Characterization and Genetic Analysis of a Novel Rice Spotted-leaf Mutant HM47 with Broad-spectrum Resistance to Xanthomonas oryzae pv. oryzae

A stable inherited rice spotted-leaf mutant HM47 derived from an EMS-induced IR64 mutant bank was identified. The mutant expressed hypersensitive response （HR）-like symptoms throughout its whole life from the first leaf to the flag leaf, without pathogen invasion. Initiation of the lesions was induced by light under natural summer field conditions. Expression of pathogenesis-related genes including PAL, PO-C1, POX22.3 and PBZ1 was enhanced significantly in association with cell death and accumulation of H2O2 at and around the site of lesions in the mutant in contrast to that in the wild-type （WT）. Disease reaction to Xanthomonas oryzae pv. oryzae from the Philippines and China showed that HM47 is a broad-spectrum disease-resistant mutant with enhanced resistance to multiple races of bacterial blight pathogens tested. An F2 progeny test showed that bacterial blight resistance to race HB-17 was cosegregated with the expression of lesions. Genetic analysis indicated that the spotted-leaf trait was controlled by a single recessive gene, tentatively named spl HM47 , flanked by two insertion/deletion markers in a region of approximately 74 kb on the long arm of chromosome 4. Ten open reading frames are predicted, and all of them are expressed proteins. Isolation and validation of the putative genes are currently underway.

Characterization and Genetic Analysis of a Light-and Temperature-sensitive Spotted-leaf Mutant in Rice

A rice spotted-leaf mutant was isolated from an ethane methyl sulfonate （EMS）-induced IR64 mutant bank. The mutant, designated as spl30 （spotted-leaf 30）, displayed normal green leaf color under shade but exhibited red-brown lesions under natural summer field conditions. Initiation of the lesions was induced by light and the symptom was enhanced at 33 ° C relative to 26°C. Histochemical staining did not show cell death around the red-brown lesions. Chlorophyll contents in the mutant were significantly lower than those of the wild type while the ratio of chlorophyll a/b remained the same, indicating that spl30 was impaired in biosynthesis or degradation of chlorophyll. Disease reaction patterns of the mutant to Xanthomonas oryzae pv. oryzae were largely unchanged to most races tested except for a few strains. Genetic analysis showed that the mutation was controlled by a single recessive gene, tentatively named spl30（t）, which co-segregated with RM15380 on chromosome 3, and was delimited to a 94 kb region between RM15380 and RM15383. Spl30（t） is likely a novel rice spotted-leaf gene since no other similar genes have been identified near the chromosomal region. The genetic data and recombination populations provided in this study will enable further fine-mapping and cloning of the gene.

Single base substitution in Os CDC48 is responsible for premature senescence and death phenotype in rice

A premature senescence and death 128 （psd128） mutant was isolated from an ethyl methane sulfonate-induced rice IR64 mutant bank. The premature senescence phenotype appeared at the six-leaf stage and the plant died at the early heading stage, psd128 exhibited impaired chloroplast develop- ment with significantly reduced photosynthetic ability, chlorophyll and carotenoid contents, root vigor, soluble protein content and increased malonaldehyde content. Furthermore, the expression of senescence-related genes was significantly altered in psd128. The mutant trait was controlled by a single recessive nuclear gene. Using map- based strategy, the mutation Oryza sativa cell division cycle 48 （OsCDC48） was isolated and predicted to encode a putative AAA-type ATPase with 809 amino-acid residuals. A single base substitution at position C2347T in psd128 resulted in a premature stop codon. Functional complementation could rescue the mutant phenotype. In addition, RNA interference resulted in the premature senescence and death phenotype. OsCDC48 was expressed constitutively in the root, stem, leaf and panicle. Subcellular analysis indicated that OsCDC48：YFP fusion proteins were located both in the cytoplasm and nucleus. OsCDC48 was highly conserved with more than 90% identity in the protein levels among plant species. Our results indicated that the impaired function of OsCDC48 was responsible for the premature senescence and death phenotype.

A substitution mutation in OsPELOTA confers bacterial blight resistance by activating the salicylic acid pathway

We previously reported a spotted-leaf mutant pelota（originally termed HM_（47）） in rice displaying arrested growth and enhanced resistance to multiple races of Xanthomonas oryzae pv. oryzae. Here, we report the mapbased cloning of the causal gene OsPELOTA（originally termed spl^（HM47））. We identified a single base substitution from T to A at position 556 in the coding sequence of OsPELOTA, effectively mutating phenylalanine to isoleucine at position 186 in the translated protein sequence. Both functional complementation and over-expression could rescue the spotted-leaf phenotype. OsPELOTA, a paralogue to eukaryotic release factor 1（eRF_1）, shows high sequence similarity to Drosophila Pelota and also localizes to the endoplasmic reticulum and plasma membrane.OsPELOTA is constitutively expressed in roots, leaves,sheaths, stems, and panicles. Elevated levels of salicylic acid and decreased level of jasmonate were detected in the pelota mutant. RNA-seq analysis confirmed that genes responding to salicylic acid were upregulated in the mutant. Our results indicate that the rice PELOTA protein is involved in bacterial leaf blight resistance by activating the salicylic acid metabolic pathway.