Distribution of the tryptophan pathway-derived defensive secondary metabolites gramine and benzoxazinones in Poaceae.

The Poaceae is a large taxonomic group consisting of approximately 12,000 species and is classified into 12 subfamilies. Gramine and benzoxazinones (Bxs), which are biosynthesized from the tryptophan pathway, are well-known defensive secondary metabolites in the Poaceae. We analyzed the presence or absence of garamine and Bxs in 64 species in the Poaceae by LC-MS/MS. We found that Hordeum brachyantherum and Hakonechloa macra accumulated gramine, but the presence of gramine was limited to small groups of species. We also detected Bxs in four species in the Pooideae and six species in the Panicoideae. In particular, four species in the Paniceae tribe in Panicoideae accumulaed Bxs, indicating that this tribe is a center of the Bx distribution. Bxs were absent in the subfamilies other than Pooideae and Panicoideae. These findings provide an overview of biased distribution of gramine and Bxs in Poaceae species.

Induced accumulation of tyramine, serotonin, and related amines in response to Bipolaris sorokiniana infection in barley.

The inducible metabolites were analyzed in barley leaves inoculated with Bipolaris sorokiniana, the causal agent of spot blotch of barley. HPLC analysis revealed that B. sorokiniana-infected leaves accumulated 4 hydrophilic compounds. They were purified by ODS column chromatography and preparative HPLC. Spectroscopic analyses revealed that they were tyramine (1), 3-(2-aminoethyl)-3-hydroxyindolin-2-one (2), serotonin (3), and 5,5'-dihydroxy-2,4'-bitryptamine (4). Among these, 2 and 4 have not been reported as natural products. They showed antifungal activity in an assay of inhibition of B. sorokiniana conidia germination, suggesting that they play a role in the chemical defense of barley as phytoalexins. The accumulation of 1-4 was examined also in the leaves of rice and foxtail millet. Rice leaves accumulated 2, 3, and 4, whereas foxtail millet leaves accumulated 3 and 4 in response to pathogen attack, suggesting the generality of accumulation of 3 and 4 in the Poaceae species.

Variation of diterpenoid phytoalexin oryzalexin A production in cultivated and wild rice.

Rice (Oryza sativa) leaves accumulate phytoalexins in response to pathogen attack. The major phytoalexins in rice are diterpenoids such as oryzalexins, momilactones, and phytocassanes. We measured the amount of oryzalexin A in leaves irradiated by UV light, treated with jasmonic acid, or inoculated with conidia of Bipolaris oryzae in the japonica cultivar Nipponbare and the indica cultivar Kasalath. Nipponbare leaves accumulated oryzalexin A at a high concentration, but Kasalath leaves did not. The locus responsible for this difference was mapped using backcrossed inbred lines and chromosome substitution lines. A region on Chr. 12 containing the KSL10 gene was responsible for the deficiency in oryzalexin A in the Kasalath cultivar. The amount of KSL10 transcript increased in Nipponbare leaves but not in Kasalath leaves in response to UV light irradiation, indicating that the suppressed expression of KSL10 caused the deficiency of oryzalexin A in Kasalath. We analyzed oryzalexin A accumulation in UV light-irradiated leaves of cultivars in the world rice core collection. There were cultivars that accumulated oryzalexin A and those that did not, and both of these chemotypes were found in japonica and indica subspecies. Furthermore, these chemotypes were found in the wild rice species Oryza rufipogon. The phylogenetic relationship of KSL10 sequences was not correlated to oryzalexin A chemotypes. These findings suggested that the biosynthesis of oryzalexin A was acquired by a common ancestor of O. rufipogon and was lost multiple times during the evolutionary process.

Induction of defense responses by extracts of spent mushroom substrates in rice.

We investigated the effect of treatment with hot water extracts from the spent mushroom substrates (SMSs) of Lentinula edodes and Hypsizygus marmoreus on the resistance of rice leaves to Pyricularia oryzae infection. The spraying of the SMS extracts clearly suppressed the development of lesions caused by Py. oryzae infection. The accumulation of phytoalexins momilactones A and B, oryzalexin A, and sakuranetin was markedly induced by the spraying of extracts. The enhanced expression of defense related genes PR1b and PBZ was also found in leaves sprayed with the extracts. Treatments with the extracts also affected phytohormone levels. The levels of N 6-(Δ2-isopentenyl)adenine and trans-zeatin markedly increased in response to treatment, whereas the levels of salicylic and jasmonic acids were largely unchanged.