly affected both the elongation period and the elongation rate, although in opposite ways; the period decreased with increasing DT, whereas the rate increased with increasing DT (Table 2). Neither the period nor the rate was significantly affected by NT, but both (especially the rate) tended to decrease as NT increased (Table 2).
|
Figure 2A and B shows A. thaliana rosette plants grown at DT/NT 22/12 °C (positive DIF) and at DT/NT 12/22 °C (negative DIF), both with ADT 17 °C. A linear regression analysis showed significant effects of DIF (P = 0·01), ADT (P
0·001) and the interaction DIF x ADT (P = 0·035) on final leaf length. The resulting regression model was used to estimate final leaf length (Fig. 3). Final leaf length was estimated to increase with increasing DIF, the effect being greater under cool than under warm ADTs (Fig. 3). Furthermore, final leaf length was estimated to be unaffected by ADT at –10 DIF. At higher DIF, however, the length was estimated to decrease with increasing ADT, the negative effect being stronger as DIF increased (Fig. 3).
|
|
Stem elongation
Final stem length decreased significantly with increasing NT in the temperature range 12–22 °C, and was 18 % longer at NT 12 °C than at NT 22 °C. However, at NT 27 °C, final stem length was about the same as under NT 12 °C. Furthermore, final stem length increased with increasing DT, being 40 % higher at DT 27 °C compared with DT 12 °C (Table 3).
|
Like leaf elongation, stem elongation followed a sigmoid pattern for all 16 DT/NT combinations (Fig. 4). Elongation period was significantly influenced by the interaction between DT and NT. Increased NT reduced the elongation period, although the effect was less at warmer DTs (Table 3). The elongation rate increased significantly with increasing DT (Table 3). It also increased significantly with increasing NT, although the increase occurred only at NT 22 °C and, even more, at NT 27 °C.
|
A linear regression analysis showed significant effects of DIF, ADT and the interaction DIF x ADT on final stem length (all with P
0·001). The resulting regression model was used to estimate final stem length (Fig. 5). At low ADT, final stem length was estimated to increase rapidly with increasing DIF, whereas under high ADT, final stem length was estimated to decrease. Furthermore, under negative and zero DIF, final stem length was estimated to increase with increasing ADT whereas, under positive DIF, the effect of ADT was estimated to be zero or only slightly negative. Removing ADT and DIF x ADT from the model, thus leaving DIF as the only variable, resulted in a model that explained only 33 % of the variation (r2 = 0·33). However, a close (r2 = 0·90) positive relationship was found when NT 27 °C values were excluded from the analysis (Fig. 6).
|
|
When comparing A. thaliana plants grown under the same ADT, plants grown at the two DT/NT combinations 22/12 °C (positive DIF) and 12/22 °C (negative DIF) showed the biggest differences in final stem length (Fig. 2C and D), and were, therefore, chosen for further cellular and hormonal studies.
Cellular studies of the stem
Flower stems grown at DT/NT 12/22 °C were significantly thinner than those grown at DT/NT 22/12 °C (Table 4 and Fig. 2E and F). However, the stems had approximately the same number of vascular bundles, and an epidermal layer surrounding a cortex of about four or five layers of chlorenchyma cells irrespective of the temperature treatment (Fig. 2E and F). To clarify the difference in cell size, the areas of pith and epidermal cells were measured from cross-sections (Table 4). The distribution of pith and epidermal cell sizes is shown in Fig. 7A and B. A higher portion of large cells of both types were produced when the stems were grown at DT/NT 22/12 °C compared with DT/NT 12/22 °C. In addition, a tendency towards fewer pith cells was found in the cross-section taken from DT/NT 12/22 °C stems, although the difference was not significant at the 5 % level (P = 0·093).
|
|
Compared with DT/NT 22/12 °C, there was a reduction in cell length in plants grown at DT/NT 12/22 °C. However, the reduction was found only in epidermal and pith cells. Both cell types were about 20 % shorter in stems grown at DT/NT 12/22 °C compared with DT/NT 22/12°C (Table 4). Cortical cell length was similar in both treatments. There was a 33 % and a 35 % reduction in the volume of epidermal and pith cells, respectively, at DT/NT 12/22 °C compared with DT/NT 22/12 °C.
