Gibberellins signal nuclear import of PHOR1, a photoperiod-responsive protein with homology to Drosophila armadillo

S. tuberosum ssp. andigena potato plants require short days (SD) for tuberization. We have isolated PHOR1 (photoperiod-responsive 1), which shows upregulated expression in induced leaves (SD). PHOR1 encodes an arm repeat protein with homology to the Drosophila segment polarity protein armadillo. Antisense inhibition of PHOR1 produces a semidwarf phenotype similar to that of GA-deficient plants, and the antisense lines show reduced GA responsiveness combined with a higher endogenous GA content than wild-type plants. Feedback regulation of GA biosynthetic genes is also altered in these lines. Conversely, transgenic lines overexpressing PHOR1 show an enhanced response to GA. GA application induces rapid migration of PHOR1-GFP protein to the nucleus. Thus, PHOR1 appears to be a general component of GA signaling pathways that relocalizes to the nucleus in the presence of GA.

Changes in GA 20-oxidase gene expression strongly affect stem length, tuber induction and tuber yield of potato plants

Gene StGA20ox1 encoding potato GA 20-oxidase is expressed to relatively high levels in leaves and regulated by daylength. To investigate whether this gene is involved in photoperiodic regulation of tuber formation, we have obtained transgenic potato plants expressing sense and antisense copies of the StGA20ox1 cDNA. Over-expression of this cDNA resulted in taller plants that required a longer duration of a short day photoperiod (SD) to tuberize. Tubers from these plants had a decreased time of dormancy and developed sprouts with elongated internodes. Plants expressing antisense copies of the StGA20ox1 cDNA had shorter stems, a decreased length of the internodes and tuberized earlier than control plants, showing increased tuber yields. Antisense inhibition of this gene had no visible effect on the time of dormancy of the tubers, although at the end of dormancy these formed sprouts with shortened internodes. Decreased levels of endogenous GA20 and GA1 were detected in the apex and first leaves of the antisense lines. These results demonstrate the involvement of the GA 20-oxidase activity encoded by StGA20ox1 in the control of stem elongation and in tuber induction but not in tuber dormancy, indicating that the latter may be regulated by another member of the gene family.

The gene pat-2, which induces natural parthenocarpy, alters the gibberellin content in unpollinated tomato ovaries

We investigated the role of gibberellins (GAs) in the effect of pat-2, a recessive mutation that induces facultative parthenocarpic fruit development in tomato (Lycopersicon esculentum Mill.) using near-isogenic lines with two different genetic backgrounds. Unpollinated wild-type Madrigal (MA/wt) and Cuarenteno (CU/wt) ovaries degenerated, but GA(3) application induced parthenocarpic fruit growth. On the contrary, parthenocarpic growth of MA/pat-2 and CU/pat-2 fruits, which occurs in the absence of pollination and hormone application, was not affected by GA(3). Pollinated MA/wt and parthenocarpic MA/pat-2 ovary development was negated by paclobutrazol, and this inhibitory effect was counteracted by GA(3). The main GAs of the early-13-hydroxylation pathway (GA(1), GA(3), GA(8), GA(19), GA(20), GA(29), GA(44), GA(53), and, tentatively, GA(81)) and two GAs of the non-13-hydroxylation pathway (GA(9) and GA(34)) were identified in MA/wt ovaries by gas chromatography-selected ion monitoring. GAs were quantified in unpollinated ovaries at flower bud, pre-anthesis, and anthesis. In unpollinated MA/pat-2 and CU/pat-2 ovaries, the GA(20) content was much higher (up to 160 times higher) and the GA(19) content was lower than in the corresponding non-parthenocarpic ovaries. The application of an inhibitor of 2-oxoglutarate-dependent dioxygenases suggested that GA(20) is not active per se. The pat-2 mutation may increase GA 20-oxidase activity in unpollinated ovaries, leading to a higher synthesis of GA(20), the precursor of an active GA.

Regulation of gibberellin biosynthesis by light

Phytochromes regulate transcript levels of gibberellin biosynthesis enzymes, GA 20-oxidases and/or GA 3beta-hydroxylases, in germinating lettuce and Arabidopsis seeds and in de-etiolating pea seedlings. Feedback regulation of GA biosynthesis by active GA is well established, but other mechanisms for regulation of these biosynthetic genes also exist, as this feedback does not operate on a GA 3beta-hydroxylase gene of Arabidopsis during seed germination.

