MicroRNAs against fungal pathogens
Like the other model plants, the two-layered immune system (PTI and ETI) have been shown to play important roles in the defense mechanisms against fungal pathogens such as Magnaporthe oryzae in rice (Chen & Ronald 2011). The Pattern Recognition Receptors (PRRs) such as CEBiP, LYP4, and LYP6 are known to recognize the pathogen-associated molecular patterns (PAMP) and induce the PAMP-triggered immunity (Shimizu et al. 2010; Liu et al. 2012a). Whereas, the products of R-genes recognize divergent pathogen effectors that activate effector triggered immunity (ETI), their functionality for recognition of the target effectors depend on a number of structural features. Most R-genes encode proteins with the nucleotide binding site/ Leucine rich repeat (NBS-LRR) domains (Białas et al. 2018). More than 102 R-genes against the rice blast fungus have been reported. Among them, 31 genes were cloned and functionally validated (Li et al. 2017b; Li, Chern, Yin, Wang & Chen 2019a). Among them, 28 genes encode NBS-LRR domain proteins.Pi5-1 , Pb1 , pi21, and Pi63  genes are pathogen-inducible, while the remaining genes constitutively expressed throughout the life-cycle (Ning, Yunyu & Aihong 2020). Complete or qualitative R-genes are controlled by single gene and are quickly overcome by emergence of new pathotype. Therefore, pyramiding of multiple R-genes in the same background is one of the easiest ways to develop rice cultivars with broad-spectrum resistance. Nonetheless, random combination of multiple R-genes in the same background may not always produce positive response. Unregulated expression of random pyramiding of R-genes causes a negative effect on the plant growth and development. Additionally, pyramiding of R-genes through conventional breeding requires multiple rounds of crossing and selection along with many unwanted genetic drags. Therefore, use of miRNAs provides an alternative strategy to develop broad-spectrum fungal resistance cultivars in rice.