Endogenous GA and IAA levels in stem tissue
There was no difference in total endogenous GA level in stem tissue between the two temperature treatments DT/NT 12/22 °C and DT/NT 22/12 °C (P = 0·486) and, of the 12 GAs measured (53, 44, 19, 20, 29, 1, 15, 37, 24, 9, 4 and 34), only GA29 was affected significantly by temperature (Table 5). The endogenous level of GA29 in stem tissue was approx. 70 % higher under DT/NT 12/22 °C than under DT/NT 22/12 °C. The mean endogenous content of the GAs in the non-13-hydroxylation pathway (GA15
GA24 GA9GA4) was five-fold higher (P 0·001) than the mean content of the GAs in the parallel 13-hydroxylation pathway (GA53GA44GA19GA20GA1). GA8 was not detected in this experiment. The endogenous level of the auxin IAA in stem tissue was significantly higher (56 %) under DT/NT 22/12 °C than under DT/NT 12/22 °C (Table 5).
|
Flowering time
Both the number of rosette leaves at visible bud and the time to visible bud were significantly influenced by the interaction between DT and NT. The number of leaves and number of days decreased as NT increased, although the effect was less at warmer DT. The effect of NT was generally greater than the effect of DT (Table 6). A linear regression analysis showed a significant effect of ADT and the interaction DIF x ADT (both with P
0·001) on flowering time (number of rosette leaves at the time of visible bud). The resulting model was used to estimate flower induction (Fig. 8). Flower induction was estimated to accelerate with increasing ADT, the effect being greater as DIF decreased. Flower induction was estimated also to accelerate as DIF decreased, although the effect of DIF was not statistically significant (P = 0·173).
|
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Elongation affected by DT and NT
Final stem length in A. thaliana increased with increasing DT and, in the temperature range 12–22 °C, it decreased with increasing NT (Table 3). Similar stem elongation responses to DT and NT were found in Lycopersicon esculentum (Went, 1944), Lilium longiflorum (Erwin et al., 1989) and Fuchsia x hybrida (Erwin et al., 1991).
The effect of DT and NT on final leaf length was somewhat different from the effect on final stem length. Like final stem length, final leaf length decreased with increasing NT (Table 2). However, DT had no effect on final leaf length. These effects of DT and NT on leaf length in A. thaliana were also seen in L. longiflorum (Erwin et al., 1989). Furthermore, temperature affected stem elongation more strongly than leaf elongation in A. thaliana; the biggest difference in final length among the 16 DT/NT combinations being only 36 % for leaves but 73 % for stems (Tables 2 and 3, respectively).
In A. thaliana, both the period and rate of elongation were influenced by temperature (Tables 2 and 3). However, which factor was decisive for the change in final length was different in the plants’ responses to DT and NT. For example, final stem length increased with increasing DT due to a faster elongation rate and not through a change in elongation period. However, the decrease in final stem length when NT increased was due to a reduction in elongation period. Similar effects of DT and NT were shown for the elongation pattern in Dendranthema grandiflora (Carvalho et al., 2002).
For cellular, hormonal and growth studies of the flower stem, all plants were pre-cultivated at the same temperature (22 °C) until the plants showed the first sign of bolting (24 d). However, such a strategy complicates the interpretation of the results since plants transferred to experimental conditions of DT and/or NT 12–17 °C, unlike plants transferred to 22–27 °C, would have required some time to adapt to these lower temperatures. This problem should be of less importance for the measurements of leaf growth and flowering time since, in this case, the pre-cultivation period was much shorter (14 d).
