Antagonistic regulation of the gibberellic acid response during stem growth in rice.

The size of plants is largely determined by growth of the stem. Stem elongation is stimulated by gibberellic acid1-3. Here we show that internode stem elongation in rice is regulated antagonistically by an 'accelerator' and a 'decelerator' in concert with gibberellic acid. Expression of a gene we name ACCELERATOR OF INTERNODE ELONGATION 1 (ACE1), which encodes a protein of unknown function, confers cells of the intercalary meristematic region with the competence for cell division, leading to internode elongation in the presence of gibberellic acid. By contrast, upregulation of DECELERATOR OF INTERNODE ELONGATION 1 (DEC1), which encodes a zinc-finger transcription factor, suppresses internode elongation, whereas downregulation of DEC1 allows internode elongation. We also show that the mechanism of internode elongation that is mediated by ACE1 and DEC1 is conserved in the Gramineae family. Furthermore, an analysis of genetic diversity suggests that mutations in ACE1 and DEC1 have historically contributed to the selection of shorter plants in domesticated populations of rice to increase their resistance to lodging, and of taller plants in wild species of rice for adaptation to growth in deep water. Our identification of these antagonistic regulatory factors enhances our understanding of the gibberellic acid response as an additional mechanism that regulates internode elongation and environmental fitness, beyond biosynthesis and gibberellic acid signal transduction.

FPX is a Novel Chemical Inducer that Promotes Callus Formation and Shoot Regeneration in Plants.

Auxin and cytokinin control callus formation from developed plant organs as well as shoot regeneration from callus. Dedifferentiation and regeneration of plant cells by auxin and cytokinin stimulation are considered to be caused by the reprogramming of callus cells, but this hypothesis is still argued to this day. Although an elucidation of the regulatory mechanisms of callus formation and shoot regeneration has helped advance plant biotechnology research, many plant species are intractable to transformation because of difficulties with callus formation. In this study, we identified fipexide (FPX) as a useful regulatory compound through a chemical biology-based screening. FPX was shown to act as a chemical inducer in callus formation, shoot regeneration and Agrobacterium infection. With regards to morphology, the cellular organization of FPX-induced calli differed from those produced under auxin/cytokinin conditions. Microarray analysis revealed that the expression of approximately 971 genes was up-regulated 2-fold after a 2 d FPX treatment compared with non-treated plants. Among these 971 genes, 598 genes were also induced by auxin/cytokinin, whereas 373 genes were specifically expressed upon FPX treatment only. FPX can promote callus formations in rice, poplar, soybean, tomato and cucumber, and thus can be considered a useful tool for revealing the mechanisms of plant development and for use in plant transformation technologies.

Arabidopsis galactinol synthase AtGolS2 improves drought tolerance in the monocot model Brachypodium distachyon.

Brachypodium distachyon (purple false brome) is a herbaceous species belonging to the grass subfamily Pooideae, which also includes major crops like wheat, barley, oat and rye. The species has been established as experimental model organism for understanding and improving cereal crops and temperate grasses. The complete genome of Bd21, the community standard line of B. distachyon, has been sequenced and protocols for Agrobacterium-mediated transformation have been published. Further improvements to the experimental platform including better evaluation systems for transgenic plants are still needed. Here we describe the growth conditions for Bd21 plants yielding highly responsive immature embryos that can generate embryogenic calli for transformation. A prolonged 20-h photoperiod produced seeds with superior immature embryos. In addition, osmotic treatment of embryogenic calli enhanced the efficiency of transfection by particle bombardment. We generated transgenic plants expressing Arabidopsis thaliana galactinol synthase 2 (AtGolS2) in these experiments. AtGolS2-expressing transgenics displayed significantly improved drought tolerance, increasing with increased expression of AtGolS2. These results demonstrate that AtGolS2 can confer drought tolerance to monocots and confirm that Brachypodium is a useful model to further explore ways to understand and improve major monocot crop species.

FOX-superroots of Lotus corniculatus, overexpressing Arabidopsis full-length cDNA, show stable variations in morphological traits.

Using the full-length cDNA overexpressor (FOX) gene-hunting system, we have generated 130 Arabidopsis FOX-superroot lines in bird's-foot trefoil (Lotus corniculatus) for the systematic functional analysis of genes expressed in roots and for the selection of induced mutants with interesting root growth characteristics. We used the Arabidopsis-FOX Agrobacterium library (constructed by ligating pBIG2113SF) for the Agrobacterium-mediated transformation of superroots (SR) and the subsequent selection of gain-of-function mutants with ectopically expressed Arabidopsis genes. The original superroot culture of L. corniculatus is a unique host system displaying fast root growth in vitro, allowing continuous root cloning, direct somatic embryogenesis and mass regeneration of plants under entirely hormone-free culture conditions. Several of the Arabidopsis FOX-superroot lines show interesting deviations from normal growth and morphology of roots from SR-plants, such as differences in pigmentation, growth rate, length or diameter. Some of these mutations are of potential agricultural interest. Genomic PCR analysis revealed that 100 (76.9%) out of the 130 transgenic lines showed the amplification of single fragments. Sequence analysis of the PCR fragments from these 100 lines identified full-length cDNA in 74 of them. Forty-three out of 74 full-length cDNA carried known genes. The Arabidopsis FOX-superroot lines of L. corniculatus, produced in this study, expand the FOX hunting system and provide a new tool for the genetic analysis and control of root growth in a leguminous forage plant.