BACKGROUND
plant growth and development are stimulated by environmental or intrinsic cues, such as hormones  <cit> .
gibberellins  are indispensable stimulators of plant growth  <cit> .
these hormones are initiated from the diterpenoid pathway.
to date, over  <dig> gas have been identified, but only a few of them are bioactive  <cit> .
nowadays, the application of exogenous gas is commonly used to regulate plant growth and development  <cit> .
ga metabolism and perception have been extensively documented  <cit> .
in vascular plants, geranyl geranyl diphosphate  is first converted to ent-kaurene by ent-copalyl diphosphate synthase  and ent-kaurene synthase .
ga <dig> is then produced with oxidation by ent-kaurene oxidase  and ent-kaurenoic acid oxidase .
stepwise oxidation is catalyzed by ga20-oxidase  and ga3-oxidase  to produce various ga intermediates and bioactive gas, whereas ga2-oxidase  is involved in inactivation of bioactive gas.
ga-mediated growth is attributed to the degradation of della proteins, which are the main restraints for plant growth  <cit> .
binding of ga to the receptor, gibberellin insensitive dwarf <dig>  triggers a close link between gid <dig> and della; subsequently, dellas are degraded via the 26s proteasome  <cit> .
other components, such as specific ubiquitin e <dig> ligase complex , sleepy <dig> , pickle, short internode  and spindly , are essentially required for ga signal transduction   <cit> .
biochemical, molecular and genetic studies have suggested that genes involved in ga metabolic and signaling pathways are essential for ga accumulation and subsequent functions .fig.
<dig> receptors and acting components in ga response in higher plants
gas are involved in various processes in plants.
ga-deficient mutants of arabidopsis and tomato cannot germinate without exogenous gas  <cit> .
by contrast, ga application can make environmental stimuli unnecessary and promote germination, indicating that gas play essential roles in germination  <cit> .
gas can promote cell elongation, and ga <dig> is the active form that regulates shoot elongation in arabidopsis  <cit> .
gas also participate in leaf expansion and fruit set and growth  <cit> .
furthermore, gas control root elongation and cell proliferation  <cit> .
carrot , a biennial plant from the apiaceae family, is a root vegetable with enriched healthy composition .
our previous work has focused on conducting carrot breeding and establishing genome and transcriptome database for carrot   <cit> .
however, the role of gas in the regulation of root plants is poorly understood because of technical reasons  <cit> .
hidden underground, root systems are often difficult to observe and quantify without any harm.
a previous research indicated that ga is essentially required for carrot somatic embryogenesis  <cit> .
however, another study suggested that applied ga inhibits the growth of carrot roots  <cit> .
thus, the functions of gas in carrot remain unclear.
the present study aimed to investigate the effects of ga treatment on the growth and development of carrot.
we attempted to gain novel insights into ga functions in carrot growth based on transcript profiles of genes involved in hormone metabolic and signaling pathways.
morphological and anatomical characteristics, along with hormone crosstalk, were also discussed to completely elucidate the roles of applied ga <dig>  this study provided novel insights into ga-mediated plant growth and development in vascular plants.
RESULTS
plant growth analysis
to determine whether ga is involved in carrot plant growth, 5-week-old carrots were treated with ga or its inhibitor paclobutrazol  weekly for five times.
the effects of the applied ga and pbz on carrot plant growth were observed after  <dig> weeks .
exogenous ga <dig> significantly increased the shoot weight but significantly decreased root diameter and the root weight of carrot.
by contrast, pbz increased the root weight and root diameter but decreased the shoot weight of carrot.
the exogenous application of ga3 + pbz resulted in a phenotype similar to that of the control, which was the intermediate between the ga <dig> and the pbz treated plants .fig.
<dig> effects of ga <dig>  pbz or ga3 + pbz on carrot growth.
black lines in the lower left corner of each plant represent 5 cm in that pixel
fig.
