Review: Plant microRNAs in pathogen defense: A panacea or a piece of the puzzle?

Plant microRNAs (miRNAs) control key agronomic traits that are associated with their conserved role(s) in development. However, despite a multitude of studies, the utility of miRNAs in plant-pathogen resistance remains less certain. Reviewing the literature identifies three general classes of miRNAs regarding plant pathogen defense. Firstly, a number of evolutionary dynamic 22 nucleotide miRNA families that repress large numbers of plant immunity genes, either directly, or through triggering the biogenesis of secondary siRNAs. However, understanding of their role in defense and of their manipulation to enhance pathogen resistance are still lacking. Secondly, highly conserved miRNAs that indirectly impact disease resistance through their targets that are primarily regulating development or hormone signaling. Any alteration of these miRNAs usually results in pleiotropic impacts, which may alter disease resistance in some plant species, and against some pathogens. Thirdly, are the comparatively diverse and evolutionary dynamic set of non-conserved miRNAs, some of which contribute to pathogen resistance, but whose narrow evolutionary presence will likely restrict their utility. Therefore, reflecting the diverse and evolving nature of plant-pathogen interactions, a complex interplay of plant miRNAs with pathogen responses exists. Any miRNA-based solution for pathogen resistance will likely be highly specific, rather than a general panacea.