Three Npr1 - Of The Rice-like Gene Cloning And Functional Studies

Three NPR1-like genes were isolated from the rice genome using the conserved domain comparison of the Arabidopsis AtNPR1, and named as OsNPR1(the same to previously report NH1), OsNPR2, and OsNPR3, respectively. OsNPR1 showed the highest identity to AtNPR1. Expression pattern and induction of the three genes by defense hormones were studied. OsNPR1 and OsNPR3 were induced by BTH, ET and JA , while OsNPR2 was induced by BTH but suppressed by JA. OsNPR1 was stronger induced in the incompatible interactions of rice-Magnaporthe grisea and rice-Xanthomonas oryzae pv.oryzar (Xoo) than in the compatible interactions. Overexpression of genomic DNA and cDNA clone of three genes showed that OsNPR1-OX rice plants exhibited enhanced disease resistance against Xoo, the resistance to Xoo was expression level-dependent. Interestingly, OsNPR3-OX plants showed high tolerant to salinity. OsNPR1-OX transgenic rice displayed a phenotype of spontaneous cell death and slightly dwarfism. OsNPR1 could complement the Arabidopsis npr1-1 mutant, confirming that OsNPR1 was the ortholog of AtNPR1 in rice.OsNPR1-GFP was located to the cytosol of transgenic rice. OsNPR1(C76A+C216A) mutants were constructed with site mutagenesis of cysteine residues proposed responsible for NPR1 polymerization. OsNPR1 (C76A+C216A) -GFP monomerized and immigrated to the nucleus. The OsNPR1-GFP-OX and OsNPR1(C76A+C216A)-GFP-OX transgenic rice showed no cell death, and less enhanced resistance to Xoo in comparision with OsNPR1-OX Plants. The regulation of these transgenic constructs to downstream PR genes expression remained the same: OsNPR1-OX and OsNPR1-GFP-OX reduced expression of the genes that are inducible by JA. The expression of the genes regulated by SA/BTH in OsNPR1(C76A+C216A) -GFP-OX transgenic rice was increased. These results demonstrated that there was a pathway in rice similar to that of dicotyledon Arabidopsis where NPR1 act as a key regulator and enhanced SA pathway would inhibit the JA pathway. However, difference was found between rice and Arabidopsis concerning the SA function in disease resistance where the OsNPR1-mediated disease resistance might be a consequence of the OsNPR1-OX induced cell death.OsNPR1 expression in rice subcellar sites regulated the concentration both free and conjugated SA, indicating that OsNPR1 plays a role in SA synthesis and accumulation in rice.