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PubMed2003

Involvement of gibberellin and cytokinin in the formation of embryogenic cell clumps in carrot (Daucus carota).

Somatic embryogenesis of carrots is a typical example of the totipotency of plant cells. However, little is known about the process of change from somatic cells to embryogenic cells. To test the involvement of plant hormones in the acquisition process of embryogenic potency, we investigated the effects of plant growth regulators and their inhibitors on auxin-induced direct somatic embryogenesis of carrots. Gibberellin (GA) inhibited the early stage of embryogenic cell differentiation/development to the globular stage and uniconazole, an inhibitor of GA synthesis, promoted the secondary embryogenesis from the primary embryo. Purine riboside, an anticytokinin, inhibited direct somatic embryogenesis, and this effect was nullified by the application of cytokinin (CK). These results show that GA and CK regulate the early stage of auxin-induced somatic embryogenesis in carrots.

Biological Transport, Active↗

PubMed2004

Involvement of endogenous gibberellins in potato tuber dormancy and early sprout growth: a critical assessment.

The role of endogenous gibberellins (GAs) in the regulation of potato (Solanum tuberosum) tuber dormancy was examined by determining: 1. changes in endogenous GA levels during natural dormancy progression, 2. the effects of GA biosynthesis inhibitors on tuber dormancy duration and 3. the dormancy status and tuber GA levels in a dwarf mutant of potato. The tubers (cv. Russet Burbank) used in these studies were still completely dormant after 98 days of storage. Between 98 and 134 days of storage, dormancy began to end and tubers exhibited limited (< 2 mm) sprout growth. Tuber dormancy weakened with further storage and tubers exhibited greater rates of sprout growth after 187 days of storage. Tubers stored for 212 days or longer were completely non-dormant and exhibited vigorous sprout growth. Immediately after harvest, the endogenous contents of GA19, GA20, and GA1 were relatively high (0.48-0.62 ng g fresh weight(-1)). The content of these GAs declined between 33 and 93 days of storage. Internal levels of GA19, GA20, and GA, rose slightly between 93 and 135 days of storage reaching levels comparable to those found in highly dormant tubers immediately after harvest. Levels of GA19, GA20, and GA1 continued to increase as sprout growth became more vigorous. Neither GA4 nor GA8 was detected in any tuber sample regardless of dormancy status. Dormant tubers exhibited a time-dependent increase in apparent GA sensitivity. Freshly harvested tubers were completely insensitive to exogenous GAs. As postharvest storage continued, exogenous GAs promoted premature dormancy release with GA1 and GA20 eliciting the greatest response. Injection of up to 5 microg tuber(-1) of kaurene, GA12, GA19 or GA8 had no effect on dormancy release. Sprout growth from non-dormant tubers was also promoted by exogenous GA in the following sequence of activity: GA1 = GA20 > GA19. Kaurene, GA12, and GA8 were inactive. Continuous exposure of developing tubers to inhibitors of GA biosynthesis (AMO-1618, ancymidol, or tetcyclasis) did not extend tuber dormancy but rather hastened dormancy release. Comparison of tuber dormancy and GA1 content in tubers of a wild-type and dwarf mutant of S. tuberosum ssp. andigena revealed a near-identical pattern of dormancy progression in spite of the absence of detectable levels of GA1 in tubers of the dwarf sibling at any time during dormancy progression. Collectively, these results do not support a role for endogenous GA in potato tuber dormancy release but are consistent with a role for GAs in the regulation of subsequent sprout growth.

Gibberellins↗

PubMed2003

Day and night temperature responses in Arabidopsis: effects on gibberellin and auxin content, cell size, morphology and flowering time.

The effect of 16 different day (DT) and night (NT) temperature combinations (DT and NT 12, 17, 22 and 27 degrees 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 degrees C, however, stem elongation rate increased greatly, resulting in the same final stem length as under NT 12 degrees 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 degrees C gave rise to longer and thicker stems compared with stems developing at DT/NT 12/22 degrees 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 degrees C compared with DT/NT 12/22 degrees C. Of the 12 gibberellins analysed, however, only the level of non-bioactive GA29 was affected by the temperature treatment.