Elongation related to DIF and ADT
Most DT and NT studies on plant growth have been performed out of horticultural interest. As DIF is a concept valuable in providing growers with a simple and effective way of calculating elongation response to temperature, most of these studies have explained the temperature responses in relation to DIF. In the present study of A. thaliana, both final leaf length and final stem length showed a poor relationship to DIF alone (r2 = 0·25 and r2 = 0·33, respectively). In the temperature range 12–27 °C, the effects of DT and NT on elongation were not equal in magnitude and not consistently opposite in sign, both of which are necessary for a high correlation to DIF. However, when all NT 27 °C values were excluded from the analysis, a close positive linear relationship (r2 = 0·90) was found between final stem length and DIF (Fig. 6). Carvalho et al. (2002) came to a similar conclusion for final internode length of D. grandiflora; the positive relationship to DIF was close only within a certain temperature interval, when the positive effect of DT on final internode length was compensated by a similar negative effect of NT, resulting in equal length at the same DIF. By contrast, Langton and Cockshull (1997) found no relationship between DIF and internode length in D. grandiflora. However, internode lengths were recorded after a fixed interval of 10 d, and these were later shown not to correlate with the final internode lengths (Carvalho et al., 2002). Likewise, the present measurements on leaf length (Fig. 1) and stem length (Fig. 4) demonstrate that comparisons between two temperature treatments are strongly time-dependent.
Whether or not NT 27 °C values were included, for a full picture of the relationship between elongation and DIF in A. thaliana, ADT also had to be taken into consideration. For both final leaf length and final stem length, linear regression analysis showed a significant effect of not only DIF, but also of ADT and the interaction DIF x ADT (Figs 3 and 5, respectively). According to their respective regression models, both final leaf length and final stem length were estimated to increase strongly with increasing DIF when ADT was low, whereas at high ADT, the effect of DIF was estimated to be almost zero (final leaf length) or even negative (final stem length). The estimated effect of ADT on final stem length was similar to that found in D. grandiflora (Karlsson et al., 1989). At negative DIF, internode length of D. grandiflora was predicted to increase as ADT increased, whereas at positive DIF, internode length was predicted to decrease. At all DIF values except –10, final leaf length of A. thaliana was predicted here to decrease with increasing ADT (Fig. 3). Likewise, in Viola x wittrockiana, leaf and petiole lengths decreased as ADT increased (Niu et al., 2000).
Cellular studies of the stem
In agreement with previous work, plants that developed at DT/NT 12/22 °C showed a reduction in epidermal and pith cell length compared with the opposite temperature regime (Table 4). In L. longiflorum, pith parenchyma and epidermal cell elongation was found to increase as DIF increased (Erwin et al., 1994), and, in Campanula isophylla pith parenchyma cells (Strøm and Moe, 1997), length decreased when the plants were grown at negative DIF compared with positive DIF. In the studies cited, cell width, measured on longitudinal sections, was unaffected by DIF. In contrast, the cells in the present study showed an enhancement of radial expansion. Similarly, in A. thaliana seedlings an enhanced transverse expansion of hypocotyl and cotyledon cells was found under high day and low night temperatures compared with the opposite regime (S. Torre, 2002, pers. comm.). This confirms that, in A. thaliana, alternating DT and NT affects cell expansion in both transverse and longitudinal directions.
In the study by Strøm and Moe (1997), it was concluded that in C. isophylla the growth stimulation of positive DIF was a result of the production of more and longer cells. However, in a study of L. longiflorum it was stated that the increased stem length of plants developed at positive DIF was due solely to longer, and not more, stem cells (Erwin et al., 1994). The samples in the present study were taken only from the middle part of the stem, which gives an approximate estimate for the entire stem. Even so, the entire stem was almost 40 % longer at positive DIF than at negative DIF, and the measured cells were not more than 20 % longer. This suggests both more and longer stem cells, indicating that alternating DT and NT affects both cell elongation and cell division in A. thaliana flower stems. The findings in L. longiflorum (Erwin et al., 1994) may be explained as most stem cells had already developed in the shoots, and the growth that occurred during cultivation was mostly expansion (Miller, 1993).
Temperature and endogenous levels of GAs and IAA
The endogenous content of the GAs in the non-13-hydroxylation pathway (GA15GA37GA24GA9 GA4) was generally higher than the content of the 13-hydroxylated GAs. This is in accordance with other studies of A. thaliana (Talon et al., 1990). Of all the 12 GAs analysed, only the endogenous content of the biologically inactive GA29 was significantly affected by DIF (Table 5).
