CLONING AND MAPPING OF A PUTATIVE BARLEY NADPH-DEPENDENT HC-TOXIN REDUCTASE

The NADPH-dependent HC-toxin reductase (HCTR), encoded by Hm1 in maize , inactivates HC-toxin produced by the fungus Cochliobolus carbonum, a nd thus confers resistance to the pathogen. The fact that e. carbonum only infects maize (Zea mays) and is the only species known to produce HC-toxin raises the question: What are the biological functions of HC TR in other plant species? An HCTR-like enzyme may function to detoxif y toxins produced by pathogens which infect other plant species (R. B. Meeley; G. S. Johal, S. E. Briggs, and J. D. Walton, Plant Cell, 4:71 -77, 1992). Hm1 homolog in rice (Y. Hihara, M. Umeda, C. Hara, Q. Liu, S. Aotsuka, K. Toriyama, and H. Uchimiya, unpublished) and HCTR activ ity in barley, wheat, oats and sorghum have been reported (R. B. Meele y and J. D. Walton, Plant Physiol. 97:1080-1086, 1993). To investigate the sequence conservation of Hm1 and HCTR in barley and the possible relationship of barley Hm1 homolog to the known disease resistance gen es, we droned and mapped a barley (Hordeum vulgare) Hm1-like gene. A p utative full-length cDNA clone, Bhm1-18, was isolated from a cDNA libr ary consisting of mRNA from young leaves, inflorescences, and immature embryos. This 1,297-bp clone encodes 363 amino acids which show great similarity (81.6%) with the amino acid sequence of HM1 in maize. Two loci were mapped to barley molecular marker linkage maps with Bhm1-18 as the probe; locus A (Bhm1A) on the long arm of chromosome 1, and loc us B (Bhm1B) on the short arm of chromosome 1 which is syntenic to mai ze chromosome 9 containing the Hm2 locus. The Bhm1-18 probe hybridized strongly to a Southern blot of a wide range of grass species, indicat ing high conservation of HCTR at the DNA sequence level among grasses. The HCTR mRNA was detected in barley roots, leaves, inflorescences, a nd immature embryos. The conservation of the HCTR sequence, together w ith its expression in other plant species (R. B. Meeley and J. D. Walt on, Plant Physiol. 97:1080-1086, 1993), suggests HCTR plays an importa nt functional role in other plant species.