Osamu Kodama

Involvement of jasmonic acid in elicitor-induced phytoalexin production in suspension-cultured rice cells

It has been suggested that jasmonic acid (JA) could be an integral part of a general signal transduction system regulating inducible defense genes in plants. It was reported that treatment with an elicitor (N-acetylchitoheptaose) induced production of phytoalexin in suspension-cultured rice (Oryza sativa L.) cells. In this study, the role of JA in the induction of phytoalexin production by N-acetylchitoheptaose was investigated. Exogenously applied ([plus or minus])-JA (10–4 M) clearly induced the production of momilactone A, a major phytoalexin, in suspension-cultured rice cells. On the other hand, in rice cells treated with N-acetylchitoheptaose, endogenous JA was rapidly and transiently accumulated prior to accumulation of momilactone A. Treatment with ibuprofen, an inhibitor of JA biosynthesis, reduced production of momilactone A in the cells treated with N-acetylchitoheptaose, but the addition of ([plus or minus])-JA increased production of momilactone A to levels higher than those in the elicited rice cells. These results strongly suggest that JA functions as a signal transducer in the induction of biosynthesis of momilactone A by N-acetylchitoheptaose in suspension-cultured rice cells.

Sakuranetin, a flavanone phytoalexin from ultraviolet-irradiated rice leaves

Abstract A new rice phytoalexin, sakuranetin (5,4′-dihydroxy-7-methoxyflavanone) was isolated from ultraviolet-irradiated rice leaves. The ED 50 value of sakuranetin against spore germination of Pyricularia oryzae was approximately 15 ppm. A marked incorporation of [ 14C ]phenylalanine into sakuranetin was observed in ultraviolet-irradiated rice leaves. Sakuranetin has also been observed in blast-infected rice leaves and the content in a resistant cultivar after infection with P. oryzae was much higher than in a susceptible cultivar.

Identification of a Biosynthetic Gene Cluster in Rice for Momilactones

Rice diterpenoid phytoalexins such as momilactones and phytocassanes are produced in suspension-cultured rice cells treated with a chitin oligosaccharide elicitor and in rice leaves irradiated with UV light. The common substrate geranylgeranyl diphosphate is converted into diterpene hydrocarbon precursors via a two-step sequential cyclization and then into the bioactive phytoalexins via several oxidation steps. It has been suggested that microsomal cytochrome P-450 monooxygenases (P-450s) are involved in the downstream oxidation of the diterpene hydrocarbons leading to the phytoalexins and that a dehydrogenase is involved in momilactone biosynthesis. However, none of the enzymes involved in the downstream oxidation of the diterpene hydrocarbons have been identified. In this study, we found that a putative dehydrogenase gene (AK103462) and two functionally unknown P-450 genes (CYP99A2 and CYP99A3) form a chitin oligosaccharide elicitor- and UV-inducible gene cluster, together with OsKS4 and OsCyc1, the diterpene cyclase genes involved in momilactone biosynthesis. Functional analysis by heterologous expression in Escherichia coli followed by enzyme assays demonstrated that the AK103462 protein catalyzes the conversion of 3β-hydroxy-9βH-pimara-7,15-dien-19,6β-olide into momilactone A. The double knockdown of CYP99A2 and CYP99A3 specifically suppressed the elicitor-inducible production of momilactones, strongly suggesting that CYP99A2, CYP99A3, or both are involved in momilactone biosynthesis. These results provide strong evidence for the presence on chromosome 4 of a gene cluster involved in momilactone biosynthesis.

Oryzalexin E, A diterpene phytoalexin from UV-irradiated rice leaves

Abstract The novel diterpene phytoalexin, oryzalexin E was isolated from UV-irradiated rice leaves. The structure and relative configuration of oryzalexin E wer

Purification and Identification of Antimicrobial Sesquiterpene Lactones from Yacon (Smallanthus sonchifolius) Leaves

The extraction of yacon [Smallanthus sonchifolius (Poepp. and Endl.) H. Robinson; Asteraceae] leaves and chromatographic separation yielded two new antibacterial melampolide-type sesquiterpene lactones, 8β-tigloyloxymelampolid-14-oic acid methyl ester and 8β-methacryloyloxymelampolid-14-oic acid methyl ester, as well as the four known melampolides, sonchifolin, uvedalin, enhydrin and fluctuanin. The newly identified compound, 8β-methacryloyloxymelampolid-14-oic acid methyl ester, exhibited potent antimicrobial activity against Bacillus subtilis and Pyricularia oryzae, while 8β-tigloyloxymelampolid-14-oic acid methyl ester showed lower activity. Fluctuanin exhibited the strongest antibacterial activity against B. subtilis among these six sesquiterpene lactones.