This is not in accordance with earlier findings for C. isophylla (Jensen et al., 1996), Pisum sativum (Grindal et al., 1998b) and Begonia x hiemalis (Myster et al., 1997a). In all these studies it was determined that under positive or zero DIF, which gave increased internode or petiole elongation, the endogenous level of the bioactive GA (GA1) was higher than under negative DIF. In P. sativum, a higher total GA concentration was found in apical buds, unexpanded leaves, and tendrils compared with GA1-responsive stem tissue (Smith et al., 1992). Due to low GA concentrations in A. thaliana in the present study, it was impossible to analyse the GA content in apical buds alone. This might have diluted possible GA concentration gradients in the tissue and masked possible differences in GA content between the two treatments.
Although the present findings indicate that the thermoperiodic flower stem elongation response was not mediated through altered GA levels, GA could still be involved through a change in GA sensitivity of the tissue. This has, however, not been supported in Fuchsia x hybrida (Maas and van Hattum, 1998). Alternatively, or in addition to GAs, other phytohormones such as IAA might be involved. In P. sativum, it has been suggested that, in addition to GA, IAA is an essential factor for stem elongation, and that the two hormones may control separate processes that together contribute to stem elongation (Yang et al., 1996). A decapitation and application study has implied a role for auxin in the flower stem elongation rate of A. thaliana, and analysis of endogenous IAA-levels has suggested that IAA metabolism is an essential factor in the regulation of the circadian growth rhythm of the flower stem in this species (Jouve et al., 1999). Furthermore, Gray et al. (1998) found that high temperature promoted dramatic hypocotyl elongation in light-grown A. thaliana seedlings, and that this temperature-induced growth response depended on IAA. The endogenous level of IAA in flower stems was 56 % higher under DT/NT 22/12 °C than under DT/NT 12/22 °C (Table 5), indicating that the observed thermoperiodic response on stem elongation in A. thaliana could be mediated through changed levels of IAA.
Brassinosteroids are another class of plant hormones that stimulates stem elongation (Mandava, 1988), but they were not analysed in the present study. These hormones appear to promote not only cell expansion but also cell division in A. thaliana (Hu et al., 2000; Nakaya et al., 2002), and could be involved in thermoperiodic stem elongation. The auxin and brassinosteroid deficient lkb mutant of P. sativum was found to respond to DIF in a similar pattern to that of wild-type plants, but the response in the stem was weak compared with wild-type plants (Grindal et al., 1998b).
Temperature and flowering time
Genetic and molecular studies have revealed a network of interacting pathways controlling flowering time in A. thaliana (Simpson and Dean, 2002). This network includes pathways for environmental signals such as photoperiod, an extensive period of cold (vernalization), and light quality. Current work also suggests a pathway for control by ambient growth temperature. A study of wild-type plants and different flowering time mutants of A. thaliana, grown under constant 23 or 16 °C has suggested a role for the so-called autonomous pathway in sensing changes in ambient growth temperature that can affect the expression of the floral pathway integrator FT (flowering locus) (Blázquez et al., 2003). Work by Halliday et al. (2003) has revealed that the classical early-flowering phenotype of the phyB monogenic mutant of A. thaliana is temperature-dependent, occurring at 22 °C but not at 16 °C. This temperature-sensitive phytochrome control of flowering also seems to operate through regulation of FT, but unlike the thermosensory pathway described by Blázquez et al. (2003), this pathway appears to operate independently of the floral repressor FLC (flowering locus C) in the autonomous pathway. In the present work, NT had a stronger effect on flowering time than DT (Table 6). This could mean that NT is perceived and transduced by genes other than DT. It certainly would be of interest to clarify which genes in the flower induction pathway are affected by daily temperature alternations.
|
ACKNOWLEDGEMENTS |
|---|
We thank Marit Siira and Jørn Medlien for technical assistance, Dr Jinqiao Xiong for help with the daily transfer of plants, and Dr Lars Snipen for useful comments and help with the statistics. We are also grateful to Svein Kristian Stormo for performing the hormone analysis. This study was carried out with financial support from the Norwegian Research Council (project no. 121604/111).
