MicroR159 is a genetic switch between growth and pathogen defense in Plants

Abstract

MicroR159 (miR159) is an evolutionarily conserved microRNA family present across all terrestrial plants. It targets a subset of genes encoding R2R3 MYB-domain transcription factors known as GAMYB through highly conserved binding sites, ensuring their strong post transcriptional repression. Although GAMYB genes are constitutively transcribed, their expression is completely silenced by miR159, a process reinforced by conserved RNA secondary structures flanking the binding site. Disruption of this silencing, either through inhibition of miR159 function or by expressing miR159-resistant GAMYB transgenes, results in severe growth inhibition, emphasizing the importance of GAMYB silencing for normal development. To investigate the transcriptional consequences of miR159 loss of function, RNA-seq datasets from mir159 loss-of-function plants were analysed. These included the 35S- MIM159 Nicotiana benthamiana lines constitutively expressing the miR159 MIMIC (a miR159 inhibitor) and the mir159ab Arabidopsis T-DNA mutant. In Arabidopsis, miR159 disruption caused strong suppression of stress and defense related biological processes, with 18 of the 20 downregulated gene ontology categories associated with response pathways, including defense response and plant-pathogen interaction. Also, defense genes such as PR1 were among the most downregulated differentially expressed genes. This transcriptional profile closely mirrored that of the 35S-MIM159 N. benthamiana, indicating conserved defense gene repression upon miR159 inhibition in these two species. However, this pattern contrasted sharply with previous findings from the 35S-MIM159 N. tabacum, where defense genes and pathways, were strongly induced. These contrasting transcriptomic outcomes, despite all three species exhibiting similar growth inhibition upon loss of miR159 function, suggest species-specific regulatory processes operating downstream of GAMYB. To further dissect GAMYB associated transcriptional responses, a transgenic miR159-resistant and estradiol inducible GAMYB construct was generated and introduced into Arabidopsis and N. tabacum. Induced GAMYB expression in both species led to transcriptomic enrichment for genes associated with abiotic and biotic stress responses. This association suggests that GAMYB expression activates pathways involved in defense against microbial and insect attack, positioning miR159 as a molecular switch between growth and broad-spectrum defense. The persistent yet silenced GAMYB transcript may function as a latent regulator, rapidly activating defense-associated transcriptional programs upon derepression, indicating a potential role in stress-responsive priming.