The Arabidopsis LOS5/ABA3 locus encodes a molybdenum cofactor sulfurase and modulates cold stress- and osmotic stress-responsive gene expression

To understand low temperature and osmotic stress signaling in plants, we is olated and characterized two allelic Arabidopsis mutants, los5-1 and los5-2 , which are impaired in gene induction by cold and osmotic stresses. Expres sion of RD29A-LUC (the firefly luciferase reporter gene under the control o f the stress-responsive RD29A promoter) in response to cold and salt/drough t is reduced in the los5 mutants, but the response to abscisic acid (ABA) r emains unaltered. RNA gel blot analysis indicates that the los5 mutation re duces the induction of several stress-responsive genes by cold and severely diminishes or even completely blocks the induction of RD29A, COR15, COR47, RD22, and P5CS by osmotic stresses. los5 mutant plants are compromised in their tolerance to freezing, salt, or drought stress. los5 plants are ABA d eficient, as indicated by increased transpirational water loss and reduced accumulation of ABA under drought stress in the mutant. A comparison with a nother ABA-deficient mutant, aba1, reveals that the impaired low-temperatur e gene regulation is specific to the los5 mutation. Genetic tests suggest t hat los5 is allelic to aba3. Map-based cloning reveals that LOS5/ABA3 encod es a molybdenum cofactor (MoCo) sulfurase. MoCo sulfurase catalyzes the gen eration of the sulfurylated form of MoCo, a cofactor required by aldehyde o xidase that functions in the last step of ABA biosynthesis in plants. The L OS5/ABA3 gene is expressed ubiquitously in different plant parts, and the e xpression level increases in response to drought, salt, or ABA treatment. O ur results show that LOS5/ABA3 is a key regulator of ABA biosynthesis, stre ss-responsive gene expression, and stress tolerance.