<dig> effects of ga <dig>  pbz or ga3 + pbz on carrot fresh weight  and root diameter .
values are means ± sd from three replicates, and the bars represent sd.
statistical differences were evaluated by student t test at the  <dig>  probability level.
the asterisk indicates a significant difference compared with control group
anatomical structure changes in the roots, petioles and leaves
in the roots
carrot roots without any treatment were approximately  <dig>  cm in diameter .
ga <dig> dramatically reduced the root diameter, which was slightly relieved in the presence of pbz .
interestingly, the ratio of xylem area to total root area was significantly higher under ga <dig> treatment compared with control conditions .
pbz alone significantly increased the thickness of root diameter, but decreased the ratio of xylem area to total area, which was relieved by application of ga <dig> .fig.
<dig> effects of ga <dig>  pbz or ga3 + pbz on the root anatomical structure of carrot.
a , d , g  and j  represent the cross sections of roots under control, ga <dig>  pbz and ga3 + pbz treatments, respectively.
epidermis , parenchymal cell , phellogen , starch granule  and vessel  are marked in the figure.
scale bars in b, c, e, f, h, i, k and l are 100 μm in length, whereas bars in a, d, g and j are 1 cm in length
fig.
<dig> effects of ga <dig> and pbz treatments on the ratio of xylem area to total root area.
values are means ± sd from three replicates, and the bars represent sd.
statistical differences were evaluated by student t test at the  <dig>  probability level.
the asterisk indicates a significant difference compared with control group
in the petioles
ga <dig> increased the number of vascular bundles in the petioles, which may contribute to influxes of nutrients and water towards the leaves .
this effect was also observed when ga <dig> was applied together with pbz.
however, pbz did not evidently change the number of vascular bundles .fig.
<dig> effects of ga <dig>  pbz or ga3 + pbz on the anatomical structure in carrot petioles.
a, b, c and d represent the cross sections of petioles under control, ga <dig>  pbz and ga3 + pbz treatments, respectively.
collenchyma , epidermis , exodermis , phloem  and xylem  are marked in the figure.
scale bars in a, b, c, d and e are 40 μm in length
fig.
<dig> number of vascular bundles in carrot petioles under different treatments.
data were calculated and presented as mean ± sd.
student’s t test was used to determine the difference between two treatments.
* p <  <dig>  was considered to be statistically significant compared with control group
in the leaves
palisade tissue  and spongy tissue  are two main components of carrot leaves.
pt, which contains the largest number of chloroplasts, may be the principal site for photosynthesis in the leaves.
st is another leaf tissue that facilitates gas exchange.
however, no obvious difference was detected in the anatomical structure of the leaves under different treatments .fig.
<dig> effects of ga <dig>  pbz or ga3 + pbz on the anatomical structure in carrot leaves.
a, b, c and d represent the cross sections of leaves under control, ga <dig>  pbz and ga3 + pbz treatments, respectively.
epidermis , palisade tissue , spongy tissue  and vascular  are marked in the figure
effects of ga <dig> treatment on the expression levels of ga biosynthetic pathway genes
to illustrate the effects of gibberellin application on ga biosynthesis, we investigated the changes in the expression levels of ga metabolic genes.
dcks, dcko, dckao <dig>  dcga20ox <dig>  dcga20ox <dig>  dcga3ox <dig>  dcga2ox <dig>  dcga2ox <dig> and dcga2ox <dig> were annotated as ga pathway-related genes based on carrotdb, a transcriptomic and genomic database for carrot .
the expression levels of these selected genes were measured by quantitative real-time pcr .
the biosynthetic pathway-related genes were strongly regulated by ga or pbz treatment .table  <dig> nucleotide sequences of primers specific to gibberellin biosynthetic and signaling genes used for qrt-pcr
dcks
dcko
dckao1
dcga20ox1
dcga20ox2
dcga3ox1
dcga2ox1
dcga2ox2
dcga2ox3
dcgid1b
dcgid1c
dcdella
dcsly1
dcpickle1
dcpickle2
dcspy
dcgamyb
dcshi
fig.