Phytoalexin production elicited by exogenously applied jasmonic acid in rice leaves (Oryza sativa L.) is under the control of cytokinins and ascorbic acid.

Jasmonic acid (JA) has been shown to be a signaling compound which elicits the production of secondary metabolites including phytoalexins in plants. It has been shown that the phytoalexin production is elicited by exogenously applied JA in rice leaves. We now show that this phytoalexin production by exogenously applied JA is significantly counteracted by cytokinins, kinetin and zeatin. Kinetin and zeatin also inhibit the induction of naringenin‐7‐O‐methyltransferase (a key enzyme in rice phytoalexin production) by JA. A natural free radical scavenger, ascorbic acid (AsA) shows both counteractive and enhancing effects on JA‐inducible phytoalexin production, depending on its concentration. This effect of AsA suggests that active oxygen species (AOS) may play important roles in phytoalexin production by JA in rice leaves.

Coronatine elicits phytoalexin production in rice leaves (Oryza sativa L.) in the same manner as jasmonic acid.

The phytotoxin coronatine induced the accumulation of the flavonoid phytoalexins sakuranetin and momilactone A in rice leaves. Coronatine-inducible sakuranetin production was under the control of kinetin and ascorbic acid (AsA), as observed with jasmonic acid (JA). The effects of kinetin and AsA on the activity of coronatine indicated that coronatine might elicit sakuranetin production in a manner similar to JA. The similarity of both their structures and the manner of elicitation of coronatine and JA suggest that they might interact at the same active site(s) to lead to phytoalexin production.

Phytoalexin production by amino acid conjugates of jasmonic acid through induction of naringenin-7-O-methyltransferase, a key enzyme on phytoalexin biosynthesis in rice (Oryza sativa L.)

Amino acid conjugates of jasmonic acid are found to elicit production of the flavonoid phytoalexin, sakuranetin in rice leaves. The elicitation is shown to arise from induction of naringenin 7‐O‐methyltransferase, a key enzyme of sakuranetin biosynthesis. The (−)‐phenylalanine conjugate, one of the active compounds, is characterized by high activity for both sakuranetin and enzyme induction and low phytotoxicity against rice growth. Both (+)‐enantiomers of the conjugates and free amino acids do not show any activity. The amino acid conjugate of jasmonic acid is speculated to be the later component in the signaling transduction chain in stressed rice plants.

Antifungal melampolides from leaf extracts of Smallanthus sonchifolius

Abstract A new antifungal melampolide, 8-angeloyl-1(10),4,11(13)-germacuratrien-12,6-olid-14-oic acid methyl ester, named sonchifolin, as well as three known melampolides, polymatin B, uvedalin and enhydrin, were isolated from leaf extracts of Yacon [ Smallanthus sonchifolius (Poepp. and Endl.) H. Robinson]. Sonchifolin exhibited the highest fungicidal activity against Pyricularia oryzae , a fungus causing rice blast disease and the ED 50 value for the spore germination was 22 ppm. This is the first report of these melampolides as fungicidal compounds.

Quantification of amino acid conjugates of jasmonic acid in rice leaves by high-performance liquid chromatography–turboionspray tandem mass spectrometry

Abstract HPLC–turboionspray MS–MS (TIS-MS–MS) was shown to be useful in the quantification of the amino acid conjugates of jasmonic acid (JA) as well as free JA in rice leaves. With the TIS-MS–MS system, high sensitivity was obtained under multiple reaction monitoring conditions. Using this quantification method, rapid increase and subsequent decrease in the leucine conjugate and valine conjugate of JA were observed in rice leaves stressed by wounding. These amino acid conjugates of JA may work as endogenous signalling compounds as well as free JA.