Gibberellin Metabolism and Signal Transduction

118

Signals produced in leaves are transported to the shoot apex where they cause flowering. Protein of the gene FLOWERING LOCUS T (FT) is probably a long day (LD) signal in Arabidopsis. In the companion paper, rapid LD increases in FT expression associated with flowering driven photosynthetically in red light were documented. In a far red (FR)-rich LD, along with FT there was a potential role for gibberellin (GA). Here, with the GA biosynthesis dwarf mutant ga1-3, GA4-treated plants flowered after 26 d in short days (SD) but untreated plants were still vegetative after 6 months. Not only was FT expression low in SD but applied GA bypassed some of the block to flowering in ft-1. On transfer to LD, ga1-3 only flowered when treated simultaneously with GA, and FT expression increased rapidly (<19.5 h) and dramatically (15-fold). In contrast, in the wild type in LD there was little requirement for GA for FT increase and flowering so its endogenous GA content was near to saturating. Despite this permissive role for endogenous GA in Columbia, RNA interference (RNAi) silencing of the GA biosynthesis gene, GA 20-OXIDASE2, revealed an additional, direct role for GA in LD. Flowering took twice as long after silencing the LD-regulated gene, GA 20-OXIDASE2. Such independent LD input by FT and GA reflects their non-sympatric expression (FT in the leaf blade and GA 20-OXIDASE2 in the petiole). Overall, FT acts as the main LD floral signal in Columbia and GA acts on flowering both via and independently of FT.

Section: Resultsmentioning

confidence: 99%

The nature of floral signals in Arabidopsis. II. Roles for FLOWERING LOCUS T (FT) and gibberellin

Hashizume

King

2008

118

“…This view has also been reported for Arabidopsis seeds ( Yamaguchi et al , 1998 , 2001; Ogawa et al , 2003 ; Seo et al , 2006 ; Yamauchi et al , 2007 ). Three classes of GA metabolism enzymes have been identified in several higher plant species, including Arabidopsis and rice ( Thomas and Hedden, 2006 ). These include diterpene cyclases (CPS and KS), P450 mono-oxygenases (KO and KAO), and 2ODDs (GA20ox and GA3ox for GA biosynthesis and GA2ox for GA deactivation).…”

Section: Discussionmentioning

confidence: 99%

“…Gibberellins are tetracyclic diterpenoid phytohormones that regulate various aspects of plant growth and development such as seed germination, leaf expansion, stem elongation, flowering, and seed development ( Thomas and Hedden, 2006 ). Bioactive GAs are biosynthesized from geranylgeranyl diphosphate (GGDP), a common diterpenoid precursor, through several steps ( Fig.…”

Section: Introductionmentioning

confidence: 99%

“…Bioactive GAs are biosynthesized from geranylgeranyl diphosphate (GGDP), a common diterpenoid precursor, through several steps ( Fig. 1 ), as reviewed by Thomas and Hedden (2006) . Geranylgeranyl diphosphate is converted to the tetracyclic hydrocarbon ent -kaurene through ent -copalyl diphosphate ( ent -CDP) by two distinct diterpene cyclases in plastids: ent -CDP synthase (CPS) and ent -kaurene synthase (KS).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Germination of photoblastic lettuce seeds is regulated via the control of endogenous physiologically active gibberellin content, rather than of gibberellin responsiveness

Sawada

Katsumata

Kitamura

et al. 2008

Phytochrome regulates lettuce (Lactuca sativa L. cv. Grand Rapids) seed germination via the control of the endogenous level of bioactive gibberellin (GA). In addition to the previously identified LsGA20ox1, LsGA20ox2, LsGA3ox1, LsGA3ox2, LsGA2ox1, and LsGA2ox2, five cDNAs were isolated from lettuce seeds: LsCPS, LsKS, LsKO1, LsKO2, and LsKAO. Using an Escherichia coli expression system and functional assays, it is shown that LsCPS and LsKS encode ent-copalyl diphosphate synthase and ent-kaurene synthase, respectively. Using a Pichia pastoris system, it was found that LsKO1 and LsKO2 encode ent-kaurene oxidases and LsKAO encodes ent-kaurenoic acid oxidase. A comprehensive expression analysis of GA metabolism genes using the quantitative reverse transcription polymerase chain reaction suggested that transcripts of LsGA3ox1 and LsGA3ox2, both of which encode GA 3-oxidase for GA activation, were primarily expressed in the hypocotyl end of lettuce seeds, were expressed at much lower levels than the other genes tested, and were potently up-regulated by phytochrome. Furthermore, LsDELLA1 and LsDELLA2 cDNAs that encode DELLA proteins, which act as negative regulators in the GA signalling pathway, were isolated from lettuce seeds. The transcript levels of these two genes were little affected by light. Lettuce seeds in which de novo GA biosynthesis was suppressed responded almost identically to exogenously applied GA, irrespective of the light conditions, suggesting that GA responsiveness is not significantly affected by light in lettuce seeds. It is proposed that lettuce seed germination is regulated mainly via the control of the endogenous content of bioactive GA, rather than the control of GA responsiveness.

“…Such a strategy can modulate the signal produced and subsequently prevent the progressive accumulation of the hormones. A number of inactivation pathways have been identified for GA (Thomas and Hedden, 2006; Zhu et al , 2006). However, based on the prevalence of 2β-hydroxylated GAs in many plant species (MacMillan, 2002), the most widespread mechanism for GA inactivation seems to be via 2-oxidation (Thomas et al , 1999).…”

Section: Introductionmentioning

confidence: 99%

Identification and genetic characterization of a gibberellin 2-oxidase gene that controls tree stature and reproductive growth in plum

El‐Sharkawy¹,

Kayal²,

Prasath³

et al. 2011

Several dwarf plum genotypes (Prunus salicina L.), due to deficiency of unknown gibberellin (GA) signalling, were identified. A cDNA encoding GA 2-oxidase (PslGA2ox), the major gibberellin catabolic enzyme in plants, was cloned and used to screen the GA-deficient hybrids. This resulted in the identification of a dwarf plum hybrid, designated as DGO24, that exhibits a markedly elevated PslGA2ox signal. Grafting ‘Early Golden’ (EG), a commercial plum cultivar, on DGO24 (EG/D) enhanced PslGA2ox accumulation in the scion part and generated trees of compact stature. Assessment of active GAs in such trees revealed that DGO24 and EG/D accumulated relatively much lower quantities of main bioactive GAs (GA1 and GA4) than control trees (EG/M). Moreover, the physiological function of PslGA2ox was studied by determining the molecular and developmental consequences due to ectopic expression in Arabidopsis. Among several lines, two groups of homozygous transgenics that exhibited contrasting phenotypes were identified. Group-1 displayed a dwarf growth pattern typical of mutants with a GA deficiency including smaller leaves, shorter stems, and delay in the development of reproductive events. In contrast, Group-2 exhibited a ‘GA overdose’ phenotype as all the plants showed elongated growth, a typical response to GA application, even under limited GA conditions, potentially due to co-suppression of closely related Arabidopsis homologous. The studies reveal the possibility of utilizing PslGA2ox as a marker for developing size-controlling rootstocks in Prunus.