<dig> effects of ga <dig>  pbz or ga3 + pbz on the expression levels of ga pathway-related genes.
error bars represent standard errors among three independent replicates.
data are the means ± sd of three replicates
in the roots, exogenous ga <dig> application upregulated dcks, dcga2ox <dig>  dcga2ox <dig> and dcga2ox <dig> expression but reduced the mrna levels of dcko, dcga20ox <dig> and dcga20ox <dig>  similarly, ga3 + pbz treatment markedly decreased the expression levels of dcko, dcga20ox <dig> and dcga20ox <dig> but obviously increased that of dcga2ox <dig>  in the petioles, exogenous ga <dig> application reduced the mrna levels of dcks, dcko, dckao <dig>  dcga2ox <dig>  dcga2ox <dig> and dcga2ox <dig> but did not significantly alter the expression levels of dcga20ox <dig>  dcga20ox <dig> and dcga3ox <dig>  pbz alone upregulated dcko, dckao <dig>  dcga20ox <dig>  dcga20ox <dig>  and dcga3ox <dig> but downregulated dcga2ox <dig> and dcga2ox <dig>  in the leaves, ga <dig> application upregulated dcga2ox <dig> but downregulated dcks and dcga20ox <dig> expression.
pbz alone upregulated dcga20ox <dig> but downregulated dcga2ox <dig> and dcga2ox <dig> .
effects of ga <dig> application on the expression levels of ga response genes
the proteins encoded by dcgid1b, dcgid1c, dcdella, dcsly <dig>  dcpickle <dig>  dcpickle <dig>  dcspy, dcgamyb and dcshi were recognized as ga receptors or acting components by carrotdb.
thus, these genes were selected and investigated for qrt-pcr analysis .fig.
<dig> effects of ga <dig>  pbz or ga3 + pbz on the expression levels of genes involved in ga signaling.
error bars represent the standard errors among three independent replicates.
data are the means ± sd of three replicates
in the roots, ga <dig> treatment downregulated dcgid1b and dcsly <dig> but upregulated dcdella, dcspy and dcshi.
in the petioles, ga <dig> treament alone downregulated dcgid1c and dcpickle <dig>  by contrast, dcgid1b, dcgid1c and dcpickle <dig> showed increased expression after pbz treatment.
in the leaves, exogenous ga <dig> upregulated della and dcspy.
pbz increased dcgid1c but suppressed dcdella .
effects of ga <dig> application on the expression levels of genes implicated in other hormone pathway
to verify whether ga <dig> altered metabolism of other hormones, a total of  <dig> genes from auxin, cytokinin, abscisic acid, brassinolide biosynthetic pathways were identified and their expression under ga <dig> and pbz treatments was determined .
in carrot roots, transcript levels of most genes were upregulated by ga <dig> treatment.
by contrast, inhibited transcription was detected when pbz was applied, which was ameliorated by application of ga <dig>  in the petioles, pbz resulted in obvious increases in transcript levels of dcipt <dig>  dcaba <dig>  dcmoco, dcopr <dig>  dcdad <dig>  dcdwf <dig> and dcdwf <dig>  which was quite different from that in the roots.
in the leaves, ga <dig> application increased transcription of dcipt <dig>  dcaba <dig> and dcdwf <dig> but reduced the mrna abundance of dcyucca, dccyp83b <dig>  dcopr <dig>  dcdad <dig> and dcdwf <dig> .table  <dig> description of genes implicated in auxin, cytokinin, abscisic acid, brassinolide biosynthesis and primers used for qrt-pcr
dcyucca
dccyp83b1
dcipt3
dcaba2
dcmoco
dcopr2
dcdad1
dcdwf4
dcdwf5
fig.
