Global transcription analysis of Krebs tricarboxylic acid cycle mutants reveals an alternating pattern of gene expression and effects on hypoxic and oxidative genes

To understand the many roles of the Krebs tricarboxylic acid (TCA) cycle in cell function, we used DNA microarrays to examine gene expression in response to TCA cycle dysfunction. mRNA was analyzed from yeast strains harboring defects in each of 15 genes that encode subunits of the eight TCA cycle enzymes. The expression of >400 genes changed at least threefold in response to TCA cycle dysfunction. Many genes displayed a common response to TCA cycle dysfunction indicative of a shift away from oxidative metabolism. Another set of genes displayed a pairwise, alternating pattern of expression in response to contiguous TCA cycle enzyme defects: expression was elevated in aconitase and isocitrate dehydrogenase mutants, diminished in alpha-ketoglutarate dehydrogenase and succinyl-CoA ligase mutants, elevated again in succinate dehydrogenase and fumarase mutants, and diminished again in malate dehydrogenase and citrate synthase mutants. This pattern correlated with previously defined TCA cycle growth-enhancing mutations and suggested a novel metabolic signaling pathway monitoring TCA cycle function. Expression of hypoxic/anaerobic genes was elevated in alpha-ketoglutarate dehydrogenase mutants, whereas expression of oxidative genes was diminished, consistent with a heme signaling defect caused by inadequate levels of the heme precursor, succinyl-CoA. These studies have revealed extensive responses to changes in TCA cycle function and have uncovered new and unexpected metabolic networks that are wired into the TCA cycle.     

Diurnal oscillation of SBE expression in sorghum endosperm

Spatial and temporal expression patterns of the sorghum SBEI, SBEIIA and SBEIIB genes, encoding, respectively, starch branching enzyme (SBE) I, IIA and IIB, in the developing endosperm of sorghum (Sorghum bicolor) were studied. Full-length genomic and cDNA clones for sorghum were cloned, and the SBEIIA cDNA was used together with gene-specific probes for sorghum SBEIIB and SBEI. In contrast to sorghum SBEIIB, which was expressed primarily in endosperm and embryo, SBEIIA was also expressed in vegetative tissues. All three genes shared a similar temporal expression profile during endosperm development, with a maximum activity at 15-24 d after pollination. This differed from barley and maize, in which SBEI gene activity showed a significantly later onset compared to that of SBEIIA and SBEIIB. Expression of the three SBE genes in the sorghum endosperm exhibited a diurnal rhythm during a 24-h cycle.     

Oocyte control of metabolic cooperativity between oocytes and companion granulosa cells: energy metabolism

Intercellular communication between oocytes and granulosa cells is essential for normal follicular differentiation and oocyte development. Subtraction hybridization was used to identify genes more highly expressed in cumulus cells than in mural granulosa cells of mouse antral follicles. This screen identified six genes involved in glycolysis: Eno1, Pkm2, Tpi, Aldoa, Ldh1, and Pfkp. When oocytes were microsurgically removed from cumulus cell-oocyte complexes, the isolated cumulus cells exhibited decreased expression levels of genes encoding glycolytic enzymes, glycolysis and activity of the tricarboxylic acid (TCA) cycle. These decreases were prevented by culturing the cumulus cells with paracrine factors secreted by fully grown oocytes. Paracrine factors from fully grown oocytes exhibited greater ability than those from growing oocytes to promote expression of genes encoding glycolytic enzymes and glycolysis in the granulosa cells of preantral follicles. However, neither fully grown nor growing oocytes secreted paracrine factors affecting activity of the TCA cycle. These results indicate that oocytes regulate glycolysis and the TCA cycle in granulosa cells in a manner specific to the population of granulosa cells and to the stage of growth and development of the oocyte. Oocytes control glycolysis in granulosa cells by regulating expression levels of genes encoding glycolytic enzymes. Therefore, mouse oocytes control the intercellular metabolic cooperativity between cumulus cells and oocytes needed for energy production by granulosa cells and required for oocyte and follicular development.     

Sorghum phytochemicals and their potential impact on human health

Sorghum is a rich source of various phytochemicals including tannins, phenolic acids, anthocyanins, phytosterols and policosanols. These phytochemicals have potential to significantly impact human health. Sorghum fractions possess high antioxidant activity in vitro relative to other cereals or fruits. These fractions may offer similar health benefits commonly associated with fruits. Available epidemiological evidence suggests that sorghum consumption reduces the risk of certain types of cancer in humans compared to other cereals. The high concentration of phytochemicals in sorghum may be partly responsible. Sorghums containing tannins are widely reported to reduce caloric availability and hence weight gain in animals. This property is potentially useful in helping reduce obesity in humans. Sorghum phytochemicals also promote cardiovascular health in animals. Such properties have not been reported in humans and require investigation, since cardiovascular disease is currently the leading killer in the developed world. This paper reviews available information on sorghum phytochemicals, how the information relates to current phytonutrient research and how it has potential to combat common nutrition-related diseases including cancer, cardiovascular disease and obesity.     

