Asexual and sexual reproduction occur jointly in many angiosperms. Stolons (elongated stems) are used for asexual reproduction in the crop species potato (Solanum tuberosum) and strawberry (Fragaria spp), where they produce tubers and clonal plants, respectively. In strawberry, stolon production is essential for vegetative propagation at the expense of fruit yield, but the underlying molecular mechanisms are unknown. Here, we show that the stolon deficiency trait of the runnerless (r) natural mutant in woodland diploid strawberry (Fragaria vesca) is due to a deletion in the active site of a gibberellin 20-oxidase (GA20ox) gene, which is expressed primarily in the axillary meristem dome and primordia and in developing stolons. This mutation, which is found in all r mutants, goes back more than three centuries. When FveGA20ox4 is mutated, axillary meristems remain dormant or produce secondary shoots terminated by inflorescences, thus increasing the number of inflorescences in the plant. The application of bioactive gibberellin (GA) restored the runnering phenotype in the r mutant, indicating that GA biosynthesis in the axillary meristem is essential for inducing stolon differentiation. The possibility of regulating the runnering-flowering decision in strawberry via FveGA20ox4 provides a path for improving productivity in strawberry by controlling the trade-off between sexual reproduction and vegetative propagation.

Touch can lead to a reduction in plant growth and a delay in flowering time. Touch-induced changes in plant morphology, termed thigmomorphogenesis, have been shown to depend on the phytohormone jasmonate(1). However, touch-induced phenotypes are also reminiscent of plants deficient in the phytohormone gibberellin(2). Here we assess the effect of touch on wild-type Arabidopsis plants and mutants deficient in gibberellin signalling. We show that touch leads to stunted growth and delayed flowering in wild-type plants, as expected. These touch-induced changes in morphology are accompanied by a reduction in gibberellin levels, and can be reversed through the application of a bioactive form of gibberellin. We further show that touch induces the expression of AtGA2ox7, which encodes an enzyme involved in gibberellin catabolism. Arabidopsis ga2ox7 loss-of-function mutants do not respond to touch, suggesting that this gene is a key regulator of thigmomorphogenesis. We conclude that touch-induced changes in Arabidopsis morphology depend on gibberellin catabolism. Given that AtGA2ox7 helps to confer resistance to salt stress, and that touch can increase plant resistance to pathogens, we suggest that gibberellin catabolism could be targeted to improve plant resistance to abiotic and biotic stress.

