Biochemical synthesis of uniformly ¹³C-labeled diterpene hydrocarbons and their bioconversion to diterpenoid phytoalexins <i>in planta</i>

<p>Phytocassanes and momilactones are the major diterpenoid phytoalexins inductively produced in rice as bioactive substances. Regardless of extensive studies on the biosynthetic pathways of these phytoalexins, bioconversion of diterpene hydrocarbons is not shown <i>in planta</i>. To elucidate the entire biosynthetic pathways of these phytoalexins, uniformly ¹³C-labeled <i>ent</i>-cassadiene and <i>syn</i>-pimaradiene were enzymatically synthesized with structural verification by GC–MS and ¹³C-NMR. Application of the ¹³C-labeled substrates on rice leaves led to the detection of ¹³C-labeled metabolites using LC-MS/MS. Further application of this method in the moss <i>Hypnum plumaeforme</i> and the nearest out-group of <i>Oryza</i> species <i>Leersia perrieri</i>, respectively, resulted in successful bioconversion of these labeled substrates into phytoalexins in these plants. These results demonstrate that genuine biosynthetic pathways from these diterpene hydrocarbons to the end product phytoalexins occur in these plants and that enzymatically synthesized [U-¹³C₂₀] diterpene substrates are a powerful tool for chasing endogenous metabolites without dilution with naturally abundant unlabeled compounds.</p> <p>Uniformly labeled ¹³C-diterpene: A powerful tool for tracing endogenous metabolites without dilution with naturally abundant unlabeled compounds</p

Transmission electron microscopy of parasitized erythrocytes treated with inhibitors.

<p>(A) Sections through an erythrocyte containing a trophozoite-stage parasite exposed to 0.1% DMSO for 6 h, 50 µM INA for 6 h or 250 µM AMO-1618 for 8 h, respectively. (B) Asynchronized parasites were treated with 0.1% DMSO (a–d) or 50 µM INA for 6 h (e–h). Sections from representative stages during intraerythrocytic development: ring- (a and e), early trophozoite- (b and f), mature trophozoite- (c and g) and schizont-stage parasites (d and h) are shown. Nuclei (n), food vacuoles (fv), merozoites (m), nuclei of merozoites (mn) and abnormal gaps between the nuclei and the nuclear envelopes (arrowheads) are indicated. Scale bar is indicated at the bottom of the images.</p

ED₅₀ of the gibberellin biosynthetic inhibitors.

<p>Growth inhibitory effects of gibberellin biosynthetic inhibitors and enantiomers of INA to <i>P. falciparum</i> and <i>T. gondii</i> are shown. Values are the mean ± standard deviations (SD) from three independent experiments, with each treatment duplicated twice. N.D.; not determined.</p

GC-MS analysis of isoprenoids in <i>P. falciparum in vitro</i> cultures.

<p>(A) Total ion chromatograms of extracts from unparasitized erythrocytes (Unparasitized), parasites treated with 0.1% DMSO, 250 µM AMO-1618, and 50 µM INA for 6 h. Triangles and circles indicate the peaks of geranylgeraniol mixed as an internal control (retention time = 6.57 min) and cholesterol (11.23 min), respectively. These compounds were identified by direct comparison with authentic samples. (B) Quantification of cholesterol in each treated sample. Cholesterol amount was calculated from the peak areas and normalized relative to the ratio of the internal control geranylgeraniol. Values are means and SD of triplicate measurements of a representative experiment.</p

Effects of osmotic pressure to intraerythrocytic parasites treated with INA.

<p>(A) Effects of hyperosmotic stress in INA-treated parasites. <i>P. falciparum</i> cultures treated with 50 µM INA were observed in various dilution ratios of PBS for 8 h. Parasitemia was determined by Giemsa-stained thin blood smears. Student's <i>t</i>-test: *P>0.005. Values are means ± SD of n = 6 in two independent experiments. Data were normalized relative to control cultures (1× PBS) in DMSO- and INA-treated samples, respectively. Representative parasite morphologies are shown for each treatment. Scale bar, 3 µm. (B) Effects of hyposmotic stress, experimentally induced by the addition of water to the culture medium of <i>P. falciparum</i>, after 8 h of 50 µM INA treatment. N = 6 smears each in two independent experiments.</p

Isoprenoid biosynthetic pathways and known inhibitors in various organisms.