Adaptation, Physiological↗

PubMed2003

Genomics approach to abscisic acid- and gibberellin-responsive genes in rice.

We used an 8987-EST collection to construct a cDNA microarray system with various genomics information (full-length cDNA, expression profile, high accuracy genome sequence, phenotype, genetic map, and physical map) in rice. This array was used as a probe to hybridize target RNAs prepared from normally grown callus of rice and from callus treated for 6 hr or 3 days with the hormones abscisic acid (ABA) or gibberellin (GA). We identified 509 clones, including many clones that had never been annotated as ABA-or GA-responsive. These genes included not only ABA- or GA-responsive genes but also genes responsive to other physiological conditions such as pathogen infection, heat shock, and metal ion stress. Comparison of ABA- and GA-responsive genes revealed antagonistic regulation for these genes by both hormones except for one defense-related gene, thionin. The gene for thionin was up-regulated by both hormone treatments for 3 days. The upstream regions of all the genes that were regulated by both hormones had cis-elements for ABA and GA response. We performed a clustering analysis of genes regulated by both hormones and various expression profiles that showed three notable clusters (seed tissues, low temperature and sugar starvation, and thionin-gene related). A comparison of the cis-elements for hormone response genes between rice and Arabidopsis thaliana, we identified cis-elements for dehydration-stress response or for expression of amylase gene as Arabidopsis gene-specific or rice gene-specific, respectively.

Abscisic Acid↗

PubMed1997

The new RGA locus encodes a negative regulator of gibberellin response in Arabidopsis thaliana.

We have identified a new locus involved in gibberellin (GA) signal transduction by screening for suppressors of the Arabidopsis thaliana GA biosynthetic mutant gal-3. The locus is named RGA for repressor of gal-3. Based on the recessive phenotype of the digenic rga/gal-3 mutant, the wild-type gene product of RGA is probably a negative regulator of GA responses. Our screen for suppressors of gal-3 identified 17 mutant alleles of RGA as well as 10 new mutant alleles at the previously identified SPY locus. The digenic (double homozygous) rga/gal-3 mutants are able to partially repress several defects of gal-3 including stem growth, leaf abaxial trichome initiation, flowering time, and apical dominance. The phenotype of the trigenic mutant (triple homozygous) rga/spy/gal-3 shows that rga and spy have additive effects regulating flowering time, abaxial leaf trichome initiation and apical dominance. This trigenic mutant is similar to wild type with respect to each of these developmental events. Because rga/spy/gal-3 is almost insensitive to GA for hypocotyl growth and its bolting stem is taller than the wild-type plant, the combined effects of the rga and spy mutations appear to allow GA-independent stem growth. Our studies indicate that RGA lies on a separate branch of the GA signal transduction pathway from SPY, which leads us to propose a modified model of the GA response pathway.

Arabidopsis↗

PubMed2001

Gibberellins are not required for normal stem growth in Arabidopsis thaliana in the absence of GAI and RGA.

The growth of Arabidopsis thaliana is quantitatively regulated by the phytohormone gibberellin (GA) via two closely related nuclear GA-signaling components, GAI and RGA. Here we test the hypothesis that GAI and RGA function as "GA-derepressible repressors" of plant growth. One prediction of this hypothesis is that plants lacking GAI and RGA do not require GA for normal stem growth. Analysis of GA-deficient mutants lacking GAI and RGA confirms this prediction and suggests that in the absence of GAI and RGA, "growth" rather than "no growth" is the default state of plant stems. The function of the GA-signaling system is thus to act as a control system regulating the amount of this growth. We also demonstrate that the GA dose dependency of hypocotyl elongation is altered in mutants lacking GAI and RGA and propose that increments in GAI/RGA repressor function can explain the quantitative nature of GA responses.

Alleles↗

PubMed2001

Synergistic derepression of gibberellin signaling by removing RGA and GAI function in Arabidopsis thaliana.