The wheat Lr34 multipathogen resistance gene confers resistance to anthracnose and rust in sorghum

The ability of the wheat Lr34 multipathogen resistance gene (Lr34res) to function across a wide taxonomic boundary was investigated in transgenic Sorghum bicolor. Increased resistance to sorghum rust and anthracnose disease symptoms following infection with the biotrophic pathogen Puccinia purpurea and the hemibiotroph Colletotrichum sublineolum, respectively, occurred in transgenic plants expressing the Lr34res ABC transporter. Transgenic sorghum lines that highly expressed the wheat Lr34res gene exhibited immunity to sorghum rust compared to the low‐expressing single copy Lr34res genotype that conferred partial resistance. Pathogen‐induced pigmentation mediated by flavonoid phytoalexins was evident on transgenic sorghum leaves following P. purpurea infection within 24–72 h, which paralleled Lr34res gene expression. Elevated expression of flavone synthase II, flavanone 4‐reductase and dihydroflavonol reductase genes which control the biosynthesis of flavonoid phytoalexins characterized the highly expressing Lr34res transgenic lines 24‐h post‐inoculation with P. purpurea. Metabolite analysis of mesocotyls infected with C. sublineolum showed increased levels of 3‐deoxyanthocyanidin metabolites were associated with Lr34res expression, concomitant with reduced symptoms of anthracnose.     

Functional characterization of desaturases involved in the formation of the terminal double bond of an unusual 16:3Delta(9,12,150) fatty acid isolated from Sorghum bicolor root hairs

Sorgoleone, produced in root hair cells of sorghum (Sorghum bicolor), is likely responsible for much of the allelopathic properties of sorghum root exudates against broadleaf and grass weeds. Previous studies suggest that the biosynthetic pathway of this compound initiates with the synthesis of an unusual 16:3 fatty acid possessing a terminal double bond. The corresponding fatty acyl-CoA serves as a starter unit for polyketide synthases, resulting in the formation of 5-pentadecatrienyl resorcinol. This resorcinolic intermediate is then methylated by an S-adenosylmethionine-dependent O-methyltransferase and subsequently dihydroxylated, yielding the reduced (hydroquinone) form of sorgoleone. To characterize the corresponding enzymes responsible for the biosynthesis of the 16:3 fatty acyl-CoA precursor, we identified and cloned three putative fatty acid desaturases, designated SbDES1, SbDES2, and SbDES3, from an expressed sequence tag (EST) data base prepared from isolated root hairs. Quantitative real-time RT-PCR analyses revealed that these three genes were preferentially expressed in sorghum root hairs where the 16:2 and 16:3 fatty acids were exclusively localized. Heterologous expression of the cDNAs in Saccharomyces cerevisiae revealed that recombinant SbDES2 converted palmitoleic acid (16:1Delta(9)) to hexadecadienoic acid (16:2Delta(9,12)), and that recombinant SbDES3 was capable of converting hexadecadienoic acid into hexadecatrienoic acid (16:3Delta(9,12,15)). Unlike other desaturases reported to date, the double bond introduced by SbDES3 occurred between carbons 15 and 16 resulting in a terminal double bond aliphatic chain. Collectively, the present results strongly suggest that these fatty acid desaturases represent key enzymes involved in the biosynthesis of the allelochemical sorgoleone.     

Biosynthesis of the leucine derived α‐, β‐ and γ‐hydroxynitrile glucosides in barley (Hordeum vulgare L.)

Barley (Hordeum vulgare L.) produces five leucine‐derived hydroxynitrile glucosides (HNGs), of which only epiheterodendrin is a cyanogenic glucoside. The four non‐cyanogenic HNGs are the β‐HNG epidermin and the γ‐HNGs osmaronin, dihydroosmaronin and sutherlandin. By analyzing 247 spring barley lines including landraces and old and modern cultivars, we demonstrated that the HNG level varies notably between lines whereas the overall ratio between the compounds is constant. Based on sequence similarity to the sorghum (Sorghum bicolor) genes involved in dhurrin biosynthesis, we identified a gene cluster on barley chromosome 1 putatively harboring genes that encode enzymes in HNG biosynthesis. Candidate genes were functionally characterized by transient expression in Nicotiana benthamiana. Five multifunctional P450s, including two CYP79 family enzymes and three CYP71 family enzymes, and a single UDP‐glucosyltransferase were found to catalyze the reactions required for biosynthesis of all five barley HNGs. Two of the CYP71 enzymes needed to be co‐expressed for the last hydroxylation step in sutherlandin synthesis to proceed. This observation, together with the constant ratio between the different HNGs, suggested that HNG synthesis in barley is organized within a single multi‐enzyme complex.