<dig> effects of ga <dig>  pbz or ga3 + pbz on the expression levels of genes involved in auxin, cytokinin, abscisic acid and brassinolide metabolism.
error bars represent the standard errors among three independent replicates.
data are the means ± sd of three replicates
discussion
modifying plant growth, stature and yield has been a farming goal in professional agronomy and horticulture  <cit> .
the use of plant growth regulators, such as synthetic hormones, has achieved great progress in recent years  <cit> .
gas, a class of plant hormones, have been widely used to regulate seed germination, plant growth and fruit yield .
the root crop carrot is popular worldwide and has great value  <cit> .
thus, regulating the root and shoot growth of carrot to increase its yield has been a critical goal.
the results of the present study provided new insights into the roles of ga in the growth and development of carrot.
gas stimulate cell elongation, and this effect has been well studied in ga-deficient mutants  <cit> .
previous studies on gas focused on aerial parts because the root is not always economically important.
however, the roots of some plants, such as carrot, need intensive attention.
the results of the present study showed that ga <dig> application increased shoot growth but impaired root growth in carrot , which is in agreement with the results obtained by michel-wolwertz and his collegues  <cit> .
in higher plants, ga could promote cell elongation at the expense of lateral expansion  <cit> .
here, enlargement of the ga-treated roots was suppressed, again supporting this statement.
this treatment also dramatically altered matter distribution.
therefore, excessive ga <dig> negatively controls root growth in carrot.
this interesting observation agrees with a previous research in carrot  <cit> , although some studies indicate that gas play essential roles in root growth  <cit> .
xylem is an important tissue for water and solute transport; this tissue also provides structural support  <cit> .
however, the phloem tissue in carrot root provides more nutrients and metabolites than the xylem tissue  <cit> .
as a result, an appropriate phloem/xylem ratio in carrot root is important.
a previous study indicated that mobile ga promotes xylem expansion in the hypocotyl of arabidopsis  <cit> .
similarly, the present study found that the xylem region in carrot roots treated with ga <dig> or ga3 + pbz was evidently multiplied .
this alteration may weaken the root taste, texture and even quality.
ga accumulation within plants may mostly be regulated by biosynthetic genes, and the signals are perceived by receptors and related acting components   <cit> .
in the present research, we observed a feedback regulation of ga-related genes.
ga <dig> application decreased the transcript levels of genes encoding ga20-oxidase and ga3-oxidase but increased those of genes encoding ga2-oxidase, whereas pbz induced opposite effects on these genes.
indeed, feedback regulation of ga biosynthesis is firmly established as a mechanism to maintain ga homeostasis in higher plants  <cit> .
treatment with ga <dig> or its inhibitor pbz can elevate or reduce ga accumulation, thus exerting different effects on plant growth and development  <cit> .
however, we cannot attribute all the alterations to the changing levels of gas.
indeed, ga-mediated plant growth often involves complex interaction among hormones  <cit> .
previous studies revealed that altered ga levels can influence accumulation, signaling, transport and even functions of other hormones  <cit> .
in this study, ga <dig> induced obvious changes in expression of hormone-related genes, suggesting gas may interact with other hormones to regulate carrot plant growth through crosstalk mechanisms .
in addition, there seems to be an organ-specific regulation of hormone-related genes in response to ga <dig> or pbz.
for example, dcga2ox <dig> was higher expressed after ga <dig> treatment in carrot roots and leaves, but was reduced in the petioles.
all these results together suggested that hormonal regulation of plant growth is a complicated regulatory network.
CONCLUSIONS
in the present study, ga <dig> or pbz treatment altered the morphological parameters, anatomical structure and transcriptional regulatory networks of hormones in carrot plants.
ga <dig> treatment restrained root growth but enhanced shoot growth possibly because of thickened xylem region in the roots and increased area of vascular bundles in the petioles.
excessive or reduced gibberellin also altered hormone homeostasis by changing transcription of related genes, thus exerting effects on carrot plant growth.