The characterization of gio, a new pea mutant, shows the role of indoleacetic acid in the control of fruit development by the apical shoot

Fruit-set and fruit growth in pea (Pisum sativum L.) depend on gibberellins (GAs). The authors have isolated a new pea mutant, gio, which appeared spontaneously within the population of the cultivar Alaska, characterized by unpollinated ovaries much less sensitive to applied GAs. The mutant also has elongated peduncles, and is taller than the wild-type (WT) because the upper plant internodes are longer. Contrary to WT, the gio ovaries respond very little to benzylaminopurine (BAP) and 2,4-dichlorophenoxyacetic acid, but become fully sensitive to GA(3) when this hormone is applied together with BAP. The gio phenotype is determined by a mutation at a single mendelian locus. The mutation is recesive, shows incomplete penetrance, and its expression depends on environmental culture conditions. The sensitivity of the ovaries to GA(3) can be recovered by removing the apical shoot (plant decapitation) and by blocking the transport of indoleacetic acid (IAA) from the apical shoot with 2,3,5-triiodobenzoic acid. The content of IAA in methanolic extracts and phloematic exudates of the apical shoot of gio is about double that in the WT. The rate of transport of [(3)H]IAA applied to the apex of the mutant is also twice that in the WT. This indicates that the insensitivity of the gio ovaries to GAs is due to the inhibitory effect of the higher basipetal IAA transport from the shoot. The interaction between the fruit and the apical shoot mediated by IAA probably also involves cytokinins transported from the basal part of the plant.

Hormonal Control of Parthenocarpic Ovary Growth by the Apical Shoot in Pea

The role of the apical shoot as a source of inhibitors preventing fruit growth in the absence of a stimulus (e.g. pollination or application of gibberellic acid) has been investigated in pea (Pisum sativum L.). Plant decapitation stimulated parthenocarpic growth, even in derooted plants, and this effect was counteracted by the application of indole acetic acid (IAA) or abscisic acid (ABA) in agar blocks to the severed stump. The treatment of unpollinated ovaries with gibberellic acid blocked the effect of IAA or ABA applied to the stump. [3H]IAA and [3H]ABA applied to the stump were transported basipetally, and [3H]ABA but not [3H]IAA was also detected in unpollinated ovaries. The concentration of ABA in unpollinated ovaries increased significantly in the absence of a promotive stimulus. The application of IAA to the stump enhanced by 2- to 5-fold the concentration of ABA in the inhibited ovary, whereas the inhibition of IAA transport from the apical shoot by triiodobenzoic acid decreased the ovary content of ABA (to approximately one-half). Triiodobenzoic acid alone, however, was unable to stimulate ovary growth. Thus, in addition to removing IAA transport from the apical shoot, the accumulation of a promotive factor is also necessary to induce parthenocarpic growth in decapitated plants.

Isolation and transcript analysis of gibberellin 20-oxidase genes in pea and bean in relation to fruit development

PCR was used with degenerate primers based on conserved amino acid sequences in gibberellin (GA) 20-oxidases to isolate cDNA clones for these enzymes from young seeds of pea (Pisum sativum) and developing embryos of French bean (Phaseolus vulgaris). One GA 20-oxidase cDNA (Ps27-12) was obtained from pea and three (Pv 15-11, Pv73-1 and Pv85-26) from bean. Their identities were confirmed by demonstrating that fusion proteins expressed in Escherichia coli exhibited GA 20-oxidase activity, converting [14C]GA12 to [14C]GA9. The intermediates in this three-step reaction, GA15 and GA24, were also identified as products. The expression proteins from three of the clones (Ps27-12, Pv15-11 and Pv73-1) were also shown to convert GA53 to GA20, as effectively as they did GA12. On the basis of transcript levels measured by northern blot analysis, the pea GA 20-oxidase gene is most highly expressed in young leaves, fully expanded internodes, very young seeds (until 4 days after anthesis) and expanding pods (from 3 days after anthesis at least until day 6). Expression in pods from 3-day-old unpollinated ovaries is higher than in those from pollinated ovaries. Treatment of unpollinated ovaries with GA3 to induce parthenocarpic fruit-set severely reduced the amount of GA 20-oxidase mRNA, whereas treatment with 2,4-D, although inducing fruit-set, did not reduce the levels of these transcripts. Plant decapitation above an unpollinated ovary resulted in very high levels of GA 20-oxidase mRNA in the pod. The three GA 20-oxidase genes from French bean showed very different patterns of expression: Pv 15-1 was expressed in the roots, young leaves, and developing seeds, but most highly in immature cotyledons, while Pv73-1 has a similar expression pattern to Ps27-12, with transcripts found only in young seeds and young leaves, where it was particularly abundant. Transcripts corresponding to Pv85-26 were detected in developing seeds, and just traces in the young leaves. Southern blot analysis indicated that the bean GA 20-oxidases are each encoded by single-copy genes, whereas one more gene, homologous to Ps27-12, could also exist in pea.

