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A quick incorporation of 13N into the soluble high-molecular compound in rice (Oryza sativa L.) roots by application of 13N-labeled nitrate/nitrite

The nitrate/nitrite response, which occurs within minutes for the gene expression of the primary nitrogen metabolism in higher-plant roots, has almost been described, but the intermediate signaling molecule that exerts in signal transduction remains to be elucidated. It is hypothesized that the constitutively present signaling molecule may bind quickly to exogenously applied nitrate/nitrite to transduce the nitrate/nitrite signal. To examine the quick binding of nitrate/nitrite to high-molecular compounds, which may include intermediate signaling molecules, in the roots of the rice plant (Oryza sativa L.), the segmented roots of rice seedlings were exposed to nitrogen-13 (13N)-labeled nitrate/nitrite for either 2 or 5 min at 25°C. The supernatant of the Tris-buffer root extract was separated by a desalting pre-packed column. The incorporation of 13N into the high-molecular fraction was identified by 13N feeding for 2 min, and by 13N feeding for 5 min, however, the amount of 13N incorporation was not increased. Although the findings are preliminary, it was clearly demonstrated that 13N-labeled nitrate/nitrite was quickly incorporated into a limited amount of the soluble high-molecular compound(s). The formation of NO x -bound proteins, which may play a role in nitrate/nitrite signal transduction, is discussed.