MicroRNAs against bacterial pathogens
A number of miRNAs involved in the regulation of rice immunity against bacterial pathogens have been identified. Of these, five miRNAs have been shown to modulate bacterial blight (BB) disease caused byXanthomonas oryzae pv. oryzae (Xoo ) and one miRNA has been shown to positively regulate bacterial foot rot disease caused by Dickeya zeae . Of the miRNAs regulating BB, Osa-miR159bacts in positive manner, while the others are involved in negative regulation. The Osa-miR159b enhances resistance to BB by repressing its target gene OsGAMYB, with concomitant effects in terms of reduced nitrogen concentration and significantly repressed activity of the GA signaling pathway (Jia et al. 2020).
The Osa-miR167 has also been shown to be involved in rice immunity (Li et al. 2010; Yao, Withers & He 2013). TheOsa-miR167d abates the rice immunity upon Xanthomonas oryzae pv. oryzae (Xoo) infection by suppressing the target gene OsWD40-174, which function in maintaining the downregulation of OsKNOX gene during leaf development (Jiaet al. 2020). Downregulation of KNOX hinders lignin biosynthesis (Mele, Ori, Sato & Hake 2003). Transgenic plants overexpressingOsa-miR164a enhances susceptibility to Xoo by virtue of the reduced accumulation of OsNAC60 (Jia et al. 2020). The role of miR164a/OsNAC60 regulatory module in blast resistance has been discussed earlier in this review. The Osa-miR169o represents the coordinated regulation of the crosstalk between BB and N-use efficiency in rice. Overexpression of the miR169o enhances N use efficiency and susceptibility to BB by suppressing the Nuclear factor Y-A (NF-YA ) which lead to significant reduction in the transcript abundance of several defense related genes includingPR10b, PR1b, PR10a and PAL (Yu et al. 2018). TheOsa-miR156 negatively regulates rice immunity against the bacterial blight (Liu et al. 2019). Transgenic plants overexpressing the target gene IPA1 (Ideal Plant Architecture 1 ) under the control of pathogen-inducible promoter ofOsHEN1 has been shown to enhance both disease resistance and yield in rice.
More recently, Osa-miR396f was shown to play a positive role in conferring resistance against bacterial foot rot disease caused byDickeya zeae. Overexpression of Osa-miR396f in the susceptible rice variety Nipponbare showed enhanced immunity to D. zeae by suppressing target gene OsGRFs (Growth-Regulating Factors ). However, the molecular mechanism underpinning the immunity against bacterial foot root mounted by OsGRFs remains to be elusive (Li et al. 2019b).