Effect of the growth retardant LAB 198 999, an acylcyclohexanedione compound, on epicotyl elongation and metabolism of gibberellins A1 and A 20 in cowpea

The effect of LAB 198 999 [3,5-dioxo-4-butyryl-cyclohexane carboxylic acid ethyl ester; a new plant growth retardant which competitively inhibits 2-oxoglutarate-dependent gibberellin (GA) dioxygenases] on elongation and in-vivo [(3)H]GA1 and [(3)H]GA20 metabolism in cowpea (Vigna sinensis L. cv Blackeye pea No. 5) epicotyls has been investigated. Gibberellins and LAB 198 999 were injected into the epicotyl at 25-30 mm from the apex. In intact seedlings, epicotyl elongation was inhibited by LAB 198 999 (25 μg · epicotyl(-1)), and the inhibition was counteracted by GA1 but not by GA20. In contrast to intact seedlings, the inhibitor enhanced epicotyl elongation in de-bladed seedlings and expiants, in the latter case proportionally to the amount of inhibitor applied (up to 50 μg · epicotyl(-1)), but not in explants made from paclobutrazol-treated seedlings. The inhibitor also enhanced dramatically the elongation induced in paclobutrazol-treated expiants by GA1, but not by GA20. The promotive effect of LAB 198 999 was associated with increased contents of GA1 and GA8 in the growing region of the epicotyl, indicating a dependence on endogenous GAs. The effect of LAB 198999 decreased progressively with the age of the seedlings, probably as a consequence of a decreased level of GAs in the epicotyl. Gibberellin substrates and metabolites in the growing region of the epicotyl (upper 20 mm) were fractionated and identified tentatively by high-performance liquid chromatography and radiocounting using a homogeneous on-line radioactivity detector. The metabolism of [(3)H]GA1(t) (tentative) to [(3)H]GA8(t), and that of [(3)H]GA20(t) to [(3)H]GA1(t) and [(3)H]GA29(t) in the epicotyl were blocked by LAB 198 999, that of the former more efficiently than the latter. The results presented support the hypothesis that GA1 is the active GA controlling elongation of cowpea epicotyls. They also show that both the promotion of epicotyl elongation in explants and the enhancement of the effect of exogenous GA1 by LAB 198 999 are the result of the inhibitor blocking the in-vivo 2β-hydroxylation of GA1.

The source of gibberellins in the parthenocarpic development of ovaries on topped pea plants

The role and source of gibberellins (GAs) involved in the development of parthenocarpic fruits of Pisum sativum L. has been investigated. Gibberellins applied to the leaf adjacent to an emasculated ovary induced parthenocarpic fruit development on intact plants. The application of gibberellic acid (GA3) had to be done within 1 d of anthesis to be fully effective and the response was concentration-dependent. Gibberellin A1 and GA3 worked equally well and GA20 was less efficient. [(3)H]Gibberellin A1 applied to the leaf accumulated in the ovary and the accumulation was related to the growth response. These experiments show that GA applied to the leaf in high enough concentration is translocated to the ovary. Emasculated ovaries on decapitated pea plants develop without application of growth hormones. When [(3)H] GA1 was applied to the leaf adjacent to the ovary a substantial amount of radioactivity accumulated in the growing shoot of intact plants. In decapitated plants, however, this radioactivity was mainly found in the ovary. There it caused growth proportional to the accumulation of CA1. Application of LAB 150978, an inhibitor of GA biosynthesis, to decapitated plants inhibited parthenocarpic fruit development and this inhibition was counteracted by the application of GA3 (either to the fruit, or the leaf adjacent to the ovary, or through the lower cut end of the stem). All evidence taken together supports the view that parthenocarpic pea fruit development on topped plants depends on the import of gibberellins or their precursors, probably from the vegetative aerial parts of the plant.

Phytochrome Regulation of the Response to Exogenous Gibberellins by Epicotyls of Vigna sinensis

The elongation rate of cowpea epicotyls from whole cowpea (Vigna sinensis) seedlings and derooted and debladed plants (explants) increased after the main light period (8-hour duration) was extended with either continuous low intensity tungsten light or brief (5 minutes) far-red (FR) irradiation. This end-of-day FR effect was reversed by red (R) irradiation suggesting the involvement of phytochrome. These results confirm and extend those obtained previously with other species. Localization studies indicate the epicotyl to be the site of the photoreceptor. Treatment of cowpea seedlings with paclobutrazol, a gibberellin (GA) biosynthetic inhibitor, abolished the FR promoted epicotyl elongation, indicating a role for GAs in this process. There was no significant difference in epicotyl elongation rates of R plus FR irradiated explants treated with GA(1) or GA(20) and R irradiated explants treated with GA(1). However, R irradiation inhibited subsequent epicotyl elongation of GA(20) treated explants. Moreover, the observation, using GC-MS, that GA(1) and GA(20) are native GAs in cowpea lends support to the concept that phytochrome may control the conversion of endogenous GA(20) to GA(1) in cowpea.

