Os02g57310

Rice blast, caused by the fungal pathogen Magnaporthe grisea, is one of the most serious diseases of rice. Os02g57310, namely Magnaporthe grisea resistance-b, pib, the first blast-resistance gene to be cloned belonging to the nucleotide binding site (NBS) and leucine-rich repeat (LRR) superfamily. pib has been mapped to the distal end of the long arm of chromosome 2 (Shinoda et al., 1971),and has been cloned and identified at the molecular level (Wang et al. 1999).

Annotated Information

Gene structure

The Pib gene encodes a polypeptide of 1251 amino acids, it contains an NBS region and C-terminal LRRs, but no distinct transmembrane domain. Thus, the Pib gene belongs to the NBS-LRR class of plant disease resistance genes and is predicted to encode a cytoplasmic protein. Interestingly, the Pib protein contains an N-terminal duplication of the conserved kinase 1a (P-loop), 2 and 3a domains of the NBS region. The duplicated segments shared 32% identity and 55% similarity to each other. It is interesting that only the kinase 1a (P-loop), 2 and 3a domains are duplicated.
The C-terminal LRR domain of Pib contains 17 repeats with similarity to the cytoplasmic LRR consensus sequence. Interestingly, there are eight cysteine residues clustered in LRR 7 and 8 which have not been reported for other R genes. Of these, two fall at LRR consensus positions indicating that they are likely to be buried in the hydrophobic core of the protein.

Function

The inheritance of host resistance to rice blast has been studied extensively. At least 68 resistance loci have been identified in rice. Many genes for blast resistance have been mapped by using restriction fragment length polymorphism (RFLP) markers.
The rice blast resistance genes confer a high race specific resistance to blast fungus races. The specific interaction between resistance genes in rice plant and avirulence genes in the fungal pathogen can be well explained by the gene-for-gene system.
Pib, on chromosome 2, shows resistant reaction to 5 of 20 standard differential blast isolates from Philippines selected by Telebanco-Yanoria et al. (2008).
Another rice blast resistance genes, Pish, was identified by Imbe and Matsumoto from a Japonica-type rice variety, Shin 2. As the monogenic resistance line with Pish developed by Tsunematsu et al showed a moderate and a broad spectrum resistant reaction to the isolates from Philippines. There has been a research confirming the additive effect of Pish and Pib in the pyramided lines by their reaction patterns to blast isolates, suggesting the potential availabilities of the combinations of these genes.

Expression

The expression of Pib was investigated by Northern blot analysis. Tohoku IL9 and Sasanishiki plants were inoculated with rice blast incompatible strain ARCP90±18C. Pib mRNA was detected in all Tohoku IL9 samples, but not in Sasanishiki samples in both cases. Pib was expressed at a low level at the zero time point, was further induced at 12 and 24 h after inoculation (HAI) with incompatible strain.
There has been extensive investigation of the expression of the Pib gene family (Pib, PibH8, HPibH8-1, HPibH8-2) under various environmental and chemical treatments. PibH8 shows 42% identity and 60% similarity to Pib and, like Pib, has a duplication of the kinase 1a, 2, and 3a motifs of the NBS region in the N-terminal half of the protein. Interestingly, genes of the Pib family exhibit a diurnal rhythm of expression. Their expression was regulated dramatically by environmental signals, such as temperature, light and water availability. Their expression was also induced by chemical treatments, such as jasmonic acid, salicylic acid, ethylene and probenazole. The findings suggest that expression of the Pib gene family is up-regulated by environmental conditions that would favour pathogen infection. This may reflect the evolution of anticipatory control of R gene expression.
Expression of Pib was low under non-challenged conditions, but strongly induced by the blast-causing fungal pathogen Magnaporthe grisea, thereby conferring resistance to the pathogen. It is generally established that cytosine methylation of the promoter-region often plays a repressive role in modulating expression of the gene in question. Two critical regions of the Pib promoter were heavily CG cytosine-methylated in both cultivars studied.Surprisingly, induced expression of Pib by M. grisea infection did not entail its promoter demethylation,and partial demethylation by 5-azacytidine-treatment actually reduced Pib expression relative to wildtype plants. Accordingly, the blast disease-resistance was compromised in the 5’-azaC-treated plants relative to wild-type. In contrast, the disease susceptibility was not affected by the 5’-azaC treatment in another two rice cultivars that did not contain the Pib gene, ruling out effects of other R genes and non-specific genotoxic effects by the drug-treatment as a cause for the compromised Pib-conditioned blast-resistance. Taken together, these results suggest that promoter DNA methylation plays a novel enhancing role in conditioning high-level of induced expression of the Pib gene in times of M. grisea infection, and its conferred resistance to the pathogen.

References

1.Shinoda, H., Toriyama, K., Yunoki, T., Ezuka, A. and Sakurai, Y., 1971, Studies in the varietal resistance of rice to blast. 6. Linkage relationship of blast resistance genes (in Japanese with English summary) Bull. Chugoku Agric. Exp. Stn. Ser. A, 20, 1-25.
2.Wang, Z. X., M. Yano, U. Yamanouchi, M. Iwamoto, L. Monna, H. Hayasaka, Y. Katayose, and T. Sasaki, 1999, The Pib gene for riceblast resistance belongs to the nucleotide binding and leucine-rich repeat class of plant disease resistance genes. Plant J. 19, 55—64.
3.Y. Koide;A. Kawasaki;M. J. Telebanco-Yanoria;A. Hairmansis;N. T. M. Nguyet;J. Bigirimana;D. Fujita;N. Kobayashi;Y. Fukut, 2010, Development of pyramided lines with two resistance genes, Pish and Pib, for blast disease (Magnaporthe oryzae B. Couch) in rice (Oryza sativa L.)Plant Breeding, 129(6): 670-675</ref> 4.Wang Z X，Yamanouchi U，Katayose Y，et al., 2001, Expression of the pib rice blast resistance gene family is up － regulated by environmental conditions favoring infection and by chemical signals that trigger secondary plant defenses［J］ ． Plant Molecular Biology， 47: 653－ 661．
5.Yuan Li, Qiong Xia, Hongping Kou, Dan Wang, Xiuyun Lin, Ying Wu, Chunming Xu, Shaochen Xing and Bao Liu, 2011, Induced Pib Expression and Resistance to Magnaporthe grisea are Compromised by Cytosine Demethylation at Critical Promoter Regions in Rice, J Integr Plant Biol, doi: 10.1111/