RGA and GAI are negative regulators of the gibberellin (GA) signal transduction pathway in Arabidopsis thaliana. These genes may have partially redundant functions because they are highly homologous, and plants containing single null mutations at these loci are phenotypically similar to wild type. Previously, rga loss-of-function mutations were shown to partially suppress defects of the GA-deficient ga1-3 mutant. Phenotypes rescued include abaxial trichome initiation, rosette radius, flowering time, stem elongation, and apical dominance. Here we present work showing that the rga-24 and gai-t6 null mutations have a synergistic effect on plant growth. Although gai-t6 alone has little effect, when combined with rga-24, they completely rescued the above defects of ga1-3 to wild-type or GA-overdose phenotype. However, seed germination and flower development defects were not restored. Additionally, rga-24 and rga-24/gai-t6 but not gai-t6 alone caused increased feedback inhibition of expression of a GA biosynthetic gene in both the ga1-3 and wild-type backgrounds. These results demonstrate that RGA and GAI have partially redundant functions in maintaining the repressive state of the GA-signaling pathway, but RGA plays a more dominant role than GAI. Removing both RGA and GAI function allows for complete derepression of many aspects of GA signaling.

Arabidopsis↗

PubMed2001

The expression of tgas118, encoding a defensin in Lycopersicon esculentum, is regulated by gibberellin.

A flower specific cDNA, tgas118, has been isolated after differential screening of a gib-1 anther cDNA library of Lycopersicon esculentum. The corresponding mRNA was present in all tissues analysed. Northern blot analysis revealed that in wild-type tomato the gene was predominantly expressed throughout flower development, while in the gibberellin (GA)-deficient mutant of tomato (gib-1) the abundance declined. Treatment of the mutant with GA resulted in an accumulation of the tgas118 mRNA within hours in leaf and bud tissues. In the leaf, GA1, GA3 and GA9 were effective in enhancing the expression while GA4 was not. In addition to GA, wounding and dehydration also increased the accumulation of tgas118 mRNA in leaf tissue. In situ hybridization showed that application of 50 ng GA3 bud(-1) induced a similar spatial expression of the tgas118 mRNA in gib-1 buds 24 h post treatment to that found in wild-type flower buds. The deduced TGAS118 protein displays up to 77% similarity with defensins and as its expression is up-regulated by stimuli such as wounding it is proposed that it may play a role in protection against pathogens.

Amino Acid Sequence↗

PubMed2002

Regulation of expression of two novel flower-specific genes from tomato (Solanum lycopersicum) by gibberellin.

Two flower-specific cDNAs have been isolated after differential screening of an anther cDNA library. This library was constructed 48 h after GA(3) treatment of buds of the GA-deficient gib-1 mutant of tomato. Northern blot analysis during flower development in tomato demonstrated that the expression of both genes is regulated by gibberellins (GAs). Application of GA(3) to developmentally arrested gib-1 flower buds induced new expression of tgas100 mRNA 48 h post-treatment, while an increased accumulation of tgas105 mRNA was found after 8 h. In situ analyses showed the spatial distribution of the expression of both genes within the tomato flower. One of the deduced polypeptides (TGAS105) displays similarities to cysteine-rich extensin-like proteins, while the other (TGAS100) shows significant homology with a stamen-specific gene of Antirrhinum majus. Based on the deduced protein sequences, the possible function of the encoded proteins is discussed.

Amino Acid Sequence↗

PubMed2002

A gibberellin-regulated xyloglucan endotransglycosylase gene is expressed in the endosperm cap during tomato seed germination.

