Phenotypic expression of blast resistance gene Pi54 is not affected by its chromosomal position

Characterization and Evaluation of Transgenic Rice Pyramided with the Pi Genes Pib, Pi25 and Pi54

BACKGROUND

Emergence of new pathogen strains of Magnaporthe oryzae is a major reason for recurrent failure of the resistance mediated by a single resistance gene (Pi) in rice. Stacking various Pi genes in the genome through marker-assisted selection is thus an effective strategy in rice breeding for achieving durable resistance against the pathogen. However, the effect of pyramiding of multiple Pi genes using transgenesis still remains largely unknown.

RESULTS

Three Pi genes Pib, Pi25 and Pi54 were transferred together into two rice varieties, the indica variety Kasalath and the japonica variety Zhenghan 10. Transgenic plants of both Kasalath and Zhenghan 10 expressing the Pi transgenes showed imparted pathogen resistance. All the transgenic lines of both cultivars also exhibited shorter growth periods with flowering 2-4 days early, and shorter plant heights with smaller panicle. Thus, pyramiding of the Pi genes resulted in reduced grain yields in both rice cultivars. However, tiller numbers and grain weight were generally similar between the pyramided lines and corresponding parents. A global analysis of gene expression by RNA-Seq suggested that both enhancement and, to a lesser extent, inhibition of gene transcription occurred in the pyramided plants. A total of 264 and 544 differentially expressed genes (DEGs) were identified in Kasalath and Zhenghan 10, respectively. Analysis of the DEGs suggested that presence of the Pi transgenes did not alter gene expression only related to disease resistance, but also impacted many gene transcriptions in the pathways for plant growth and development, in which several were common for both Kasalath and Zhenghan 10.

CONCLUSION

Pyramiding of the Pi genes Pib, Pi25 and Pi54 via transgenesis is a potentially promising approach for improving rice resistance to the pathogen Magnaporthe oryzae. However, pleiotropic effects of the Pi genes could potentially result in yield loss. These findings support the idea that immunity is often associated with yield penalties. Rational combination of the Pi genes based on the genetic background may be important to balance yield and disease resistance.

Deciphering the role of microRNAs during Pi54 gene mediated Magnaporthe oryzae resistance response in rice

The broad-spectrum resistance gene Pi54 confers resistance to multiple isolates of Magnaporthe oryzae in rice. In order to decipher the molecular mechanism underlying the Pi54 mediated resistance in rice line Taipei309 ^Pi54 (carrying Pi54), miRNAome study was performed at 24 h post-inoculation (hpi) with M. oryzae. A total of 222 known miRNAs representing 101 miRNA families were found in this study. Of these, 29 and 24 miRNAs were respectively up- and down-regulated in the resistant Taipei309 ^Pi54 . Defence response (DR) genes, like, NBSGO35, and OsWAK129b, and genes related to transcription factors were up-regulated in Taipei309 ^Pi54 line. The vast array of miRNA candidates identified here are miR159c, miR167c, miR2100, miR2118o, miR2118l, miR319a, miR393, miR395l, miR397a, miR397b, miR398, miR439g, miR531b, miR812f, and miR815c, and they manifest their role in balancing the interplay between various DR genes during Pi54 mediated resistance. We also validated miRNA/target gene pairs involved in hormone signalling, and cross-talk among hormone pathways regulating the rice immunity. This study suggests that the Pi54 gene mediated blast resistance is influenced by several microRNAs through PTI and ETI components in the rice line Taipei309 ^Pi54 , leading to incompatible host-pathogen interaction.

Blast resistance gene Pi54 over-expressed in rice to understand its cellular and sub-cellular localization and response to different pathogens

Rice blast resistance gene, Pi54 provides broad-spectrum resistance against different strains of Magnaporthe oryzae. Understanding the cellular localization of Pi54 protein is an essential step towards deciphering its place of interaction with the cognate Avr-gene. In this study, we investigated the sub-cellular localization of Pi54 with Green Fluorescent Protein (GFP) as a molecular tag through transient and stable expression in onion epidermal cells (Allium cepa) and susceptible japonica cultivar rice Taipei 309 (TP309), respectively. Confocal microscopy based observations of the onion epidermal cells revealed nucleus and cytoplasm specific GFP signals. In the stable transformed rice plants, GFP signal was recorded in the stomata, upper epidermal cells, mesophyll cells, vascular bundle, and walls of bundle sheath and bulliform cells of leaf tissues. These observations were further confirmed by Immunocytochemical studies. Using GFP specific antibodies, it was found that there was sufficient aggregation of GFP::Pi54protein in the cytoplasm of the leaf mesophyll cells and periphery of the epidermal cells. Interestingly, the transgenic lines developed in this study could show a moderate level of resistance to Xanthomonas oryzae and Rhizoctonia solani, the causal agents of the rice bacterial blight and sheath blight diseases, respectively. This study is a first detailed report, which emphasizes the cellular and subcellular distribution of the broad spectrum blast resistance gene Pi54 in rice and the impact of its constitutive expression towards resistance against other fungal and bacterial pathogens of rice.

Functional validation of the Pi54 gene by knocking down its expression in a blast-resistant rice line using RNA interference and its effects on other traits

Rice blast disease caused by Magnaporthe oryzae is one of the major diseases affecting the rice (Oryza sativa L.) crop. A major blast resistance gene, Pi54, has already been cloned and deployed in different rice varieties. To understand the role of Pi54 in providing rice blast resistance, we used the RNA interferences (RNAi) approach to knock down the expression of this gene. We showed a high frequency of Agrobacterium tumefaciens-mediated transformation of rice line Taipei 309 containing a single gene (Pi54) for blast resistance. Pi54 RNAi leads to a decreased level of Pi54 transcripts, leading to the susceptibility of otherwise M. oryzae-resistant rice lines. However, among the RNAi knockdown plants, the severity of blast disease varied between the lines. Histochemical analysis of the leaves of knockdown plants inoculated with M. oryzae spores also showed typical cell death and blast lesions. Additionally, Pi54 RNAi also showed an effect on the Hda3 gene, a florigen gene playing a role in rice flowering. By using the RNAi technique, for the first time, we showed that the directed degradation of Pi54 transcripts results in a significant reduction in the rice blast resistance response, suggesting that RNAi is a powerful tool for functional validation of genes.

Rice Improvement Through Genome-Based Functional Analysis and Molecular Breeding in India

Rice is one of the main pillars of food security in India. Its improvement for higher yield in sustainable agriculture system is also vital to provide energy and nutritional needs of growing world population, expected to reach more than 9 billion by 2050. The high quality genome sequence of rice has provided a rich resource to mine information about diversity of genes and alleles which can contribute to improvement of useful agronomic traits. Defining the function of each gene and regulatory element of rice remains a challenge for the rice community in the coming years. Subsequent to participation in IRGSP, India has continued to contribute in the areas of diversity analysis, transcriptomics, functional genomics, marker development, QTL mapping and molecular breeding, through national and multi-national research programs. These efforts have helped generate resources for rice improvement, some of which have already been deployed to mitigate loss due to environmental stress and pathogens. With renewed efforts, Indian researchers are making new strides, along with the international scientific community, in both basic research and realization of its translational impact.