 plant growth and development are stimulated by environmental or intrinsic cues, such as hormones. gibberellins are indispensable stimulators of plant growth. these hormones are initiated from the diterpenoid pathway. to date, over gas have been identified, but only a few of them are bioactive. nowadays, the application of exogenous gas is commonly used to regulate plant growth and development. ga metabolism and perception have been extensively documented. in vascular plants, geranyl geranyl diphosphate is first converted to ent-kaurene by ent-copalyl diphosphate synthase and ent-kaurene synthase. ga is then produced with oxidation by ent-kaurene oxidase and ent-kaurenoic acid oxidase. stepwise oxidation is catalyzed by ga20-oxidase and ga3-oxidase to produce various ga intermediates and bioactive gas, whereas ga2-oxidase is involved in inactivation of bioactive gas. ga-mediated growth is attributed to the degradation of della proteins, which are the main restraints for plant growth. binding of ga to the receptor, gibberellin insensitive dwarf triggers a close link between gid and della; subsequently, dellas are degraded via the 26s proteasome. other components, such as specific ubiquitin e ligase complex, sleepy, pickle, short internode and spindly, are essentially required for ga signal transduction. biochemical, molecular and genetic studies have suggested that genes involved in ga metabolic and signaling pathways are essential for ga accumulation and subsequent functions.fig. receptors and acting components in ga response in higher plants gas are involved in various processes in plants. ga-deficient mutants of arabidopsis and tomato cannot germinate without exogenous gas. by contrast, ga application can make environmental stimuli unnecessary and promote germination, indicating that gas play essential roles in germination. gas can promote cell elongation, and ga is the active form that regulates shoot elongation in arabidopsis. gas also participate in leaf expansion and fruit set and growth. furthermore, gas control root elongation and cell proliferation. carrot, a biennial plant from the apiaceae family, is a root vegetable with enriched healthy composition. our previous work has focused on conducting carrot breeding and establishing genome and transcriptome database for carrot. however, the role of gas in the regulation of root plants is poorly understood because of technical reasons. hidden underground, root systems are often difficult to observe and quantify without any harm. a previous research indicated that ga is essentially required for carrot somatic embryogenesis. however, another study suggested that applied ga inhibits the growth of carrot roots. thus, the functions of gas in carrot remain unclear. the present study aimed to investigate the effects of ga treatment on the growth and development of carrot. we attempted to gain novel insights into ga functions in carrot growth based on transcript profiles of genes involved in hormone metabolic and signaling pathways. morphological and anatomical characteristics, along with hormone crosstalk, were also discussed to completely elucidate the roles of applied ga this study provided novel insights into ga-mediated plant growth and development in vascular plants.  plant growth analysis to determine whether ga is involved in carrot plant growth, 5-week-old carrots were treated with ga or its inhibitor paclobutrazol weekly for five times. the effects of the applied ga and pbz on carrot plant growth were observed after weeks. exogenous ga significantly increased the shoot weight but significantly decreased root diameter and the root weight of carrot. by contrast, pbz increased the root weight and root diameter but decreased the shoot weight of carrot. the exogenous application of ga3 + pbz resulted in a phenotype similar to that of the control, which was the intermediate between the ga and the pbz treated plants.fig. effects of ga pbz or ga3 + pbz on carrot growth. black lines in the lower left corner of each plant represent 5 cm in that pixel fig. effects of ga pbz or ga3 + pbz on carrot fresh weight and root diameter. values are means ± sd from three replicates, and the bars represent sd. statistical differences were evaluated by student t test at the probability level. the asterisk indicates a significant difference compared with control group anatomical structure changes in the roots, petioles and leaves in the roots carrot roots without any treatment were approximately  cm in diameter. ga dramatically reduced the root