UVB sensitivity and cyclobutane pyrimidine dimer (CPD) photolyase genotypes in cultivated and wild rice species.

We investigated the UVB-sensitivity in 12 rice strains belonging to two cultivated species (O. sativa and O. glaberrima) and three wild species (O. barthii, O. meridionalis and O. rufipogon) of rice possessing the AA genome, while focusing on the CPD photolyase activity and the genotypes of CPD photolyase. Although the UVB sensitivity, CPD photolyase activity, and CPD photolyase genotype varied widely among these rice species, the sensitivity to UVB radiation depended on the activity of the CPD photolyase, regardless of grass shape, habitat, or species. The rice strains examined here clearly divided into three groups based on the CPD photolyase activity, and the activity of the strains greatly depended on amino acid residues at positions 126 and 296, with the exception of the W1299 strain (O. meridionalis). The amino acid residues 126 and 296 of CPD photolyase in Sasanishiki strain (O. sativa), which showed higher enzymatic activity and more resistance to UVB, were glutamine (Gln) and Gln, respectively. An amino acid change at position 126 from Gln to arginine ("Nori"-type) in the photolyase led to a reduction of enzymatic activity. Additionally, an amino acid change at position 296 from Gln to histidine led to a further reduction in activity. The activity of the W1299 strain, which possesses a "Nori"-type CPD photolyase, was the highest among the strains examined here, and was similar to that of the Sasanishiki. The CPD photolyase of the W1299 contains ten amino acid substitutions, compared to Sasanishiki. The alterations in amino acid residues in the W1299 CPD photolyase compensated for the reduction in activity caused by the amino acid substitutions at positions 126. Knowledge of the activity of different CPD photolyase genotypes will be useful in developing improved rice cultivars.

Spontaneously occurring mutations in the cyclobutane pyrimidine dimer photolyase gene cause different sensitivities to ultraviolet-B in rice.

Sensitivity to ultraviolet-B (UVB) radiation (280-320 nm) varies widely among rice cultivars. We previously indicated that UV-resistant rice cultivars are better able to repair cyclobutane pyrimidine dimers (CPDs) through photorepair than are UV-sensitive cultivars. In this paper, we report that UVB sensitivity in rice, in part, is the result of defective CPD photolyase alleles. Surjamkhi (indica) exhibited greater sensitivity to UVB radiation and was more deficient in CPD photorepair ability compared with UV-resistant Sasanishiki (japonica). The deficiency in CPD photorepair in Surjamkhi resulted from changes in two nucleotides at positions 377 and 888 in the photolyase gene, causing alterations of two deduced amino acids at positions 126 and 296 in the photolyase enzyme. A linkage analysis in populations derived from Surjamkhi and Sasanishiki showed that UVB sensitivity is a quantitative inherited trait and that the CPD photolyase locus is tightly linked with a quantitative trait locus that explains a major portion of the genetic variation for this trait. These results suggest that spontaneously occurring mutations in the CPD photolyase gene cause different degrees of sensitivity to UVB in rice, and that the resistance of rice to UVB radiation could be increased by increasing the photolyase function through conventional breeding or bioengineering.

Ultraviolet-B sensitivities in Japanese lowland rice cultivars: cyclobutane pyrimidine dimer photolyase activity and gene mutation.

There is a cultivar difference in the response to ultraviolet-B (UVB: 280-320 nm) in rice (Oryza sativa L.). Among Japanese lowland rice cultivars, Sasanishiki, a leading Japanese rice cultivar, is resistant to the damaging effects of UVB while Norin 1, a close relative, is less resistant. We found previously that Norin 1 was deficient in cyclobutane pyrimidine dimer (CPD) photorepair ability and suggested that the UVB sensitivity in rice depends largely on CPD photorepair ability. In order to verify that suggestion, we examined the correlation between UVB sensitivity and CPD photolyase activity in 17 rice cultivars of progenitors and relatives in breeding of UV-resistant Sasanishiki and UV-sensitive Norin 1. The amino acid at position 126 of the deduced amino acid sequence of CPD photolyase in cultivars including such as Norin 1 was found to be arginine, the CPD photolyase activities of which were lower. The amino acid at that position in cultivars including such as Sasanishiki was glutamine. Furthermore, cultivars more resistant to UVB were found to exhibit higher photolyase activities than less resistant cultivars. These results emphasize that single amino acid alteration from glutamine to arginine leads to a deficit of CPD photolyase activity and that CPD photolyase activity is one of the main factors determining UVB sensitivity in rice.