The enzymes gibberellin (GA) 20-oxidase and 3-oxidase are major sites of regulation in GA biosynthesis. We have characterised one member of each of the gene families encoding these enzymes that are highly expressed in elongating stems and in developing and germinating grains of wheat and are therefore likely to have prominent developmental roles in these tissues. We mapped the three homoeologues of the GA 20-oxidase gene TaGA20ox1 to chromosomes 5BL, 5DL and 4AL. TaGA20ox1 is expressed mainly in the nodes and ears of the elongating stem, and also in developing and germinating embryos. Expression in the nodes, ears and germinating embryos is predominantly from the A and D genomes. Each homoeologous cDNA encodes a functional enzyme that catalyses the multi-step conversions of GA12-GA9, and GA53-GA20. Time course and enzyme kinetic studies indicate that the initial oxidation steps from GA12 and GA53 to the free alcohol forms of GA15 and GA44, respectively, occur rapidly but that subsequent steps occur more slowly. The intermediate GA19 has an especially low affinity for the enzyme, consistent with its accumulation in wheat tissues. The three homoeologous cDNAs for the 3-oxidase gene TaGA3ox2 encode functional enzymes, one of which was shown to possess low levels of 2beta-hydroxylase, 2,3-desaturase, 2,3-epoxidase and even 13-hydroxylase activities in addition to 3beta-hydroxylase activity. In contrast to TaGA20ox1, TaGA3ox2 is expressed in internodes, as well as nodes and the ear of the elongating stem. It is also highly expressed in developing and germinated embryos.

Ectopic expression of a gibberellin 2-oxidase gene (PcGA2ox1) decreased the content of bioactive gibberellins (GAs) in transgenic wheat, producing a range of dwarf plants with different degrees of severity. In at least one case, a single transformation event gave rise to T(1) plants with different degrees of dwarfism, the phenotypes being stably inherited over at least four generations. The dwarf phenotype, which included dark-green leaves, increased tillering and, in severe cases, a prostrate growth habit, was replicated by the application of a GA biosynthesis inhibitor to the wild type. Ear rachis length, grain set, and grain size were also decreased in the wheat transformants, compared with an azygous (null) line. The extent of post-germination alpha-amylase production in grains reflected the severity of the shoot phenotype of the transformants and both developmental processes were restored to normal by the application of gibberellic acid (GA(3)). Expression of two GA biosynthesis genes (TaGA20ox1 and TaGA3ox2) was up-regulated, and that of two alpha-amylase gene families (alpha-Amy1 and alpha-Amy2) down regulated, in scutella of semi-dwarf lines, compared with controls. The marked decline in transcript abundance of both alpha-amylase gene families in aleurone was associated with a decreased content of bioactive GAs in grains of the semi-dwarf lines.

The mechanisms which contribute to the plastic deformation of the intermetallic compound MoSiz have been determined for singlecrystal specimens of MoSi2, (Moo.~Nbo.ol)Siz and ( M O~.~~~T~~.~~~)~~ oriented along [13i] compressed at 1400°C and at a strain rate of 10-s-in vacuum. The results of compression testing of the three single crystals show significant differences between the yield stress values. For the binary MoSi2 single crystal, the yield stress was found to be 34MPa, and for the and (Moo.wzsT&.oo7s)Si2 crystals values of 79 and 38MPa respectively were recorded. An apparent violation of Schmid's law was found in deformation of samples of two of the crystals, namely the binary and Ti-containing version. Thus, while in the Nb-containing compound, the expected slip system, f (1 11){ 1 lo), was activated; this was not the case in the other two compounds. An alternative explanation involves the influence of climb, this being similar to the conclusions drawn in part I for polycrystalline MoSiz. 1. INTRODUCTIONIn part I (Evans, Scheltens, Woodhouse and Fraser 1993, it was shown that experimental results, including mechanical testing and microstructural observations, were consistent with the notion that the contribution to plastic strain from dislocation climb increased significantly with temperature and inverse strain rate at temperatures above 1200°C. Thus the brittle-to-ductile transition appears to be influenced more by climb than by increased dislocation mobilities in glide. Part of the evidence used to determine the contribution from climb involved the identification of active 'slip systems' in which not only are the slip planes physically unreasonable (i.e. not close packed) but also where their Schmid factors are so small that their activation would be considered to be unlikely. Such analysis based on Schrnid factors estimated in polycrystalline samples represents an approximation, and more precise conclusions may be derived from experiments involving single crystals. Hence, the 0 1997 US Government. research described in the following was performed in an attempt to determine unambiguously the factors influencing the brittle-to-ductile transition above 1200°C in MoSi2.The deformation behaviour of single crystals of MoSi2 has been studied by several workers (for example Umakoshi, Sakagami, Hirano and Yamane (1990b), Kimura, Nakamura and Hirano (1390), Boldt, Embury and Weatherley (1992), Maloy, Mitchell, Lewandowski and Heuer (1993a) and Ito, Inui, Shirai and Yamapchi (1995)). The latter paper reports the results of deformation of single crystals of MoSi2 as a function of crystal orientation over a wide range of temperatures, It is found that (111){110), (lOO]{Oll), (100){010), (100]{023) and (331]{013)are activated, depending on orientation. These determinations were the results of analyses of Burgers vectors using diffraction contrast in the transmission electron microscope and also surface slip markings following deformation (up to about 1200"C, since surface contamination precluded the observation of ...

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