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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.

Brassinosteroids (BRs) are required for various aspects of plant growth and development, but also participate in stress responses. The hormones convey their activity through transcriptional regulation and posttranslational modification of transcription factors and one class are basic helix-loop-helix (bHLH) proteins of the BR Enhanced Expression (BEE) subfamily, which in Arabidopsis thaliana include BEE1-3 and CESTA (CES). CES and the BEEs promote the expression of different BR-responsive genes, including genes encoding gibberellin (GA) biosynthetic and catabolizing enzymes, as well as cold-responsive genes. Interestingly, in terms of an application, CES could promote both fruit growth and cold stress tolerance when over-expressed in A. thaliana and here it was investigated, if this function is conserved in the fruit crop Solanum lycopersicum (cultivated tomato). Based on amino acid sequence similarity and the presence of regulatory motifs, a CES orthologue of S. lycopersicum , SlCES, was identified and the effects of its over-expression were analysed in tomato. This showed that SlCES, like AtCES, was re-localized to nuclear bodies in response to BR signaling activation and that it effected GA homeostasis, with related phenotypes, when over-expressed. In addition, over-expression lines showed an increased chilling tolerance and had altered fruit characteristics. The possibilities and potential limitations of a gain of SlCES function as a breeding strategy for tomato are discussed.

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. Touch can lead to a reduction in plant growth and a delay in flowering time. Experiments with wild-type Arabidopsis plants, and mutants impaired in gibberellin signalling and breakdown, suggest that touch-induced changes in plant morphology depend on gibberellin catabolism.

Plants are able to sense a rise in temperature of several degrees, and appropriately adapt their metabolic and growth processes. To this end, plants produce various signalling molecules that act throughout the plant body. Here, we report that root-derived GA 12 , a precursor of the bioactive gibberellins, mediates thermo-responsive shoot growth in Arabidopsis . Our data suggest that root-to-shoot translocation of GA 12 enables a flexible growth response to ambient temperature changes. Gibberellin precursor GA 12 moves from root to shoot and participates in hypocotyl elongation at warmer temperatures.

The consequences of altered abscisic acid (ABA) sensitivity in gray poplar (Populus x canescens [Ait.] Sm.) development were examined by ectopic expression of the Arabidopsis (Arabidopsis thaliana) mutant abi1 (for abscisic acid insensitive1) gene. The expression resulted in an ABA-insensitive phenotype revealed by a strong tendency of abi1 poplars to wilt, impaired responsiveness of their stomata to ABA, and an ABA-resistant bud outgrowth. These plants therefore required cultivation under very humid conditions to prevent drought stress symptoms. Morphological alterations became evident when comparing abi1 poplars with poplars expressing Arabidopsis nonmutant ABI1 or wild-type plants. abi1 poplars showed increased stomatal size, enhanced shoot growth, and retarded leaf and root development. The increased stomatal size and its reversion to the size of wild-type plants by exogenous ABA indicate a role for ABA in regulating stomatal development. Enhanced shoot growth and retarded leaf and root development support the hypothesis that ABA acts independently from drought stress as a negative regulator of growth in shoots and as a positive regulator of growth in leaves and roots. In shoots, we observed an interaction of ABA with ethylene: abi1 poplars exhibited elevated ethylene production, and the ethylene perception inhibitor Ag⁺ antagonized the enhanced shoot growth. Thus, we provide evidence that ABA acts as negative regulator of shoot growth in nonstressed poplars by restricting ethylene production. Furthermore, we show that ABA has a role in regulating shoot branching by inhibiting lateral bud outgrowth.

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 GA₄ 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 GA₄. 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 GA₄ and increased levels of physiological inactive GA₁₇ and GA₂₅ and unexpected GA₃₄ 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 GA₄ and an increase in the corresponding inactivation product GA₃₄ 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.

Brassinosteroids (BRs) are plant steroid hormones with structural similarity to mammalian sex steroids and ecdysteroids from insects. The BRs are synthesized from sterols and are essential regulators of cell division, cell elongation and cell differentiation. In this work we show that voriconazole, an antifungal therapeutic drug used in human and veterinary medicine, severely impairs plant growth by inhibiting sterol-14α-demethylation and thereby interfering with BR production. The plant growth regulatory properties of voriconazole and related triazoles were identified in a screen for compounds with the ability to alter BR homeostasis. Voriconazole suppressed growth of the model plant Arabidopsis thaliana and of a wide range of both monocotyledonous and dicotyledonous plants. We uncover that voriconazole toxicity in plants is a result of a deficiency in BRs that stems from an inhibition of the cytochrome P450 CYP51, which catalyzes a step of BR-dependent sterol biosynthesis. Interestingly, we found that the woodland strawberry Fragaria vesca, a member of the Rosaceae, is naturally voriconazole resistant and that this resistance is conferred by the specific CYP51 variant of F. vesca. The potential of voriconazole as a novel tool for plant research is discussed.

Plant growth and development are highly regulated processes that are coordinated by hormones including the brassinosteroids (BRs), a group of steroids with structural similarity to steroid hormones of mammals. Although it is well understood how BRs are produced and how their signals are transduced, BR targets, which directly confer the hormone’s growth-promoting effects, have remained largely elusive. Here, we show that BRs regulate the biosynthesis of gibberellins (GAs), another class of growth-promoting hormones, in Arabidopsis thaliana. We reveal that Arabidopsis mutants deficient in BR signaling are severely impaired in the production of bioactive GA, which is correlated with defective GA biosynthetic gene expression. Expression of the key GA biosynthesis gene GA20ox1 in the BR signaling mutant bri1-301 rescues many of its developmental defects. We provide evidence that supports a model in which the BR-regulated transcription factor BES1 binds to a regulatory element in promoters of GA biosynthesis genes in a BR-induced manner to control their expression. In summary, our study underscores a role of BRs as master regulators of GA biosynthesis and shows that this function is of major relevance for the growth and development of vascular plants.