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
-
Blázquez MA, Ahn JH, Weigel D. 2003. A thermosensory pathway controlling flowering time in Arabidopsis thaliana. Nature Genetics 33: 168–171.[CrossRef][Web of Science][Medline]
Carvalho SMP, Heuvelink E, Cascais R, Van Kooten O. 2002. Effect of day and night temperature on internode and stem length in chrysanthemum: is everything explained by DIF? Annals of Botany 90: 111–118.
del Cerro M, Cogen J, Del Cerro C. 1980. Stevenel’s blue, an excellent stain for optical microscopical study of plastic embedded tissues. Microscopica Acta 83: 117–121.[Web of Science][Medline]
Efron B, Tibshirani RJ. 1993. An introduction to the bootstrap. New York: Chapman & Hall.
Erwin JE, Heins RD. 1995. Thermomorphogenic responses in stem and leaf development. Horticultural Science 30: 940–949.
Erwin JE, Heins RD, Karlsson MG. 1989. Thermomorphogenesis in Lilium longiflorum. American Journal of Botany 76: 47–52.[CrossRef][Web of Science]
Erwin JE, Heins RD, Moe R. 1991. Temperature and photoperiod effects on Fuchsia x hybrida morphology. Journal of the American Society for Horticultural Science 116: 955–960.
Erwin JE, Velguth P, Heins RD. 1994. Day/night temperature environment affects cell elongation but not division in Lilium longiflorum Thumb. Journal of Experimental Botany 45: 1019–1025.
Gray WM, Östin A, Sandberg G, Romano CP, Estelle M. 1998. High temperature promotes auxin-mediated hypocotyl elongation in Arabidopsis. Proceedings of the National Academy of Sciences of the USA 95: 7197–7202.
Grindal G, Junttila O, Reid JB, Moe R. 1998a. The response to gibberellin in Pisum sativum grown under alternating day and night temperature. Journal of Plant Growth Regulation 17: 161–167.
Grindal G, Ernstsen A, Reid JB, Junttila O, Lindgård B, Moe R. 1998b. Endogenous gibberellin A1 levels control thermoperiodic stem elongation in Pisum sativum. Physiologia Plantarum 102: 523–531.[CrossRef]
Halliday KJ, Salter MG, Thingnaes E, Whitelam GC. 2003. Phytochrome control of flowering is temperature sensitive and correlates with expression of the floral integrator FT. Plant Journal 33: 875–885.[CrossRef][Web of Science][Medline]
Hu Y, Bao F, Li J. 2000. Promotive effect of brassinosteroids on cell division involves a distinct CycD3-induction pathway in Arabidopsis. Plant Journal 24: 693–701.[CrossRef][Web of Science][Medline]
Jensen E, Eilertsen S, Ernstsen A, Junttila O, Moe R. 1996. Thermoperiodic control of stem elongation and endogenous gibberellins in Campanula isophylla. Journal of Plant Growth Regulation 15: 167–171.
Jouve L, Gaspar T, Kevers C, Greppin H, Degli Agosti R. 1999. Involvement of indole-3-acetic acid in the circadian growth of the first internode of Arabidopsis. Planta 209: 136–142.[CrossRef][Web of Science][Medline]
Junttila O, Heide OM, Lindgård B, Ernstsen A. 1997. Gibberellins and the photoperiodic control of leaf growth in Poa pratensis. Physiologia Plantarum 101: 599–605.[CrossRef]
Karlsson MG, Heins RD, Erwin JE, Berghage RD, Carlson WH, Biernbaum JA. 1989. Temperature and photosynthetic photon flux influence chrysanthemum shoot development and flower initiation under short-day conditions. Journal of the American Society for Horticultural Science 114: 158–163.
