Role of gibberellic acid in tomato defence against potato purple top phytoplasma infection

Infection of tomato by potato purple top (PPT) phytoplasma causes disruption of gibberellin (GA) homeostasis in the plant host. Such pathologically‐induced GA deficiency can be partially reversed by exogenous application of GA. This study was designed to explore the role of GA in tomato defence response against phytoplasmal disease, and to determine whether pretreatment with GA would protect healthy tomato seedlings from subsequent phytoplasmal infection and disease development. Our results revealed that, following exogenous GA application and subsequent PPT phytoplasma graft inoculation, there was an apparently coordinated down‐regulation of the gene encoding a key GA signalling component and growth repressor known as DELLA protein (GAI) and up‐regulation of genes involved in salicylic acid (SA) synthesis (ICS1), signalling (NIM1) and downstream defence responses (PRP‐1). Our results also indicated that differential regulation of the above genes was correlated with an increase in activities of defence‐related enzymes β‐1,3‐glucanase and chitinase. The data presented in this communication provide evidence to suggest that GA may act via DELLA and SA signalling pathways to modulate host defence in response to PPT phytoplasma infection. Although the GA pretreatment‐induced defence was not sufficient to prevent a systemic infection, it reduced phytoplasma titre and significantly attenuated disease symptoms. While the actual molecular mechanism underlying the GA‐induced plant defence remains elusive, findings from the current study open new opportunities for in‐depth studies of the functional role of the GA signalling network during defence response against phytoplasmal infection.     

Down regulation of StGA3ox genes in potato results in altered GA content and affect plant and tuber growth characteristics

GA biosynthesis and catabolism has been shown to play an important role in regulating tuberization in potato. Active GAs are inactivated in the stolon tips shortly after induction to tuberization. Overexpression of a GA inactivation gene results in an earlier tuberization phenotype, while reducing expression of the same gene results in delayed tuberization. In addition, overexpression of genes involved in GA biosynthesis results in delayed tuberization, while decreased expression of those genes results in earlied tuberization. The final step in GA biosynthesis is catalysed by StGA3ox1 and StGA3ox2 activity, that convert inactive forms of GA into active GA₁ and GA₄. In this study we cloned StGA3ox2 gene in an RNAi construct and used this construct to transform potato plants. The StGA3ox2 silenced plants were smaller and had shorter internodes. In addition, we assayed the concentrations of various GAs in the transgenic plants and showed an altered GA content. No difference was observed on the time point of tuber initiation. However, the transgenic clones had increased number of tubers with the same yield, resulting in smaller average tuber weight. In addition, we cloned the promoter of StGA3ox2 to direct expression of the GUS reporter gene to visualize the sites of GA biosynthesis in the potato plant. Finally, we discuss how changes of several GA levels can have an impact on shoot, stolon and tuber development, as well as the possible mechanisms that mediate feed-forward and feed-back regulation loops in the GA biosynthetic pathway in potato.     

2,4‐D‐induced parthenocarpy in pear is mediated by enhancement of GA4 biosynthesis

Parthenocarpy, the productions of seedless fruit without pollination or fertilization, is a potentially desirable trait in many commercially grown fruits, especially in pear, which is self‐incompatible. Phytohormones play important roles in fruit set, a process crucial for parthenocarpy. In this study, 2,4‐dichlorophenoxyacetic acid (2,4‐D), an artificially synthesized plant growth regulator with functions similar to auxin, was found to induce parthenocarpy in pear. Histological observations revealed that 2,4‐D promoted cell division and expansion, which increased cortex thickness, but the effect was weakened by paclobutrazol (PAC), a gibberellin (GA) biosynthesis inhibitor. Phenotypic differences in pear may therefore be due to different GA contents. Hormone testing indicated that 2,4‐D mainly induced the production of bioactive GA₄, rather than GA_3. Three key oxidase genes function in the GA biosynthetic pathway: GA20ox, GA3ox and GA2ox. In a pear group treated with only 2,4‐D, PbGA20ox2‐like and PbGA3ox‐1 were significantly upregulated. When treated with 2,4‐D supplemented with PAC, however, expression levels of these genes were significantly downregulated. Additionally, PbGA2ox1‐like and PbGA2ox2‐like expression levels were significantly downregulated in pear treated with either 2,4‐D only or 2,4‐D supplemented with PAC. We thus hypothesize that 2,4‐D can induce parthenocarpy by enhancing GA₄ biosynthesis.     