diameter, which was slightly relieved in the presence of pbz. interestingly, the ratio of xylem area to total root area was significantly higher under ga treatment compared with control conditions. pbz alone significantly increased the thickness of root diameter, but decreased the ratio of xylem area to total area, which was relieved by application of ga.fig. effects of ga pbz or ga3 + pbz on the root anatomical structure of carrot. a, d, g and j represent the cross sections of roots under control, ga pbz and ga3 + pbz treatments, respectively. epidermis, parenchymal cell, phellogen, starch granule and vessel are marked in the figure. scale bars in b, c, e, f, h, i, k and l are 100 μm in length, whereas bars in a, d, g and j are 1 cm in length fig. effects of ga and pbz treatments on the ratio of xylem area to total root area. values are means ± sd from three replicates, and the bars represent sd. statistical differences were evaluated by student t test at the probability level. the asterisk indicates a significant difference compared with control group in the petioles ga increased the number of vascular bundles in the petioles, which may contribute to influxes of nutrients and water towards the leaves. this effect was also observed when ga was applied together with pbz. however, pbz did not evidently change the number of vascular bundles.fig. effects of ga pbz or ga3 + pbz on the anatomical structure in carrot petioles. a, b, c and d represent the cross sections of petioles under control, ga pbz and ga3 + pbz treatments, respectively. collenchyma, epidermis, exodermis, phloem and xylem are marked in the figure. scale bars in a, b, c, d and e are 40 μm in length fig. number of vascular bundles in carrot petioles under different treatments. data were calculated and presented as mean ± sd. student’s t test was used to determine the difference between two treatments. * p <  was considered to be statistically significant compared with control group in the leaves palisade tissue and spongy tissue are two main components of carrot leaves. pt, which contains the largest number of chloroplasts, may be the principal site for photosynthesis in the leaves. st is another leaf tissue that facilitates gas exchange. however, no obvious difference was detected in the anatomical structure of the leaves under different treatments.fig. effects of ga pbz or ga3 + pbz on the anatomical structure in carrot leaves. a, b, c and d represent the cross sections of leaves under control, ga pbz and ga3 + pbz treatments, respectively. epidermis, palisade tissue, spongy tissue and vascular are marked in the figure effects of ga treatment on the expression levels of ga biosynthetic pathway genes to illustrate the effects of gibberellin application on ga biosynthesis, we investigated the changes in the expression levels of ga metabolic genes. dcks, dcko, dckao dcga20ox dcga20ox dcga3ox dcga2ox dcga2ox and dcga2ox were annotated as ga pathway-related genes based on carrotdb, a transcriptomic and genomic database for carrot. the expression levels of these selected genes were measured by quantitative real-time pcr. the biosynthetic pathway-related genes were strongly regulated by ga or pbz treatment.table nucleotide sequences of primers specific to gibberellin biosynthetic and signaling genes used for qrt-pcr dcks dcko dckao1 dcga20ox1 dcga20ox2 dcga3ox1 dcga2ox1 dcga2ox2 dcga2ox3 dcgid1b dcgid1c dcdella dcsly1 dcpickle1 dcpickle2 dcspy dcgamyb dcshi fig. effects of ga pbz or ga3 + pbz on the expression levels of ga pathway-related genes. error bars represent standard errors among three independent replicates. data are the means ± sd of three replicates in the roots, exogenous ga application upregulated dcks, dcga2ox dcga2ox and dcga2ox expression but reduced the mrna levels of dcko, dcga20ox and dcga20ox similarly, ga3 + pbz treatment markedly decreased the expression levels of dcko, dcga20ox and dcga20ox but obviously increased that of dcga2ox in the petioles, exogenous ga application reduced the mrna levels of dcks, dcko, dckao dcga2ox dcga2ox and dcga2ox but did not significantly alter the expression levels of dcga20ox dcga20ox and dcga3ox pbz alone upregulated dcko, dckao dcga20ox dcga20ox and dcga3ox but downregulated dcga2ox and dcga2ox in the leaves, ga application upregulated dcga2ox but downregulated dcks and dcga20ox expression. pbz alone upregulated dcga20ox but downregulated dcga2ox and dcga2ox. effects of ga application on the expression levels of ga response genes the proteins encoded by dcgid1b, dcgid1c, dcdella, dcsly