Photocontrol of nitrite reductase gene expression in the barley seedling (Hordeum vulgare L.)

Abstract

Nitrite reductase (NIR, EC 1.7.7.1) — a key enzyme of nitrate assimilation — is known to be induced by nitrate and light. In the present work with barley (Hordeum vulgare L.) seedlings, control of NIR gene expression by light and its dependency on intact plastids was studied. The major results were as follows: (i) It was confirmed that a single isoform of NIR occurs in shoot and root. (ii) Any light-mediated increase of NIR activity is strictly correlated with an increase in cross-reacting 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 response. (v) While a small light effect possibly occurs in the root, a strong action of phytochrome was observed in the shoot. (vi) Photooxidative treatments of plastids led to a strong decrease of the NIR level and almost abolished the appearance of NIR transcripts. (vii) With plastids intact, application of nitrate (10 mM) caused the appearance of a considerable transcript level even in darkness. Light treatments in the presence of nitrate stimulated the transcript level further. (viii) Since a correlation between transcript level and rate of NIR increase was found it was concluded that in barley — in contrast to mustard, spinach and tobacco — NIR gene expression is controlled predominantly, if not exclusively, at the level of transcript accumulation.