Comparison of the role of gibberellins and ethylene in response to submergence of two lowland rice cultivars, Senia and Bomba

We examined the gibberellin (GA) and ethylene regulation of submergence-induced elongation in seedlings of the submergence-tolerant lowland rice (Oryza sativa L.) cvs Senia and Bomba. Elongation was enhanced after germination to facilitate water escape and reach air. We found that submergence-induced elongation depends on GA because it was counteracted by paclobutrazol (an inhibitor of GA biosynthesis), an effect that was negated by GA₃. Moreover, in the cv Senia, submergence increased the content of active GA₁ and its immediate precursors (GA₅₃, GA₁₉ and GA₂₀) by enhancing expression of several GA biosynthesis genes (OsGA20ox1 and -2, and OsGA3ox2), but not by decreasing expression of several OsGA2ox (GA inactivating genes). Senia seedlings, in contrast to Bomba seedlings, did not elongate in response to ethylene or 1-aminocyclopropane-1-carboxylic-acid (ACC; an ethylene precursor) application, and submergence-induced elongation was not reduced in the presence of 1-methylcyclopropene (1-MCP; an ethylene perception inhibitor). Ethylene emanation was similar in Senia seedlings grown in air and in submerged-grown seedlings following de-submergence, while it increased in Bomba. The expression of ethylene biosynthesis genes (OsACS1, -2 and -3, and OsACO1) was not affected in Senia, but expression of OsACS5 was rapidly enhanced in Bomba upon submergence. Our results support the conclusion that submergence elongation enhancement of lowland rice is due to alteration of GA metabolism leading to an increase in active GA (GA₁) content. Interestingly, in the cv Senia, in contrast to cv Bomba, this was triggered through an ethylene-independent mechanism.     

Expression of novel rice gibberellin 2-oxidase gene is under homeostatic regulation by biologically active gibberellins

We have cloned two genes for gibberellin (GA) 2-oxidase from rice (Oryza sativa L.). Expression of OsGA2ox2 was not observed. The other gene, OsGA2ox3, was expressed in every tissue examined and was enhanced by the application of biologically active GA. Recombinant OsGA2ox3 protein catalyzed the metabolism of GA₁ to GA₈ and GA₂₀ to GA₂₉-catabolite. These results indicate that OsGA2ox3 is involved in the homeostatic regulation of the endogenous level of biologically active GA in rice. Electronic Publication     

The Arabidopsis Myb genes MYR1 and MYR2 are redundant negative regulators of flowering time under decreased light intensity

Changes in the duration, quality and intensity of light affect flowering time. Compared with the effects of light duration and quality, less is known about the effects of light intensity on flowering. Here we describe two paralogous single Myb domain genes, MYB‐RELATED PROTEIN 1 (MYR1) and MYB‐RELATED PROTEIN 2 (MYR2), and their roles as repressors of responses to decreased light intensity in Arabidopsis. Homozygous myr1 myr2 double mutants flowered early under low light intensities. Additionally, myr1 myr2 mutants exhibited increases in petiole length, leaf angle and apical dominance. Genetic analyses involving mutants in the long‐day, gibberellin (GA) and phyB flowering pathways indicated that all aspects of the myr1 myr2 phenotype required GA biosynthesis. The early‐flowering phenotype of myr1 myr2 also required FLOWERING LOCUS T, and myr1 myr2 mutants showed an epistatic interaction with the phyB‐9 mutant. Over‐expression of MYR1 or MYR2 produced GA‐deficiency symptoms that were rescued by application of gibberellic acid (GA₃). Loss of MYR1 and MYR2 function was associated with a twofold increase in GA20ox2 expression and a 30% increase in GA₄ levels, while over‐expression of MYR2 led to a threefold decrease in GA20ox2 expression and a 50% decrease in GA₄ levels. Considered together, these results suggest that the ability of MYR1 and MYR2 to repress flowering and organ elongation is at least partly due to their negative effect on levels of bioactive GA.     

Cytokinin-Induced Parthenocarpic Fruit Development in Tomato Is Partly Dependent on Enhanced Gibberellin and Auxin Biosynthesis

Fruit set of plants largely depends on the biosynthesis and crosstalk of phytohormones. To date the role of cytokinins (CKs) in the fruit development is less understood. Here, we showed that parthenocarpic fruit could be induced by 1-(2-chloro-4-pyridyl)-3-phenylurea (CPPU, an active CK) in tomato ( Solanum lycopersicum cv. Micro-Tom). The fresh weight of CPPU-induced parthenocarpic fruits was comparable with that induced by GA₃. Importantly, CPPU-induced parthenocarpy was found to be compromised by simultaneous application of paclobutrazol (a GA biosynthesis inhibitor), and this effect could be restored by exogenous GA₃. Like pollination, CPPU-induced fruit showed enhanced accumulation of GA₁₊₃ and indole-3-acetic (IAA), which were accompanied by elevated expression of GA biosynthesis genes like SlGPS, SlGA20ox1, SlGA20ox2 and SlGA3ox1, and IAA biosynthesis gene ToFZY. Elevated GAs level in CPPU-induced fruits was also associated with down-regulation of GA inactivation genes, namely SlGA2ox1,2,3,4,5 in comparison with untreated control. These results suggested that CKs may induce parthenocarpy in tomato partially through modulation of GA and IAA metabolisms.     