dcpickle dcpickle dcspy, dcgamyb and dcshi were recognized as ga receptors or acting components by carrotdb. thus, these genes were selected and investigated for qrt-pcr analysis.fig. effects of ga pbz or ga3 + pbz on the expression levels of genes involved in ga signaling. error bars represent the standard errors among three independent replicates. data are the means ± sd of three replicates in the roots, ga treatment downregulated dcgid1b and dcsly but upregulated dcdella, dcspy and dcshi. in the petioles, ga treament alone downregulated dcgid1c and dcpickle by contrast, dcgid1b, dcgid1c and dcpickle showed increased expression after pbz treatment. in the leaves, exogenous ga upregulated della and dcspy. pbz increased dcgid1c but suppressed dcdella. effects of ga application on the expression levels of genes implicated in other hormone pathway to verify whether ga altered metabolism of other hormones, a total of genes from auxin, cytokinin, abscisic acid, brassinolide biosynthetic pathways were identified and their expression under ga and pbz treatments was determined. in carrot roots, transcript levels of most genes were upregulated by ga treatment. by contrast, inhibited transcription was detected when pbz was applied, which was ameliorated by application of ga in the petioles, pbz resulted in obvious increases in transcript levels of dcipt dcaba dcmoco, dcopr dcdad dcdwf and dcdwf which was quite different from that in the roots. in the leaves, ga application increased transcription of dcipt dcaba and dcdwf but reduced the mrna abundance of dcyucca, dccyp83b dcopr dcdad and dcdwf.table description of genes implicated in auxin, cytokinin, abscisic acid, brassinolide biosynthesis and primers used for qrt-pcr dcyucca dccyp83b1 dcipt3 dcaba2 dcmoco dcopr2 dcdad1 dcdwf4 dcdwf5 fig. effects of ga pbz or ga3 + pbz on the expression levels of genes involved in auxin, cytokinin, abscisic acid and brassinolide metabolism. error bars represent the standard errors among three independent replicates. data are the means ± sd of three replicates discussion modifying plant growth, stature and yield has been a farming goal in professional agronomy and horticulture. the use of plant growth regulators, such as synthetic hormones, has achieved great progress in recent years. gas, a class of plant hormones, have been widely used to regulate seed germination, plant growth and fruit yield. the root crop carrot is popular worldwide and has great value. thus, regulating the root and shoot growth of carrot to increase its yield has been a critical goal. the results of the present study provided new insights into the roles of ga in the growth and development of carrot. gas stimulate cell elongation, and this effect has been well studied in ga-deficient mutants. previous studies on gas focused on aerial parts because the root is not always economically important. however, the roots of some plants, such as carrot, need intensive attention. the results of the present study showed that ga application increased shoot growth but impaired root growth in carrot, which is in agreement with the results obtained by michel-wolwertz and his collegues. in higher plants, ga could promote cell elongation at the expense of lateral expansion. here, enlargement of the ga-treated roots was suppressed, again supporting this statement. this treatment also dramatically altered matter distribution. therefore, excessive ga negatively controls root growth in carrot. this interesting observation agrees with a previous research in carrot, although some studies indicate that gas play essential roles in root growth. xylem is an important tissue for water and solute transport; this tissue also provides structural support. however, the phloem tissue in carrot root provides more nutrients and metabolites than the xylem tissue. as a result, an appropriate phloem/xylem ratio in carrot root is important. a previous study indicated that mobile ga promotes xylem expansion in the hypocotyl of arabidopsis. similarly, the present study found that the xylem region in carrot roots treated with ga or ga3 + pbz was evidently multiplied. this alteration may weaken the root taste, texture and even quality. ga accumulation within plants may mostly be regulated by biosynthetic genes, and the signals are perceived by receptors and related acting components. in the present research, we observed a feedback regulation of ga-related genes. ga application decreased the transcript levels of genes encoding ga20-oxidase and ga3-oxidase but increased