<B>ABSTRACT</B><P>The myristoylated calcium sensor SOS3 and its interacting protein kinase, SOS2, play critical regulatory roles in salt tolerance. Mutations in either of these proteins render <I>Arabidopsis thaliana</I> plants hypersensitive to salt stress. We report here the isolation and characterization of a mutant called <I>enh1-1</I> that enhances the salt sensitivity of <I>sos3-1</I> and also causes increased salt sensitivity by itself. <I>ENH1</I> encodes a chloroplast-localized protein with a PDZ domain at the N-terminal region and a rubredoxin domain in the C-terminal part. Rubredoxins are known to be involved in the reduction of superoxide in some anaerobic bacteria. The <I>enh1-1</I> mutation causes enhanced accumulation of reactive oxygen species (ROS), particularly under salt stress. ROS also accumulate to higher levels in <I>sos2-1</I> but not in <I>sos3-1</I> mutants. The <I>enh1-1</I> mutation does not enhance <I>sos2-1</I> phenotypes. Also, <I>enh1-1</I> and <I>sos2-1</I> mutants, but not <I>sos3-1</I> mutants, show increased sensitivity to oxidative stress. These results indicate that ENH1 functions in the detoxification of reactive oxygen species resulting from salt stress by participating in a new salt tolerance pathway that may involve SOS2 but not SOS3.</P>
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