<p>Broken arrows indicate blocks in the biosynthesis due to specific inhibitors. “R” indicates various functional groups specific to individual compounds. CPPS, copalyl-diphosphate synthase (EC 5.5.1.13); KO, <i>ent</i>-kaurene oxidase (EC 1.14.13.78); CAS, cycloartenol synthase (EC 5.4.99.8); LS, lanosterol synthase (EC 5.4.99.7); KS, <i>ent</i>-kaurene synthase (EC 4.2.3.19); PP, pyrophosphate.</p

<i>In planta</i> functions of cytochrome P450 monooxygenase genes in the phytocassane biosynthetic gene cluster on rice chromosome 2

<p>In response to environmental stressors such as blast fungal infections, rice produces phytoalexins, an antimicrobial diterpenoid compound. Together with momilactones, phytocassanes are among the major diterpenoid phytoalexins. The biosynthetic genes of diterpenoid phytoalexin are organized on the chromosome in functional gene clusters, comprising diterpene cyclase, dehydrogenase, and cytochrome P450 monooxygenase genes. Their functions have been studied extensively using <i>in vitro</i> enzyme assay systems. Specifically, P450 genes (<i>CYP71Z6</i>, <i>Z7</i>; <i>CYP76M5</i>, <i>M6</i>, <i>M7</i>, <i>M8</i>) on rice chromosome 2 have multifunctional activities associated with <i>ent</i>-copalyl diphosphate-related diterpene hydrocarbons, but the <i>in planta</i> contribution of these genes to diterpenoid phytoalexin production remains unknown. Here, we characterized <i>cyp71z7</i> T-DNA mutant and <i>CYP76M7/M8</i> RNAi lines to find that potential phytoalexin intermediates accumulated in these P450-suppressed rice plants. The results suggested that <i>in planta</i>, CYP71Z7 is responsible for C2-hydroxylation of phytocassanes and that CYP76M7/M8 is involved in C11α-hydroxylation of 3-hydroxy-cassadiene. Based on these results, we proposed potential routes of phytocassane biosynthesis <i>in planta</i>.</p> <p>C2-hydoroxylation and C11-hydorxylation are active <i>in planta</i> catalyzed by CYP71Z7 and CYP76M7/M8 leading to phytocassanes. New intermediates are found in this study.</p

Effect of INA and AMO-1618 on intraerythrocytic development of <i>P. falciparum</i>.

<p>Tightly synchronized parasite cells that have undergone 5% sorbitol treatment and percoll density gradient centrifugation (window period: 4 h) were treated with 1 µl/ml DMSO, 50 µM INA or 250 µM AMO-1618 at different parasite stages: after 0 h (ring, A), 20 h (immature trophozoite, B), 28 h (mature trophozoite, C) and 36 h (schizont, D). Cultures were examined after 4, 8 and 12 h using Giemsa thin blood smears. Scale bar: 3 µm; all images without a scale bar are displayed at the same scale as the left uppermost image in (A).</p

Fluorescence microscopy of INA-treated parasites.

<p>Infected erythrocytes were stained with (A) acridine orange, (B) Nile Red, (C) rhodamine 123 and (D) LysoTracker™ Red DND-99. INA was introduced at: (A) 100 µM for 9 h, (B) 50 µM for 6 h, and (C and D) 50 µM for 4 h. 100 µg/ml acridine orange was applied to thin blood smears made from intraerythrocytic parasites treated with INA. For other fluorescence dyes, <i>P. falciparum</i> cultures were incubated with probes for 1 h at the following concentrations: Nile Red, 1 µg/ml; rhodamine 123, 10 ng/ml; and LysoTracker® Red DND-99, 75 nM. Cells were not washed prior to fluorescence microscopy to minimize damage due to osmotic changes. Scale bar, 3 µm. An arrow indicates a lipid body in (B).</p

Changes in the transcript levels of gibberellin (GA) metabolism genes in the cotyledon end and hypocotyl end of lettuce seeds

(A) Frozen seeds were divided into two parts: cotyledon end, including the cotyledons (Cot), fruit wall (FW), seed coat (SC), and endosperm (ES); and the hypocotyl end, including the hypocotyl (Hyp), root apical meristem (RAM), shoot apical meristem (SAM), and part of the Cot, FW, SC, and ES. (B) Expression levels of GA metabolism genes after light treatment, determined using QRT-PCR. See for light treatments. The results were normalized to the expression of 18S rRNA (internal control), and the highest value was set to 100. Two independent experiments were performed, and means with standard errors are shown.<p><b>Copyright information:</b></p><p>Taken from "Germination of photoblastic lettuce seeds is regulated via the control of endogenous physiologically active gibberellin content, rather than of gibberellin responsiveness"</p><p></p><p>Journal of Experimental Botany 2008;59(12):3383-3393.</p><p>Published online 24 Jul 2008</p><p>PMCID:PMC2529229.</p><p></p