Xyloglucan endotransglycosylases (XETs) modify xyloglucans, major components of primary cell walls in dicots. A cDNA encoding an XET (LeXET4) was isolated from a germinating tomato (Lycopersicon esculentum Mill.) seed cDNA library. DNA gel blot analysis showed that LeXET4 is a single-copy gene in the tomato genome. LeXET4 mRNA was strongly expressed in germinating seeds, was much less abundant in stems, and was not detected in roots, leaves or flower tissues. During germination, LeXET4 mRNA was detected in seeds within 12 h of imbibition with maximum mRNA abundance at 24 h. Tissue prints showed that LeXET4 mRNA was localized exclusively to the endosperm cap region. Expression of LeXET4 was dependent on exogenous gibberellin (GA) in GA-deficient (gib-1 mutant) tomato seeds, while abscisic acid, a seed germination inhibitor, had no effect on LeXET4 mRNA expression in wild-type seeds. LeXET4 mRNA disappeared after radicle emergence, even though degradation of the lateral endosperm cell walls continued. The temporal, spatial and hormonal regulation pattern of LeXET4 gene expression suggests that XET has a role in endosperm cap weakening, a key process regulating tomato seed germination.

Abscisic Acid↗

PubMed2002

The role of gibberellins in embryo axis development.

The role of gibberellins (GAs) during early embryo development was examined using microspore-derived embryos (MDEs) of Brassica napus. At the globular stage of development, 10 d after initial culture (DAC) when endogenous GA(1) levels are increasing rapidly, a triazole, uniconazole, was used at 1, 33 and 100 microM to inhibit GA biosynthesis. Within this dose range there was no apparent effect of the inhibitor on embryo growth through to the early torpedo stage. However, by 25 DAC uniconazole-treated MDEs showed significantly reduced (50%) axis elongation. Addition of GA(1) at 33 microM on 14 DAC to embryos pretreated with 1 microM uniconazole on 10 DAC prevented this reduction in axis length, giving axis elongation equivalent to untreated MDEs. Application of GA(1) alone, however, did not significantly increase axis elongation. The reduced axis growth seen with uniconazole treatment was due to reduced cell elongation, but not cell number, and the co-applied GA(1) thus prevented the uniconazole-induced reduction in cell length. The elongating axis of MDEs may thus be a useful tool for examining the role of GAs in cell elongation.

Brassica napus↗

PubMed2004

An alpha-crystallin gene, ACD31.2 from Arabidopsis is negatively regulated by FPF1 overexpression, floral induction, gibberellins, and long days.

A gene sequence was isolated from a differential display experiment to find transcripts altered in expression by overexpression of FLORAL PROMOTING FACTOR 1 (FPF1) in Arabidopsis thaliana. The gene, ACD31.2, encodes an alpha-crystallin domain containing protein with homology to small heat shock proteins. In addition to down-regulation by FPF1 overexpression, the ACD31.2 transcript is also down-regulated by long days, floral induction, and by gibberellin in wild-type plants. Expression is highest in leaves and stems.

Amino Acid Sequence↗

PubMed2005

Exogenous gibberellins inhibit coffee (Coffea arabica cv. Rubi) seed germination and cause cell death in the embryo.

The mechanism of inhibition of coffee (Coffea arabica cv. Rubi) seed germination by exogenous gibberellins (GAs) and the requirement of germination for endogenous GA were studied. Exogenous GA(4+7) inhibited coffee seed germination. The response to GA(4+7) showed two sensitivity thresholds: a lower one between 0 and 1 microM and a higher one between 10 and 100 microM. However, radicle protrusion in coffee seed depended on the de novo synthesis of GAs. Endogenous GAs were required for embryo cell elongation and endosperm cap weakening. Incubation of coffee seed in exogenous GA(4+7) led to loss of embryo viability and dead cells were observed by low temperature scanning microscopy only when the endosperm was surrounding the embryo. The results described here indicate that the inhibition of germination by exogenous GAs is caused by factors that are released from the endosperm during or after its weakening, causing cell death in the embryo and leading to inhibition of radicle protrusion.

Cell Death↗

PubMed2005

Gibberellin-regulated XET is differentially induced by auxin in rice leaf sheath bases during gravitropic bending.