Ribulose-1,5-bisphosphate carboxylase and fruit set or degeneration of unpollinated ovaries of Pisum sativum L

The polypeptide patterns obtained by sodium dodecylsulphate-polyacrylamide gel electrophoresis of undigested and autodigested extracts from pea (Pisum sativum L.) ovaries at the early stages of development or degeneration have been studied. Development of unpollinated ovaries was stimulated by application of different plant growth regulators (gibberellic acid, 2,4-dichlorophenoxyacetic acid, and N(6)-benzyladenine) or by plant topping. Polypeptide bands of similar mobility to ribulose-1,5-bisphosphate carboxylase (RuBPCase) subunits (16 and 55 kDa) could be detected in all types of autodigested extracts from stimulated ovaries. However these bands were absent in electrophoretic patterns of autodigested extracts from unstimulated ovaries after 3 d post anthesis and in patterns of autodigested mixtures of these extracts with either those from stimulated ovaries or those from unstimulated ovaries before day 3. These observations indicate that a proteolytic activity which promotes the hydrolysis of RuBPCase appears in unstimulated ovaries about 3 d after anthesis. This event coincides with the loss of the capacity of unpollinated ovaries to develop in response to gibberellic acid and with the degeneration of the ovary wall.

Uptake and subcellular compartmentation of gibberellin a(1) applied to leaves of barley and cowpea

The uptake and subcellular accumulation of gibberellin A(1) (GA(1)) by leaves and protoplasts of barley (cv. Numar) and cowpea (cv. Blackeye pea No. 5) were investigated.Uptake of GA(1) by cowpea leaves is optimal at pH 5.8 and occurs by a saturable, probably carrier-mediated process having a half-maximal velocity at 10 to 20 micromolar. Uptake by both barley and cowpea leaves is inhibited by low temperature (+4 C) and the metabolic inhibitors 2,4-dinitrophenol and azide and is stimulated by ATP. Mesophyll protoplasts isolated from leaves fed radioactive GA(1) retain 20 to 80% of the radioactivity incorporated by excised leaves.The subcellular localization of the [(3)H]GA was determined by lysing protoplasts and separating subcellular organelles by density gradient centrifugation. Less than 5% of the incorporated [(3)H]GA was found associated with chloroplasts, mitochondria, nuclei, or other organelles or membranes with densities in sucrose gradients greater than 1.15 grams per cubic centimeter. Fifty to 100% of the [(3)H]GA was found in vacuoles. Isolated vacuoles were judged to be free of contamination by cytoplasm using phosphoenolpyruvate carboxylase as a marker enzyme. Osmotic breakage of vacuoles or protoplasts released > 95% of the [(3)H]GA, suggesting that GA is associated with the vacuolar sap rather than with the tonoplast membrane.

Fruit-set of unpollinated ovaries of Pisum sativum L. : Influence of plant-growth regulators

The development of parthenocarpic fruits of Pisum sativum L. cv. Alaska was induced by the application of different plant-growth regulators in aqueous solution to the emasculated ovaries in untopped plants. At least one compound in each of the groups of auxins (2,4-dichlorophenoxyacetic acid), cytokinins (benzyladenine), and gibberellins (gibberellic acid) was found active. Gibberellic acid (GA3), however, was the only substance which produced pods similar to those of fruits with seeds. The length of the pods obtained by GA3 was a linear function of the logarithm of the concentration of GA3 in the solution. The effect of GA3 (at a concentration which produced 50% of the maximum pod length) was enhanced by a simultaneous application of 2,4-dichlorophenoxyacetic acid. Abscisic acid (ABA) counteracted the effect of GA3 and of topping. The results suggest that gibberellins and ABA may exert a major regulatory control in natural fruit-set. Peas can be used for the assay of fructigenic activity and is an advantageous material for the study of the mode of action of gibberellins on fruit-set.

Fruit-set of unpollinated ovaries of Pisum sativum L. : Influence of vegetative parts

The influence of removing the apical shoot and different leaves above and below the flower on the fruit-set of unpollinated pea ovaries (Pisum sativum L. cv. Alaska) has been studied. Unpollinated ovaries were induced to set and develop either by topping or by removing certain developing leaves of the shoot. Topping had a maximum effect when carried out before or on the day of anthesis, and up to four consecutive ovaries were induced to set in the same plant. The inhibition of fruit-set was due to the developing leaves and not to the apex. The third leaf above the first flower, which had a simultaneous development to the ovary, had the stronger inhibitory effect on parthenocarpic fruit-set. The application of different plant-growth regulators (indoleacetic acid, naphthylacetic acid, 2,4-dichlorophenoxyacetic acid, gibberellic acid, benzyladenine and abscisic acid) did not mimic the negative effect of the shoot.