Modulation of gonadotrophin-releasing hormone pulse generator activity by the pheromone in small ruminants.

In small ruminants, such as goats and sheep, a primer pheromone produced by males induces an out-of-seasonal ovulation in anoestrous females, a phenomenon known as the male effect. The male effect is unique in that an external chemical stimulus can immediately modulate the activity of the hypothalamic gonadotrophin-releasing hormone (GnRH) pulse generator. We have established a monitoring method of the GnRH pulse generator activity in Shiba goat. Using this method as a sensitive bioassay to assess the male effect pheromone activity, we have shown that the male effect pheromone is synthesised in an androgen-dependent manner in the sebaceous glands or their vicinity in specific body regions in goats. Although chemical identity of the pheromone is yet to be determined, analyses of male goat hair extracts by gas chromatography fractionation suggest that the male effect pheromone is a volatile substance with relatively small molecular weight. From morphological and molecular biological studies in goats, it is suggested that the pheromone molecule is detected by a member of the V1R family located on both the olfactory neurones and the vomeronasal sensory neurones, and the pheromone signal is conveyed to the medial nucleus of amygdala via the main olfactory and vomeronasal pathways and, subsequently, to the hypothalamic GnRH pulse generator to enhance its activity.

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.

Methionine-induced phytoalexin production in rice leaves.

The application of methionine on wounded rice leaves induced the production of rice phytoalexins, sakuranetin and momilactone A. This induction resulted from stimulation of phenylalanine ammonia-lyase and naringenin 7-O-methyltransferase activity. Jasmonic acid, ethylene, and active oxygen species are important as signal transducers in disease resistance mechanisms. However, although the endogenous level of jasmonic acid rapidly increased in reaction to wound, methionine treatment could not induced endogenous JA production. Ethylene induced the production of the flavonoid phytoalexin, sakuranetin, but did not induce the production of a terpenoid phytoalexin, momilactone A. On the other hand, a free radical scavenger, Tiron, counteracted the induction of both sakuranetin and momilactone A production in methionine-treated leaves. Active oxygen species may be important in methionine-induced production of phytoalexins.

Momilactones A and B in Rice Straw Harvested at Different Growth Stages.

Momilactones A and B in rice straw harvested at different growth stages were quantified by HPLC-MS-MS. They increased to their maximal levels at the heading stage and then gradually decreased. In addition, these phytotoxins were found to be relatively easily extracted with water.

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.

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.

Synthesis of 7-methoxyapigeninidin and its fungicidal activity against Gloeocercospora sorghi.

In the structure of sakuranetin, which was isolated as a phytoalexin from the rice plant, the methoxy group at C-7 has been shown to be important for its high activity. Apigeninidin was isolated as a phytoalexin from sorghum, but it had no methoxy group at C-7. We prepared 7-methoxyapigeninidin and compared its fungicidal activity with that of apigeninidin. The 7-methoxyapigeninidin showed higher activity against sorghum fungi than apigeninidin, suggesting that the methoxy group at C-7 was important for the high fungicidal activity.