The asymmetric distribution of auxin plays a fundamental role in plant gravitropism, yet little is understood about how its lateral distribution stimulates growth. In the present work, the asymmetric distribution not only of auxin, but also that of gibberellins (GAs), was observed in rice leaf sheath bases following gravistimulation. Gravistimulation induced the transient accumulation of greater amounts of both IAA and GA in the lower halves of the leaf sheath bases of rice seedlings. OsGA3ox1, a gene of active GA synthesis, was differentially induced by gravistimulation. Furthermore, 2,3,5-tri-iodobenzoic acid (TIBA), an inhibitor of auxin transport, substantially decreased the asymmetric distribution of IAA and the gradient of OsGA3ox1 expression. Externally applied GA(3) restored the gravitropic curvature of rice leaf sheaths inhibited by either TIBA or by ancymidol, a GA synthesis inhibitor. The expression of XET (encoding xyloglucan endotransglycosylase) was differentially induced in the lower halves of gravistimulated leaf sheath bases and was also up-regulated by exogenous IAA and GA(3). Both ancymidol and TIBA decreased the gradient of XET expression. These data suggest that the asymmetric distribution of auxin effected by gravistimulation induced a gradient of GAs via asymmetric expression of OsGA3ox1 in rice leaf sheath bases, and hence caused the asymmetric expression of XET. Cell wall loosening in the curvature site of the leaf sheath triggered by the expression of XET would contribute to gravitropic growth.

Enzyme-Linked Immunosorbent Assay↗

PubMed1998

Transcript level for a gene encoding a putative type 1a plasma membrane receptor is induced by gibberellin in deepwater rice.

In search for differentially expressed genes, a novel gene was identified whose transcript levels increased in response to gibberellin in the internodes of deepwater rice. Its expression was high in regions undergoing cell division and lower in the elongation and differentiation zones. Amino acid sequence analysis indicated that the gene may encode a type 1a receptor with an extracellular domain, a single transmembrane domain, and a short cytoplasmic domain.

Amino Acid Sequence↗

PubMed1998

Characterization of a protein kinase gene responsive to auxin and gibberellin in cucumber hypocotyls.

By means of the PCR, cDNA clones encoding putative protein kinases have been obtained from cucumber hypocotyls. The abundance of the transcript of one of these genes, which was named CsPK3, increased on treatment with gibberellin (GA4) and/or auxin (IAA). We screened a cucumber cDNA library to clone CsPK3 cDNA. The cDNA clone (cCsPK3) encodes an open reading frame of 1,413 bp (471 amino acids), and its predicted amino acid sequence showed homology with those of serine/threonine protein kinases. Northern blot analysis indicated that IAA was more active than GA4 in increasing the level of CsPK3 mRNA in cucumber hypocotyls and that the increase in the level of CsPK3 mRNA on treatment with IAA was not inhibited by pretreatment with a protein synthesis inhibitor. The level of CsPK3 mRNA was high in hypocotyls of dark-grown cucumber seedlings and decreased to less than 50% of the original level within 15 min of the start of irradiation with white light.

Amino Acid Sequence↗

PubMed2000

Gibberellin biosynthesis: its regulation by endogenous and environmental signals.

The hormone gibberellin (GA) plays an essential role in many aspects of plant growth and development, such as seed germination, stem elongation and flower development. In recent years, exciting progress has been made in understanding how the biosynthesis of this hormone is regulated by endogenous and environmental factors. This has resulted from isolation of genes encoding enzymes involved in GA biosynthesis and metabolism, which also enabled us to manipulate the pathway by modifying the expression of these genes in transgenic plants. In addition, new GA response mutants provided information about how signaling components are involved in feedback regulation of the GA biosynthetic pathway.

Environment↗

PubMed2001

Involvement of gibberellins in expression of a cysteine proteinase (SH-EP) in cotyledons of Vigna mungo seedlings.

The expression of a papain-type proteinase, designated SH-EP, in cotyledons of Vigna mungo seedlings has been shown to require some factors in the embryonic axes. Gibberellin A1 (GA(1)) and GA(20) were identified by GC-MS in embryonic axes of V. mungo seedlings. The level of accumulation of SH-EP in cotyledons of V. mungo seedlings was greatly reduced by treatment of the seeds with uniconazole-P, an inhibitor for GA biosynthesis. The reduced level of accumulation of SH-EP in cotyledons by uniconazole-P was recovered by exogenous application of GA(1) and GA(20) to the seedlings.

Cotyledon↗