Langton FA, Cockshull KE. 1997. Is stem extension determined by DIF or by absolute day and night temperatures? Scientia Horticulturaee 69: 229–237.[CrossRef]
Maas FM, van Hattum J. 1998. Thermomorphogenic and photomorphogenic control of stem elongation in Fuchsia is not mediated by changes in responsiveness to gibberellins. Journal of Plant Growth Regulation 17: 39–45.
Mandava NB. 1988. Plant growth-promoting brassinosteroids. Annual Review of Plant Physiology and Plant Molecular Biology 39: 23–52.[CrossRef][Web of Science]
Miller WB. 1993. Lilium longiflorum. In De Hertogh A, ed. The physiology of flower bulbs. Amsterdam: Elsevier, 391–422.
Myster J, Moe R. 1995. Effect of diurnal temperature alternations on plant morphology in some greenhouse crops—a mini review. Scientia Horticulturaee 62: 205–215.[CrossRef]
Myster J, Ernstsen A, Junttila O, Moe R. 1997a. Thermo- and photoperiodicity and involvement of gibberellins during day and night cycle on elongation growth of Begonia x hiemalis Fotsch. Journal of Plant Growth Regulation 16: 189–196.[CrossRef]
Myster J, Junttila O, Lindgård B, Moe R. 1997b. Temperature alternations and the influence of gibberellins and indoleacetic acid on elongation growth and flowering of Begonia x hiemalis Fotsch. Plant Growth Regulation 21: 135–144.
Nakaya M, Tsukaya H, Murakami N, Kato M. 2002. Brassinosteroids control the proliferation of leaf cells of Arabidopsis thaliana. Plant Cell Physiology 43: 239–244.
Niu G, Heins RD, Cameron AC, Carlson WH. 2000. Day and night temperatures, daily light integral, and CO2 enrichment affect growth and flower development of pansy (Viola x wittrockiana). Journal of the American Society for Horticultural Science 125: 436–441.
Simpson GG, Dean C. 2002. Arabidopsis, the Rosetta stone of flowering time? Science 296: 285–289.
Smith VA, Knatt CJ, Gaskin P, Reid JB. 1992. The distribution of gibberellins in vegetative tissues of Pisum sativum L. I. Biological and biochemical consequences of the le mutation. Plant Physiology 99: 368–371.
Strøm M, Moe R. 1997. DIF affects internode and cell extension growth and cell number in Campanula isophylla shoots. Acta Horticulturae 435: 17–24.
Talon M, Zeevaart JAD. 1990. Gibberellins and stem growth as related to photoperiod in Silene-Armeria L. Plant Physiology 92: 1094–1100.
Talon M, Koornneef M, Zeevaart JAD. 1990. Endogenous gibberellins in Arabidopsis thaliana and possible steps blocked in the biosynthetic pathway of the semidwarf ga4 and ga5 mutants. Proceedings of the National Academy of Sciences of the USA 87: 7983–7987.
Went FW. 1944. Plant growth under controlled conditions. II. Thermoperiodicity in growth and fruiting of the tomato. American Journal of Botany 31: 135–150.[CrossRef][Web of Science]
Xu YL, Gage DA, Zeevaart JAD. 1997. Gibberellins and stem growth in Arabidopsis thaliana. Effects of photoperiod on expression of the GA4 and GA5 loci. Plant Physiology 114: 1471–1476.[Abstract]
Yang T, Davies PJ, Reid JB. 1996. Genetic dissection of the relative roles of auxin and gibberellin in the regulation of stem elongation in intact light-grown peas. Plant Physiology 110: 1029–1034.[Abstract]
Zeevaart JAD, Gage DA, Talon M. 1993. Gibberellin-A(1) is required for stem elongation in spinach. Proceedings of the National Academy of Sciences of the USA 90: 7401–7405.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  P. Q. Craufurd and T. R. Wheeler Climate change and the flowering time of annual crops J. Exp. Bot., July 1, 2009; 60(9): 2529 - 2539. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. Teper-Bamnolker and A. Samach The Flowering Integrator FT Regulates SEPALLATA3 and FRUITFULL Accumulation in Arabidopsis Leaves PLANT CELL, October 1, 2005; 17(10): 2661 - 2675. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||