Identification and analysis of homoeologous segments of the genomes of rice and Arabidopsis thaliana.

Using contiguous genomic DNA sequences of Arabidopsis thaliana, we were able to identify a region of conserved structure in the genome of rice. The conserved, and presumptive homoeologous segments, are 194 kb and 219-300 kb in size in Arabidopsis and rice, respectively. They contain five homologous genes, distinguished in order by a single inversion. These represent the first homoeologous segments identified in the genomes of a dicot and a monocot, demonstrating that fine-scale conservation of genome structure exists and is detectable across this major divide in the angiosperms. The conserved framework of genes identified is interspersed with non-conserved genes, indicating that mechanisms beyond segmental inversions and translocations need to be invoked to fully explain plant genome evolution, and that the benefits of comparative genomics over such large taxonomic distances may be limited.

Studies on the biosynthesis of tropane alkaloids by Datura stramonium L. transformed root cultures : 3. The relationship between morphological integrity and alkaloid biosynthesis.

Transformed root cultures of Datura stramonium, competent in tropane-alkaloid biosynthesis, have been treated with exogenous plant growth regulators. It was found that combinations of α-naphthalene-acetic acid, kinetin (N(6)-furfurylaminopurine) and 2,4-dichlorophenoxyacetic acid induced de-differentiation, causing both the rooty phenotype and the hyoscyamine-biosynthetic capacity to be lost. Alkaloid biosynthesis disappeared rapidly and prior to the loss of morphological integrity. It was observed that the enzymes ornithine decarboxylase (EC 4.1.1.17), arginine decarboxylase (EC 4.1.1.19) and N-methylputrescine oxidase did not show the increase in level normally associated with subculturing the roots. The level of putrescine N-methyltransferase (EC 2.1.1.53) activity, the first enzyme fully committed to hyoscyamine biosynthesis, rapidly declined, about 80% being lost from the roots within 12h. This activity, although showing some temporary restoration, declined further after a few days, and was totally absent from fully dispersed cultures. N-Methylputrescine oxidase persisted at a low level. Following sub-culture of established de-differentiated lines to plant-growth-regulator-free medium, limited root regeneration occurred. The roots formed showed renewed competence in alkaloid biosynthesis and putrescine N-methyltransferase and N-methylputrescine oxidase activities were restored to their normal levels. The relationship between the morphological state and alkaloid-biosynthetic capacity of the cultures is discussed in relation to the overall control of alkaloid biosynthesis.

Studies on the biosynthesis of tropane alkaloids in Datura stramonium L. transformed root cultures : 1. The kinetics of alkaloid production and the influence of feeding intermediate metabolites.

The factors by which the endogenous regulation of tropane-alkaloid biosynthesis may be effected have been examined in a transformed root culture of Datura stramonium. Pools of intermediates showed a subculture-related maximal accumulation, as did the enzyme activities by which they are synthesised and/or metabolised. The end-products, principally hyoscyamine and apohyoscyamine, in contrast, accumulated steadily in growing cultures. Feeding putrescine, agmatine or tropine did not enhance alkaloid accumulation, but rather may even have resulted in a lowering of hyoscyamine levels. Similarly, feeding precursors for the tropate moiety of hyoscyamine either had no influence or had a detrimental effect on hyoscyamine accumulation. Under some feeding conditions, intermediates in the pathway from N-methylputrescine up to and including tropine accumulated up to 40-fold. Little effect on early intermediates was found, however, when tropinone or tropine were fed. The expression of the enzyme arginine decarboxylase (EC 4.1.1.19) was particularly sensitive to feed-back repression, both by its product agmatine and by more distant pathway intermediates, notably putrescine and tropine. Some diminution of the levels of putrescine N-methyltransferase (EC 2.1.1.53) and N-methylputrescine oxidase, the first committed enzymes of alkaloid biosynthesis, was also seen with tropine, although only at rather high levels. It is concluded that the pathway is not regulated in a simple manner and that (i) the early enzymes of the pathway are at near rate-limiting levels, (ii) there is a major limitation to flux at the level of the esterification of tropine, and (iii) the level of free tropine may be important in determining the flux into and through the tropane pathway.