Identification of Sternbin and Naringenin as Detoxified Metabolites from the Rice Flavanone Phytoalexin Sakuranetin by Pyricularia oryzae.

Sakuranetin (1) is a flavanone phytoalexin that has been reported to play an important role in disease resistance in rice plants. The rice blast fungus Pyricularia oryzae (syn. Magnaporthe oryzae) has been reported to metabolize 1 to lower its antifungal activity. Here, two flavanones, sternbin (2) and naringenin (3), were identified as metabolites of 1 in P. oryzae suspension culture by liquid chromatography tandem mass spectrometry (LC/MS/MS). The inhibition of 1, 2, and 3 on P. oryzae mycelial growth were 45%, 19%, and 19%, respectively, at a concentration of 100 µm. Thus, 2 and 3 are detoxified metabolites of 1 by P. oryzae.

Characterization and evolutionary analysis of ent-kaurene synthase like genes from the wild rice species Oryza rufipogon.

Cultivated rice (Oryza sativa) possesses various labdane-related diterpene synthase genes, homologs of ent-copalyl diphosphate synthase (CPS) and ent-kaurene synthase (KS) that are responsible for the biosynthesis of phytohormone gibberellins. The CPS homologs and KS like (KSL) homologs successively converted geranylgeranyl diphosphate to cyclic diterpene hydrocarbons via ent-copalyl diphosphate or syn-copalyl diphosphate in O. sativa. Consequently, a variety of labdane-related diterpenoids, including phytoalexin phytocassanes, momilactones and oryzalexins, have been identified from cultivated rice. Our previous report indicated that the biosynthesis of phytocassanes and momilactones is conserved in Oryza rufipogon, the progenitor of Asian cultivated rice. Moreover, their biosynthetic gene clusters, containing OsCPS2 and OsKSL7 for phytocassane biosynthesis and OsCPS4 and OsKSL4 for momilactone biosynthesis, are also present in the O. rufipogon genome. We herein characterized O. rufipogon homologs of OsKSL5, OsKSL6, OsKSL8 responsible for oryzalexin S biosynthesis, and OsKSL10 responsible for oryzalexins A-F biosynthesis, to obtain more evolutionary insight into diterpenoid biosynthesis in O. sativa. Our phytoalexin analyses showed that no accumulation of oryzalexins was detected in extracts from O. rufipogon leaf blades. In vitro functional analyses indicated that unlike OsKSL10, O. rufipogon KSL10 functions as an ent-miltiradiene synthase, which explains the lack of accumulation of oryzalexins A-F in O. rufipogon. The different functions of KSL5 and KSL8 in O. sativa japonica to those in indica are conserved in each type of O. rufipogon, while KSL6 functions (ent-isokaurene synthases) are well conserved. Our study suggests that O. sativa japonica has evolved distinct specialized diterpenoid metabolism, including the biosynthesis of oryzalexins.

Evolutionary trajectory of phytoalexin biosynthetic gene clusters in rice.

Plants frequently possess operon-like gene clusters for specialized metabolism. Cultivated rice, Oryza sativa, produces antimicrobial diterpene phytoalexins represented by phytocassanes and momilactones, and the majority of their biosynthetic genes are clustered on chromosomes 2 and 4, respectively. These labdane-related diterpene phytoalexins are biosynthesized from geranylgeranyl diphosphate via ent-copalyl diphosphate or syn-copalyl diphosphate. The two gene clusters consist of genes encoding diterpene synthases and chemical-modification enzymes including P450s. In contrast, genes for the biosynthesis of gibberellins, which are labdane-related phytohormones, are scattered throughout the rice genome similar to other plant genomes. The mechanism of operon-like gene cluster formation remains undefined despite previous studies in other plant species. Here we show an evolutionary insight into the rice gene clusters by a comparison with wild Oryza species. Comparative genomics and biochemical studies using wild rice species from the AA genome lineage, including Oryza barthii, Oryza glumaepatula, Oryza meridionalis and the progenitor of Asian cultivated rice Oryza rufipogon indicate that gene clustering for biosynthesis of momilactones and phytocassanes had already been accomplished before the domestication of rice. Similar studies using the species Oryza punctata from the BB genome lineage, the distant FF genome lineage species Oryza brachyantha and an outgroup species Leersia perrieri suggest that the phytocassane biosynthetic gene cluster was present in the common ancestor of the Oryza species despite the different locations, directions and numbers of their member genes. However, the momilactone biosynthetic gene cluster evolved within Oryza before the divergence of the BB genome via assembly of ancestral genes.

Identification of UV-Induced Diterpenes Including a New Diterpene Phytoalexin, Phytocassane F, from Rice Leaves by Complementary GC/MS and LC/MS Approaches.

Rice phytoalexins are regarded as one of the most important weapons against pathogenic microorganisms. We attempted to identify novel phytoalexins and their derivatives using GC/MS and LC/MS analyses. Diterpene derivatives, 9ß-pimara-7,15-diene-3ß,6ß,19-triol, 1, stemar-13-en-2α-ol, 2, and 1α,2α-dihydroxy-ent-12,15-cassadiene-3,11-dione, 3, were isolated from UV-irradiated rice leaves by chromatographic methods. These structures were confirmed by 1D- and 2D-NMR and MS analyses. Interestingly, all three compounds were accumulated following an infection by the rice blast pathogen Magnaporthe oryzae. Compounds 1 and 2 exhibited weak antifungal activity and may be the biosynthetic intermediates of rice phytoalexins momilactones and oryzalexin S, respectively. Compound 3 exhibited relatively high inhibitory activity against the fungal mycelial growth of M. oryzae to the same extent as the known phytoalexin phytocassane A. We conclude that 3 is a member of the cassane-type phytoalexin family and propose the name phytocassane F.

Proteomics of two cultivated mushrooms Sparassis crispa and Hericium erinaceum provides insight into their numerous functional protein components and diversity.

Mushroom can be defined as a macrofungus with a distinctive fruiting body. Mushrooms of class Basidiomycete are primarily wood degradation fungi, but serve as food and a part of traditional medicine used by humans. Although their life cycle is fairly well-established, the information on the molecular components, especially proteins are very limited. Here, we report proteomics analysis of two edible mushrooms (fruiting bodies) Sparassis crispa and Hericium erinaceum using one- and two-dimensional gel electrophoresis (1-DGE and 2-DGE) based complementary proteomics approaches. 1-DGE coupled with liquid chromatography and mass spectrometry identified 77 (60 nonredundant proteins) and 121 (88 nonredundant proteins) proteins from S. crispa and H. erinaceum, respectively. 2-DGE analysis revealed 480 and 570 protein spots stained with colloidal coomassie brilliant blue in S. crispa and H. erinaceum, respectively. Of the 71 and 115 selected protein spots from S. crispa and H. erinaceum 2D gel blots on polyvinyldifluoride (PVDF) membranes, respectively, 29 and 35 nonredundant proteins were identified by N-terminal amino acid sequencing. Identified nonredundant proteins from 1- or 2-DGE belonged to 19 functional categories. Twenty-one proteins were found common in both S. crispa and H. erinaceum proteomes, including 14-3-3 protein and septin. Together this study provides evidence for the presence of a large number of functionally diverse proteins, expressed in the fruiting body of two economically important mushrooms, S. crispa and H. erinaceum. Data obtained from 1-DGE and 2-DGE analyses is accessible through the mushroom proteomics portal http://foodfunc.agr.ibaraki.ac.jp/mushprot.html.