The appearance of glutamine synthetase in turions ofSpirodela polyrhiza(L.) Schleiden as regulated by blueand red light, nitrate and ammonium

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Abstract
Control by light and nitrogen (nitrate and ammonium) of the appearance of glutamine synthetase (GS; EC 6.3.1.2) in turions of Spirodela polyrhiza (L.) Schleiden, strain SJ, was investigated during the pregermination period, i.e. up to 48 h after onset of light. Immediately after transfer from after-ripening conditions (5°C, darkness, D) to germination conditions (25°C), GS activity did not respond to light or nitrate. After 72 h in D (25°C) activity increased in continuous light. Therefore, the regulatory role of light, nitrate and ammonium in the process of appearance of GS was mainly studied between 72 and 120 h after transfer from after-ripening to germination conditions (phase II of the pre-germination process). The inducing effect of red light is mediated by the photoreceptor phytochrome: the effect of long-term continuous red light (6 or 24 h) can be reversed, at least in part, by a subsequent far-red light pulse (‘end of day’ Irradiation). Blue light is more effective than red light in inducing the appearance of GS. Therefore, a specific blue light effect has to be assumed. This represents a novel mode of light action in regulating the level of the ammonium assimilating enzyme in an angiosperm system. lmmunoblots showed that (i) increase in the enzymatic activity is caused by de novo synthesis of the enzyme protein, (ii) two different subunits (38 and 42 kDa) contribute to the total activity which must be attributed to two different isofornis. In accordance with results from other higher plants, the 38 kDa subunit (presumably related to the cytosolic isoform) did not increase in the presence of light, whereas the 42 kDa subunit (presumably related to the plastidic isoform) was induced. The maximal enzyme level was reached only in the presence of both light (blue light) and nitrate. Light induction was also observed in the presence of ammonium; however, GS activity was decreased, when compared to nitrate-treated turions. Comparison of these results with previous observations suggest that the influence of light and nitrate on the germination response and regulation of the nitrate/ammonium assimilation pathway in turions appear to be unrelated phenomena.

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