those of genes encoding ga2-oxidase, whereas pbz induced opposite effects on these genes. indeed, feedback regulation of ga biosynthesis is firmly established as a mechanism to maintain ga homeostasis in higher plants. treatment with ga or its inhibitor pbz can elevate or reduce ga accumulation, thus exerting different effects on plant growth and development. however, we cannot attribute all the alterations to the changing levels of gas. indeed, ga-mediated plant growth often involves complex interaction among hormones. previous studies revealed that altered ga levels can influence accumulation, signaling, transport and even functions of other hormones. in this study, ga induced obvious changes in expression of hormone-related genes, suggesting gas may interact with other hormones to regulate carrot plant growth through crosstalk mechanisms. in addition, there seems to be an organ-specific regulation of hormone-related genes in response to ga or pbz. for example, dcga2ox was higher expressed after ga treatment in carrot roots and leaves, but was reduced in the petioles. all these results together suggested that hormonal regulation of plant growth is a complicated regulatory network.  in the present study, ga or pbz treatment altered the morphological parameters, anatomical structure and transcriptional regulatory networks of hormones in carrot plants. ga treatment restrained root growth but enhanced shoot growth possibly because of thickened xylem region in the roots and increased area of vascular bundles in the petioles. excessive or reduced gibberellin also altered hormone homeostasis by changing transcription of related genes, thus exerting effects on carrot plant growth. 
in addition, transcript levels of genes related to gibberellins, auxin, cytokinins, abscisic acid and brassinolides were altered in response to increased or reduced gibberellins. gibberellins may interact with other hormones to regulate carrot plant growth through crosstalk mechanisms. the results found that ga dramatically reduced the root growth but stimulated the shoot growth of carrot. the inhibited tuberous root growth but enhanced shoot growth in plants treated with ga can be principally attributed to the changes in the xylem development of carrot roots. this study provided novel insights into the functions of gibberellins in the growth and development of carrot. to investigate the effects of gibberelliins on the growth of carrot, we treated carrot plants with gibberellic acid or paclobutrazol. gibberellins stimulate cell elongation and expansion during plant growth and development. carrot is a root plant with great value and undergoes obvious alteration in organ size over the period of plant growth.
 gibberellins stimulate cell elongation and expansion during plant growth and development. carrot is a root plant with great value and undergoes obvious alteration in organ size over the period of plant growth. however, the roles of gibberellins in carrot remain unclear.  to investigate the effects of gibberelliins on the growth of carrot, we treated carrot plants with gibberellic acid or paclobutrazol. the results found that ga dramatically reduced the root growth but stimulated the shoot growth of carrot. it also significantly promoted xylem development in the tuberous root of carrot. in addition, transcript levels of genes related to gibberellins, auxin, cytokinins, abscisic acid and brassinolides were altered in response to increased or reduced gibberellins.  the inhibited tuberous root growth but enhanced shoot growth in plants treated with ga can be principally attributed to the changes in the xylem development of carrot roots. negative feedback regulation mechanism of gibberellin biosynthesis also occurred in response to altered gibberellin accumulation. gibberellins may interact with other hormones to regulate carrot plant growth through crosstalk mechanisms. this study provided novel insights into the functions of gibberellins in the growth and development of carrot. keywords gibberellinsmorphologyanatomictranscript profileshormonal crosstalkdaucus carota lhttp://dx.doi.org/ /501100004602program for new century excellent talents in university ncet-11-0670xiong ai-sheng http://dx.doi.org/ /501100004608natural science foundation of jiangsu province bk20130027xiong ai-sheng priority academic program development of jiangsu higher education institutionspapdxiong ai-sheng shanxi province coal based key scientific and technological projectft201402-07xiong ai-sheng issue-copyright-statement© the author 2015 