The Metabolism of Gibberellin A20 to Gibberellin A1 by Tall and Dwarf Mutants of Oryza sativa and Arabidopsis thaliana

SYL3-k increases style length and yield of F1 seeds via enhancement of endogenous GA4 content in Oryza sativa L. pistils

SYL3-k allele increases the outcrossing rate of male sterile line and the yield of hybrid F₁ seeds via enhancement of endogenous GA₄ content in Oryza sativa L. pistils. The change in style length might be an adaptation of rice cultivation from south to north in the northern hemisphere. The style length (SYL) in rice is one of the major factors influencing the stigma exertion, which affects the outcross rate of male sterile line and the yield of hybrid F₁ seeds. However, the biological mechanisms underlying SYL elongation remain elusive. Here, we report a map-based cloning and characterisation of the allele qSYL3-k. The qSYL3-k allele encodes a MADS-box family transcription factor, and it is expressed in various rice organs. The qSYL3-k allele increases SYL via the elongation of cell length in the style, which is associated with a higher GA₄ content in the pistil. The expression level of OsGA3ox2 in pistils with qSYL3-k alleles is significantly higher than that in pistils with qSYL3-n allele on the same genome background of Nipponbare. The yield of F₁ seeds harvested from plants with 7001S^SYL3-k alleles was 16% higher than that from plants with 7001S^SYL3-n allele. The sequence data at the qSYL3 locus in 136 accessions showed that alleles containing the haplotypes qSYL3^AA, qSYL3^AG, and qSYL3^GA increased SYL, whereas those containing the haplotype qSYL3^GG decreased it. The frequency of the haplotype qSYL3^GG increases gradually from the south to north in the northern hemisphere. These findings will facilitate improvement in SYL and yield of F₁ seeds henceforward.

Overexpression of ovate family protein 22 confers multiple morphological changes and represses gibberellin and brassinosteroid signalings in transgenic rice

OVATE family proteins (OFPs) are plant-specific transcription factors that regulate plant growth and development. OFPs interact with 3-aa loop extension (TALE) homeodomain proteins and brassinosteroid (BR) signaling components to modulate gibberellic acid (GA) biosynthesis and BR responses. Bioactive GAs are essential in regulating plant organogenesis and organ growth by promoting cell differentiation and elongation. DELLA proteins act as the central repressors of GA-regulated processes and are targeted to be degraded by the 26S proteasome in the presence of GA. We discovered that the rice OFP22 negatively regulates GA and BR signal transduction. OsOFP22 expression was rapidly up-regulated by exogenous GA and BR application, detected predominantly in the calli and spikelets. Overexpression of OsOFP22 conferred multiple morphological phenotypes, including reduced plant height, dark green leaves, and shortened and widened leaves, floral organs and grains. The GA-induced elongation of the second leaf sheath in the seedlings, and α-amylase activity in the endosperms were attenuated in transgenic lines overexpressing OsOFP22, while GA-biosynthesis gene transcripts and bioactive GA₃ and GA₄ contents were increased in the transgenic plants. OsOFP22 promotes the protein accumulation of SLR1, the single DELLA in rice protein. Furthermore, Overexpression of OsOFP22 suppresses BR response and the expression of BR-related genes. OsOFP22 is thus involved in the repression of GA and BR signal transduction and integrates GA with BR to regulate plant growth and development.