Function and Expression Analysis of Gibberellin Oxidases in Apple

Three cDNAs, encoding gibberellin (GA) 20-oxidase (MdGA20ox1, identical to AB037114), 3-oxidase (MdGA3ox1), and 2-oxidase (MdGA2ox1), were isolated from apple cv. Fuji (Malus x domestica). Southern blot analysis indicated that each of these genes belongs to a gene family. Standard enzyme assays show that the MdGA20ox1-MBP fusion protein can sequentially oxidize three times at C-20 position of GA₁₂ and GA₅₃ and generate GA₉ and GA₂₀; the MdGA3ox1-MBP fusion protein converts GA₂₀ and GA₉ to GA₄ and GA₁, and the MdGA2ox1-MBP fusion protein converts GA₄ and GA₁ to GA₃₄ and GA₈, respectively. In addition, we confirmed that MdGA20ox1 is strongly expressed in immature seeds and scarcely detected in other tissues, whereas MdGA3ox1 and MdGA2ox1 are mainly expressed in flowers. Therefore, all the three cDNAs are localized in reproductive tissues. Functional and expression analysis of the three GA oxidases would provide fundamental molecular information to analyze GA metabolic regulation in apple.     

Developmental and Hormonal Regulation of Gibberellin Biosynthesis and Catabolism in Pea Fruit

In pea (Pisum sativum), normal fruit growth requires the presence of the seeds. The coordination of growth between the seed and ovary tissues involves phytohormones; however, the specific mechanisms remain speculative. This study further explores the roles of the gibberellin (GA) biosynthesis and catabolism genes during pollination and fruit development and in seed and auxin regulation of pericarp growth. Pollination and fertilization events not only increase pericarp PsGA3ox1 message levels (codes for GA 3-oxidase that converts GA₂₀ to bioactive GA₁) but also reduce pericarp PsGA2ox1 mRNA levels (codes for GA 2-oxidase that mainly catabolizes GA₂₀ to GA₂₉), suggesting a concerted regulation to increase levels of bioactive GA₁ following these events. 4-Chloroindole-3-acetic acid (4-Cl-IAA) was found to mimic the seeds in the stimulation of PsGA3ox1 and the repression of PsGA2ox1 mRNA levels as well as the stimulation of PsGA2ox2 mRNA levels (codes for GA 2-oxidase that mainly catabolizes GA₁ to GA₈) in pericarp at 2 to 3 d after anthesis, while the other endogenous pea auxin, IAA, did not. This GA gene expression profile suggests that both seeds and 4-Cl-IAA can stimulate the production, as well as modulate the half-life, of bioactive GA₁, leading to initial fruit set and subsequent growth and development of the ovary. Consistent with these gene expression profiles, deseeded pericarps converted [¹⁴C]GA₁₂ to [¹⁴C]GA₁ only if treated with 4-Cl-IAA. These data further support the hypothesis that 4-Cl-IAA produced in the seeds is transported to the pericarp, where it differentially regulates the expression of pericarp GA biosynthesis and catabolism genes to modulate the level of bioactive GA₁ required for initial fruit set and growth.     

Ethylene-induced stem growth of deepwater rice is correlated with expression of gibberellin- and abscisic acid-biosynthetic genes

Ethylene decreases the content of endogenous abscisic acid (ABA) and increases the level of bioactive gibberellin A₁ (GA₁) in the submerged internodes of deepwater rice. During partial submergence, internodes of deepwater rice undergo rapid elongation as a result of ethylene accumulation in the internodal lacunae. In anin vitro experiment using stem sections from deepwater rice, treatment with 5 μL L^-1 ethylene promoted stem growth by up to 3.2-foId times over air treatment. Expression patterns were analyzed for genes that encode GA- and ABA-biosynthesis enzymes to determine any possible molecular basis for the changes observed in GA₁ and ABA contents as a result of ethylene action. Expression of theOsGA20ox2 andOsGA20ox4 genes, which encode GA 20-oxidase, and of theOsGA3ox2 gene, which encodes the enzyme that converts GA₂₀ to CA₁, was up-regulated, whereas that of three ABA-biosynthetic genes —OsNCED1, OsNCED2, andOsNCEDS-was down-regulated in the presence of ethylene. These results indicate that GA and ABA contribute equally to the submergence-or ethylene-induced stem elongation of deepwater rice via the coordinated and opposite regulation of biosynthesis.