Arabidopsis thaliana AtHAL3: a flavoprotein related to salt and osmotic tolerance and plant growth

We have isolated two Arabidopsis thaliana genes, AtHAL3a and AtHAL3b, showi ng homology with HAL3, a yeast protein which regulates the cell cycle and t olerance to salt stress through inhibition of the PPZ1 type-1 protein phosp hatase. Expression of AtHAL3a in yeast hal3 mutants partially complements t heir LiCl sensitivity, suggesting possible conserved functions between both proteins. AtHAL3a and AtHAL3b are induced by salt stress and AtHAL3a is th e most expressed in non-stressed plants, particularly in seeds. In situ hyb ridization demonstrates enrichment of AtHAL3a mRNA in seed embryos and in t he vascular phloem of different plant tissues. AtHAL3 proteins show strikin g homology with a group of proteins found in fungi, plants and animals and some homology with a large family of prokaryotic flavoproteins. Recombinant AtHAL3a protein purified from Escherichia coli was yellow because it conta ined a non-covalently bound chromophore revealed as flavin mononucleotide. Trans- genic Arabidopsis plants, with gain of AtHAL3a function, show altere d growth rates and improved tolerance to salt and osmotic stress.