Modeling vegetative vigour in grapevine: unraveling underlying mechanisms

Mechanistic modeling constitutes a powerful tool to unravel complex biological phenomena. This study describes the construction of a mechanistic, dynamic model for grapevine plant growth and canopy biomass (vigor). To parametrize and validate the model, the progeny from a cross of Ramsey (Vitis champinii) × Riparia Gloire (V. riparia) was evaluated. Plants with different vigor were grown in a greenhouse during the summer of 2014 and 2015. One set of plants was grafted with Cabernet Sauvignon. Shoot growth rate (b), leaf area (LA), dry biomass, whole plant and root specific hydraulic conductance (k_H and L_pr), stomatal conductance (g_s), and water potential (Ψ) were measured. Partitioning indices and specific leaf area (SLA) were calculated. The model includes an empirical fit of a purported seasonal pattern of bioactive GAs based on published seasonal evolutionary levels and reference values. The model provided a good fit of the experimental data, with R = 0.85. Simulation of single trait variations defined the individual effect of each variable on vigor determination. The model predicts, with acceptable accuracy, the vigor of a young plant through the measurement of L_pr and SLA. The model also permits further understanding of the functional traits that govern vigor, and, ultimately, could be considered useful for growers, breeders and those studying climate change.

Cloning and functional analysis of two gibberellin 3 beta -hydroxylase genes that are differently expressed during the growth of rice

We have cloned two gibberellin (GA) 3 beta-hydroxylase genes, OsGA3ox1 and OsGA3ox2, from rice by screening a genomic library with a DNA fragment obtained by PCR using degenerate primers. We have used full-scan GC-MS and Kovats retention indices to show function for the two encoded recombinant fusion proteins. Both proteins show 3 beta-hydroxylase activity for the steps GA(20) to GA(1), GA(5) to GA(3), GA(44) to GA(38), and GA(9) to GA(4). In addition, indirect evidence suggests that the OsGA3ox1 protein also has 2,3-desaturase activity, which catalyzes the steps GA(9) to 2,3-dehydro-GA(9) and GA(20) to GA(5) (2,3-dehydro GA(20)), and 2 beta-hydroxylase activity, which catalyzes the steps GA(1) to GA(8) and GA(4) to GA(34). Molecular and linkage analysis maps the OsGA3ox1 gene to the distal end of the short arm of chromosome 5; the OsGA3ox2 gene maps to the distal end of the short arm of chromosome 1 that corresponds to the D18 locus. The association of the OsGA3ox2 gene with the d18 locus is confirmed by sequence and complementation analysis of three d18 alleles. Complementation of the d18-AD allele with the OxGA3ox2 gene results in transgenic plants with a normal phenotype. Although both genes show transient expression, the highest level for OsGA3ox1 is from unopened flower. The highest level for OsGA3ox2 is from elongating leaves.

Azospirillum spp. metabolize [17,17-2H2]gibberellin A20 to [17,17-2H2]gibberellin A1 in vivo in dy rice mutant seedlings

Azospirillum spp. are endophytic bacteria with beneficial effects on cereals--effects partially attributed to gibberellin production by the microorganisms. Azospirillum lipoferum and Azospirillum brasilense inoculated to rice dy mutant reversed dwarfism in seedlings incubated with [17,17-2H2]GA20 with formation of [17,17-2H2]GA1, showing the in vivo capacity to perform the 3beta-hydroxylation. When prohexadione-Ca, an inhibitor of late steps in gibberellin biosynthesis, was added to the culture medium, no complementation was observed and no [17,17-2H2]GA1 was produced. The latter suggests that the bacterial operating enzyme may be a 2-oxoglutarate-dependent dioxygenase, similar to those of plants.

Azospirillum brasilense and Azospirillum lipoferum hydrolyze conjugates of GA20 and metabolize the resultant aglycones to GA1 in seedlings of rice dwarf mutants

Azospirillum species are plant growth-promotive bacteria whose beneficial effects have been postulated to be partially due to production of phytohormones, including gibberellins (GAs). In this work, Azospirillum brasilense strain Cd and Azospirillum lipoferum strain USA 5b promoted sheath elongation growth of two single gene GA-deficient dwarf rice (Oryza sativa) mutants, dy and dx, when the inoculated seedlings were supplied with [17,17-2H2]GA20-glucosyl ester or [17,17- 2H2]GA20-glucosyl ether. Results of capillary gas chromatography-mass spectrometry analysis show that this growth was due primarily to release of the aglycone [17,17-2H2]GA20 and its subsequent 3beta-hydroxylation to [17,17-2H2]GA1 by the microorganism for the dy mutant, and by both the rice plant and microorganism for the dx mutant.

