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.