PHOTOCONTROL OF NITRITE REDUCTASE GENE-EXPRESSION IN THE BARLEY SEEDLING (HORDEUM-VULGARE L)

Nitrite reductase (NIR, EC 1.7.7.1) - a key enzyme of nitrate assimila tion - is known to be induced by nitrate and light. In the present wor k with barley (Hordeum vulgare L.) seedlings, control of NIR gene expr ession by light and its dependency on intact plastids was studied. The major results were as follows: (i) It was confirmed that a single iso form of NIR occurs in shoot and root. (ii) Any light-mediated increase of NIR activity is strictly correlated with an increase in cross-reac ting material. (iii) Exogenous nitrate is absolutely required for NIR synthesis and the appearance of NIR transcripts. The nitrate effect is saturated at 10 mM. (iv) Light-pulse experiments and long-term light treatments indicate the involvement of phytochrome in the light respon se. (v) While a small light effect possibly occurs in the root, a stro ng action of phytochrome was observed in the shoot. (vi) Photooxidativ e treatments of plastids led to a strong decrease of the NIR level and almost abolished the appearance of NIR transcripts. (vii) With plasti ds intact, application of nitrate (10 mM) caused the appearance of a c onsiderable transcript level even in darkness. Light treatments in the presence of nitrate stimulated the transcript level further. (viii) S ince a correlation between transcript level and rate of NIR increase w as found it was concluded that in barley - in contrast to mustard, spi nach and tobacco - NIR gene expression is controlled predominantly, if not exclusively, at the level of transcript accumulation.