Expression of a gibberellin 2-oxidase gene around the shoot apex is related to phase transition in rice

A major catabolic pathway for gibberellin (GA) is initiated by 2beta-hydroxylation, a reaction catalyzed by GA 2-oxidase. We have isolated and characterized a cDNA, designated Oryza sativa GA 2-oxidase 1 (OsGA2ox1) from rice (Oryza sativa L. cv Nipponbare) that encodes a GA 2-oxidase. The encoded protein, produced by heterologous expression in Escherichia coli, converted GA(1), GA(4), GA(9), GA(20), and GA(44) to the corresponding 2beta-hydroxylated products GA(8), GA(34), GA(51), GA(29), and GA(98), respectively. Ectopic expression of the OsGA2ox1 cDNA in transgenic rice inhibited stem elongation and the development of reproductive organs. These transgenic plants were deficient in endogenous GA(1). These results indicate that OsGA2ox1 encodes a GA 2-oxidase, which is functional not only in vitro but also in vivo. OsGA2ox1 was expressed in shoot apex and roots but not in leaves and stems. In situ hybridization analysis revealed that OsGA2ox1 mRNA was localized in a ring at the basal region of leaf primordia and young leaves. This ring-shaped expression around the shoot apex was drastically decreased after the phase transition from vegetative to reproductive growth. It was absent in the floral meristem, but it was still present in the lateral meristem that remained in the vegetative phase. These observations suggest that OsGA2ox1 controls the level of bioactive GAs in the shoot apical meristem; therefore, reduction in its expression may contribute to the early development of the inflorescence meristem.

Gibberellin biosynthesis: metabolic evidence for three steps in the early 13-hydroxylation pathway of rice

[14C4]GA53, [14C4]GA44, and [2H2/14C4]GA19 were injected separately into seedlings of rice (Oryza sativa) using a dwarf mutant (d35) that has low levels of endogenous gibberellins (GAs). After 8 h incubation, the shoots were extracted and the labeled metabolites were identified by full-scan gas chromatography mass spectrometry (GC-MS) and Kovats retention indices (KRIs). Our results document the metabolic sequence, GA53-->GA44-->GA19-->GA20 and the presence of endogenous GA53, GA44, GA19, GA20 and GA1. Previous metabolic studies have shown the presence of the step, GA20-->GA1 in rice. Taken together, the data establish in vegetative shoots of rice the presence of the early 13-hydroxylation pathway, a pathway that originates from GA12 and leads to bioactive GA1.

Function and substrate specificity of the gibberellin 3beta-hydroxylase encoded by the Arabidopsis GA4 gene

cDNA corresponding to the GA4 gene of Arabidopsis thaliana L. (Heynh. ) was expressed in Escherichia coli, from which cell lysates converted [14C]gibberellin (GA)9 and [14C]GA20 to radiolabeled GA4 and GA1, respectively, thereby confirming that GA4 encodes a GA 3beta-hydroxylase. GA9 was the preferred substrate, with a Michaelis value of 1 microm compared with 15 microm for GA20. Hydroxylation of these GAs was regiospecific, with no indication of 2beta-hydroxylation or 2,3-desaturation. The capacity of the recombinant enzyme to hydroxylate a range of other GA substrates was investigated. In general, the preferred substrates contained a polar bridge between C-4 and C-10, and 13-deoxy GAs were preferred to their 13-hydroxylated analogs. Therefore, no activity was detected using GA12-aldehyde, GA12, GA19, GA25, GA53, or GA44 as the open lactone (20-hydroxy-GA53), whereas GA15, GA24, and GA44 were hydroxylated to GA37, GA36, and GA38, respectively. The open lactone of GA15 (20-hydroxy-GA12) was hydroxylated but less efficiently than GA15. In contrast to the free acid, GA25 19,20-anhydride was 3beta-hydroxylated to give GA13. 2,3-Didehydro-GA9 and GA5 were converted by recombinant GA4 to the corresponding epoxides 2, 3-oxido-GA9 and GA6.

