Accumulation of Indole‐3‐Acetic Acid in Rice sl Mutant Leaves Infected with Bipolaris oryzae

Abstract

Rice leaves accumulate serotonin in response to infection by Bipolaris oryzae. The leaves of the sl mutant, which is deficient in the gene encoding tryptamine 5-hydroxylase, accumulate tryptamine instead of serotonin upon infection by B. oryzae. Because tryptamine is a possible precursor of indole-3-acetic acid (IAA), we investigated the accumulation of IAA in sl leaves infected with B. oryzae. Liquid chromatography coupled with tandem mass spectrometry analysis indicated that IAA accumulated at approximately 1.5 lmol/gFW in the leaves of sl mutant. This accumulation was suppressed by 95% by the treatment with the tryptamine decarboxylase inhibitor, (S)-a-(fluoromethyl)tryptophan, at 100 lM, indicating that tryptamine served as the precursor of IAA. The accumulation of IAA was not reproduced by treatment with CuCl 2 or by exogenous feeding of tryptamine. Furthermore, inoculation of Magnaporthe grisea induced only a lower level of IAA accumulation. On the other hand, B. oryzae produced IAA in culture media containing tryptamine. These findings strongly suggested that the metabolism of tryptamine by B. oryzae was responsible for IAA accumulation in the leaves of the sl mutant. Serotonin added to the culture media was also converted into 5-hydroxyindole-3-acetic acid (5HIAA) at a rate similar to that of tryptamine. Considering that wild-type rice leaves accumulate serotonin for defensive purposes, reducing the concentration of serotonin by conversion into 5HIAA may be significant as a detoxification process in the interaction between B. oryzae and rice.

An Insight into the Roles of Dietary Tryptophan and Its Metabolites in Intestinal Inflammation and Inflammatory Bowel Disease

“…[25,79,99] The free IAA is associated with basal immunity, tolerance, and disease resistance in rice, grape, and cassava. [109][110][111] For instance, prevention of accumulating free IAA in rice regulated the resistance of rice to bacterial blight, and thus it promoted the basal immunity in rice. [112] The tryptamine has been confirmed to have the ability to activate the serotonergic receptor-mediated signaling pathway and thereby it affects immune system.…”

Natural variation in the expression and catalytic activity of a naringenin 7‐O‐methyltransferase influences antifungal defenses in diverse rice cultivars

“…Gibberella fujikuroi OB-178 was a stock from the Laboratory of Natural Product Chemistry, Faculty of Agriculture, Tottori University. Conidia of P. oryzae and B. oryzae were prepared according to the method described by Ishihara et al (2016). Gibberella fujikuroi was inoculated onto potato dextrose agar plates and cultured with UV irradiation for 4 weeks.…”

Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation

“…Rice leaves accumulated serotonin ( 259 ) in response to infection by Bipolaris oryzae . If serotonin ( 259 ) was added to the culture media, it was converted into 5-hydroxyindole-3-acetic acid (5HIAA), which may be a detoxification process in the interaction between B. oryzae and rice [196].…”