A gibberellin (GA) biosynthetic pathway was discovered operating in root tips of 7-d-old pumpkin (Cucurbita maxima) seedlings. Stepwise analysis of GA metabolism in cell-free systems revealed the conversion of GA 12 -aldehyde to bioactive GA 4 and inactive GA 34 . Highest levels of endogenous GA 4 and GA 34 were found in hypocotyls and root tips of 3-d-old seedlings. cDNA molecules encoding two GA oxidases, CmGA20ox3 and CmGA3ox3, were isolated from root tips of 7-d-old LAB150978-treated seedlings. Recombinant CmGA20ox3 fusion protein converted GA 12 to GA 9 , GA 24 to GA 9 , GA 14 to GA 4 , and, less efficiently, GA 53 to GA 20 , and recombinant CmGA3ox3 protein oxidized GA 9 to GA 4 . Transcript profiles were determined for four GA oxidase genes from pumpkin revealing relatively high transcript levels for CmGA7ox in shoot tips and cotyledons, for CmGA20ox3 in shoot tips and hypocotyls, and for CmGA3ox3 in hypocotyls and roots of 3-d-old seedlings. Transcripts of CmGA2ox1 were mainly found in roots of 7-d-old seedlings. In roots of 7-d-old seedlings, transcripts of CmGA7ox, CmGA20ox3, and CmGA3ox3 were localized in the cap and the rhizodermis by in situ hybridization. We conclude that hypocotyls and root tips are important sites of GA biosynthesis in the developing pumpkin seedling.Gibberellins (GAs) are signaling molecules that regulate and integrate developmental processes during the entire life cycle of higher plants, including shoot elongation and root development (Richards et al., 2001;Olszewski et al., 2002;Fu and Harberd, 2003;Reid et al., 2004;Sun, 2004).GA biosynthetic pathways are of considerable complexity (for review, see Hedden and Kamiya, 1997;Sponsel and Hedden, 2004). They are divided into nonhydroxylated, 3b-hydroxylated, and 13-hydroxylated pathways (Fig. 1, A -C, respectively;Graebe, 1987). A principal pathway to GA plant hormones can be drawn from GA 12 -aldehyde. First, 7-oxidation of GA 12 -aldehyde results in the formation of GA 12 . GA 53 is often formed by 13-hydroxylation of GA 12 . The following three oxidation steps at carbon (C)-20 are catalyzed by one enzyme, the GA 20-oxidase, which, in general, leads to the formation of a C 19 -GA (e.g. GA 9 and GA 20 ; Fig. 1). Alternatively, C-20 oxidation leads to the formation of a carboxylic acid (e.g. GA 25 and GA 17 ). The resulting C 20 -GAs usually are minor products of GA 20-oxidase activity. Finally, 3-oxidation produces GA plant hormones (GA 4 and GA 1 ), which are subsequently inactivated by 2-oxidation (GA 34 and GA 8 ; Fig. 1). In many plant species, 7-oxidation and 13-hydroxylation are catalyzed by NADPH-dependent cytochrome P450 mono-oxygenases. In Arabidopsis (Arabidopsis thaliana), a multifunctional ent-kaurenoic acid oxidase with 7-oxidation catalytic properties has been characterized (Helliwell et al., 2001). In pumpkin (Cucurbita maxima), however, 7-oxidation is catalyzed by an additional multifunctional GA 7-oxidase that belongs to the class of 2-oxoglutarate-dependent dioxygenases (Lange, 1997). Recently, recombinant pumpk...

Gibberellins (GAs) form a large family of plant growth substances with distinct functions during the whole life cycle of higher plants. The rate of GA biosynthesis and catabolism determines how the GA hormone pool occurs in plants in a tissue and developmentally regulated manner. With the availability of genes coding for GA biosynthetic enzymes, our understanding has improved dramatically of how GA plant hormones regulate and integrate a wide range of growth and developmental processes. This review focuses on two plant systems, pumpkin and Arabidopsis, which have added significantly to our understanding of GA biosynthesis and its regulation. In addition, we present models for regulation of GA biosynthesis in transgenic plants, and discuss their suitability for altering plant growth and development.

Immature pumpkin (Cucurbita maxima) seeds contain gibberellin (GA) oxidases with unique catalytic properties resulting in GAs of unknown function for plant growth and development. Overexpression of pumpkin GA 7-oxidase (CmGA7ox) in Arabidopsis (Arabidopsis thaliana) resulted in seedlings with elongated roots, taller plants that flower earlier with only a little increase in bioactive GA 4 levels compared to control plants. In the same way, overexpression of the pumpkin GA 3-oxidase1 (CmGA3ox1) resulted in a GA overdose phenotype with increased levels of endogenous GA 4 . This indicates that, in Arabidopsis, 7-oxidation and 3-oxidation are rate-limiting steps in GA plant hormone biosynthesis that control plant development. With an opposite effect, overexpression of pumpkin seed-specific GA 20-oxidase1 (CmGA20ox1) in Arabidopsis resulted in dwarfed plants that flower late with reduced levels of GA 4 and increased levels of physiological inactive GA 17 and GA 25 and unexpected GA 34 levels. Severe dwarfed plants were obtained by overexpression of the pumpkin GA 2-oxidase1 (CmGA2ox1) in Arabidopsis. This dramatic change in phenotype was accompanied by a considerable decrease in the levels of bioactive GA 4 and an increase in the corresponding inactivation product GA 34 in comparison to control plants. In this study, we demonstrate the potential of four pumpkin GA oxidase-encoding genes to modulate the GA plant hormone pool and alter plant stature and development.