MOLYBDENUM COFACTOR MUTANTS, SPECIFICALLY IMPAIRED IN XANTHINE DEHYDROGENASE-ACTIVITY AND ABSCISIC-ACID BIOSYNTHESIS, SIMULTANEOUSLY OVEREXPRESS NITRATE REDUCTASE

The molybdenum cofactor is shared by nitrate reductase (NR), xanthine dehydrogenase (XDH), and abscisic acid (ABA) aldehyde oxidase in highe r plants (M. Walker-Simmons, D.A. Kudrna, R.L. Warner [1989] Plant Phy siol 90: 728-733). In agreement with this, cnx mutants are simultaneou sly deficient for these three enzyme activities and have physiological characteristics of ABA-deficient plants. In this report we show that aba1 mutants, initially characterized as ABA-deficient mutants, are im paired in both ABA aldehyde oxidase and XDH activity but overexpress N R. These characteristics suggest that aba1 is in fact involved in the last step of molybdenum cofactor biosynthesis specific to XDH and ABA aldehyde oxidase; aba1 probably has the same function as hxB in Asperg illus. The significance of NR overexpression in aba1 mutants is discus sed.