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Decreased inputs of S have increased the incidence of S-deficiency in crops, resulting in decreased yields and quality. Remediation by fertilizer application is not always successful because this often results in an uneven supply of S. The ability to respond to S-deficiency stress varies between crops and this is a target for the genetic improvement of S-utilization efficiency. Improved capture of resources, the accumulation of greater reserves of S and improved mechanisms for the remobilization of these reserves are required. It is an inability to over-accumulate S and subsequently, effectively remobilize S-reserves, which restricts optimum S-use efficiency. Genetic manipulation of the transporters and their expression will contribute to overcoming these limitations. Control of gene expression limits excess uptake and activity of the assimilatory pathway: the endogenous expression of sulphate transporters is regulated by S-supply, with negative regulation from reduced S-containing compounds and positive regulation by O-acetylserine, the C/N skeleton precursor of cysteine. Constitutive expression of the transporter will remove this control and may enable the accumulation of sulphate reserves. Sulphate in the vacuole and other pools of reduced sulphur, such as glutathione or protein may be remobilized under S-limiting conditions. Low efficiencies of these remobilization processes, particularly the remobilization of vacuolar sulphate, suggest that the transporters involved in the remobilization are a target for modification. Transporters are involved in facilitating the multiple trans-membrane transport steps between uptake of sulphate from the soil solution, and delivery to the site of reduction in the chloroplast or plastid. A gene family has been identified and phylogenetic relationships based on primary sequence information indicate multiple sub-groups. Groups which are expressed in roots, in shoots and in both tissue types are postulated, however, the functional roles for these groups and the identification of transporters involved in recycling remain to be confirmed.

A cyanoalanine synthase and two isoforms (A, cytosolic and B, chloroplastic) of cysteine synthase (O:-acetylserine (thiol) lyase) were isolated from spinach. N-terminal amino acid sequence analysis of the cyanoalanine synthase gave 100% homology for the determined 12 residues with a published sequence for the mitochondrial cysteine synthase isoform. All three enzymes catalysed both the cysteine synthesis and cyanoalanine synthesis reactions, although with different efficiencies. Michaelis-Menten kinetics were observed for all three enzymes when substrate saturation experiments were performed varying O:-acetylserine, chloroalanine and cysteine. Negative co-operative kinetics were observed for cysteine synthases A and B when substrate saturation experiments were performed varying sulphide and cyanide, compared with the Michaelis-Menten kinetics observed for cyanoalanine synthase. The exception was negative co-operativity observed towards sulphide for cyanoalanine synthase with O:-acetylserine as co-substrate. The optimum sulphide concentration was dependent on the alanyl co-substrate used. The amino acid sequence similarity places these three enzymes in the same gene family, and whilst the close kinetic similarities support this, they also indicate distinct roles for the isoforms.

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Abstract:Wheat has been domesticated into a large number of agricultural environments and has a remarkable ability to adapt to diverse environments. To understand this process, we survey genotype, repeat content and DNA methylation across a bread wheat landrace collection representing global genetic

Cytochrome c oxidase has been purified from Zea mays mitochondria by a solubilization with dodecyl maltoside followed by a simple and rapid two

Characteristic differences exist between the mitochondrial genome organization of fertile and cytoplasmic male-sterile (CMS) lines in a range of plant species. Current evidence suggests that these characteristic mitochondrial genotypes arose by aberrant recombination events, generating chimeric mitochondrial DNA sequences which have subsequently become stabilized, possibly by selective amplification. An investigation of the variation in stoichiometry of the four atp A gene types in maize have suggested evolutionary mechanisms for the generation of mitochondrial genome diversity which are based on amplification of pre-existing, rare recombinant DNA molecules. As with a number of other well-documented examples of genome rearrangement, those involving the atp A gene appear to have no obvious phenotypic significance. However, in a number of cases, recombination events have resulted in either modification of existing mitochondrial genes, leading to the synthesis of a modified polypeptide, e.g. the cox I gene in the 9E sorghum cytoplasm, or the generation of novel open reading frames. In the latter case the unique open reading frame found in the mitochondrial DNA of CMS-T maize plants encodes a 13 kDa polypeptide, previously identified as a CMS-T-specific mitochondrial translation product. Current studies are directed towards establishing a causal link between the 13 kDa polypeptide, mitochondrial enzyme complexes, and the CMS phenotype.