Nitrate reduction in the wildtype and a nitrate reductase deficient mutant of Arabidopsis thaliana

Abstract

A chlorate‐resistant mutant B25 of Arabidopsis thaliana (L.) Heinh. was isolated, which has very little or no in vitro nitrate reductase activity and grows poorly on a substrate with nitrate as the sole nitrogen source. The mutation of B25 ( rgn ) is monogenic and recessive, tightly linked to the marker gene an on chromosome 1. Nitrate induces cytochrome‐ c reductase activity in the mutant but to a lower level than in the wildtype. After sucrose gradient centrifugation the greatest part of the cytochrome‐ c reductase from induced wildtype is found as 8s type whereas cytochrome‐ c reductase from B25 under the same conditions is found as 4s type. Nitrate reductase is found at the 8s position. It is suggested that B25 has lost the ability to assemble two 4s subunits showing cytochrome‐ c reductase activity and a Mo‐bearing co‐factor into the functional nitrate reductase. Nitrate rather than nitrite is the inducing agent for nitrite reductase, since in B25 nitrite reductase is even more rapidly induced than in the wildtype after addition of nitrate. Both the wildtype and B25 contain a nitrate reductase inhibiting factor when grown on ammonium. This inhibiting factor is a small protein, possibly similar to the nitrate reductase inactivating enzyme reported for other plants.