GIBBERELLIN BIOSYNTHESIS: Enzymes, Genes and Their Regulation

The recent impressive progress in research on gibberellin (GA) biosynthesis has resulted primarily from cloning of genes encoding biosynthetic enzymes and studies with GA-deficient and GA-insensitive mutants. Highlights include the cloning of ent-copalyl diphosphate synthase and ent-kaurene synthase (formally ent-kaurene synthases A and B) and the demonstration that the former is targeted to the plastid; the finding that the Dwarf-3 gene of maize encodes a cytochrome P450, although of unknown function; and the cloning of GA 20-oxidase and 3beta-hydroxylase genes. The availability of cDNA and genomic clones for these enzymes is enabling the mechanisms by which GA concentrations are regulated by environmental and endogenous factors to be studied at the molecular level. For example, it has been shown that transcript levels for GA 20-oxidase and 3beta-hydroxylase are subject to feedback regulation by GA action and, in the case of the GA 20-oxidase, are regulated by light. Also discussed is other new information, particularly from mutants, that has added to our understanding of the biosynthetic pathway, the enzymes, and their regulation and tissue localization.

The dwarf-1 (dt) Mutant of Zea mays blocks three steps in the gibberellin-biosynthetic pathway

In plants, gibberellin (GA)-responding mutants have been used as tools to identify the genes that control specific steps in the GA-biosynthetic pathway. They have also been used to determine which native GAs are active per se, i.e., further metabolism is not necessary for bioactivity. We present metabolic evidence that the D1 gene of maize (Zea mays L.) controls the three biosynthetic steps: GA20 to GA1, Ga20 to GA5, and GA5 to GA3. We also present evidence that three gibberellins, GA1, GA5, and GA3, have per se activity in stimulating shoot elongation in maize. The metabolic evidence comes from the injection of [17-13C,3H]GA20 and [17-13C,3H]GA5 into seedlings of d1 and controls (normal and d5), followed by isolation and identification of the 13C-labeled metabolites by full-scan GC-MS and Kovats retention index. For the controls, GA20 was metabolized to GA1,GA3, and GA5; GA5 was metabolized to GA3. For the d1 mutant, GA20 was not metabolized to GA1, GA3, or to GA5, and GA5 was not metabolized to GA3. The bioassay evidence is based on dosage response curves using d1 seedlings for assay. GA1, GA3, and GA5 had similar bioactivities, and they were 10-times more active than GA20.

The dwarf-1 (dt) Mutant of Zea mays blocks three steps in the gibberellin-biosynthetic pathway

In plants, gibberellin (GA)-responding mutants have been used as tools to identify the genes that control specific steps in the GA-biosynthetic pathway. They have also been used to determine which native GAs are active per se, i.e., further metabolism is not necessary for bioactivity. We present metabolic evidence that the D1 gene of maize (Zea mays L.) controls the three biosynthetic steps: GA20 to GA1, Ga20 to GA5, and GA5 to GA3. We also present evidence that three gibberellins, GA1, GA5, and GA3, have per se activity in stimulating shoot elongation in maize. The metabolic evidence comes from the injection of [17-13C,3H]GA20 and [17-13C,3H]GA5 into seedlings of d1 and controls (normal and d5), followed by isolation and identification of the 13C-labeled metabolites by full-scan GC-MS and Kovats retention index. For the controls, GA20 was metabolized to GA1,GA3, and GA5; GA5 was metabolized to GA3. For the d1 mutant, GA20 was not metabolized to GA1, GA3, or to GA5, and GA5 was not metabolized to GA3. The bioassay evidence is based on dosage response curves using d1 seedlings for assay. GA1, GA3, and GA5 had similar bioactivities, and they were 10-times more active than GA20.