Abstract
In a particular high-altitude area of Sasebo city, Nagasaki, a unique disorder in paddy rice has been observed since the 1960s. This disorder is called suitou hakare shou in Japanese, and we refer to it here as rice hakare symptom. Despite decades of research on rice hakare symptom, the particular environmental factors and mechanisms causing rice leaf damage are unknown. We therefore conducted a metabalomic analysis with the aim of clarifying the environmental stresses inducing this disorder. We collected healthy and damaged uppermost leaves from rice plants at different developmental stages and analyzed the various stress-related metabolites by means of gas chromatography-mass spectrometry. We found that glucose and fructose, early metabolites in photosynthesis, tended to be greater in rice leaves collected from paddy fields displaying signs of damage from rice hakare symptom, especially in late August. Furthermore, non-targeted metabolic profiles derived from mass spectral data led us to select γ-amino-butyric acid, trehalose, sorbitol, and serotonin as stress-related metabolites. Sorbitol and serotonin, minor components in healthy leaves, were accumulated specifically in damaged leaves at any growth stage, and further increased with development of the leaf lesion. These results indicate the significance of sorbitol and serotonin in rice leaves as biomarkers of rice hakare symptom. Both compounds are generally known to accumulate in response to drought stress or desiccation of plant tissue, suggesting the possibility that changes in leaf-water relations are an important part of the mechanism that causes rice hakare symptom.
