miR156/SPL9 Regulates Reactive Oxygen Species Accumulation and Immune Response in Arabidopsis thaliana.

The functions of microRNA156 (miR156) and its targeted SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factor genes in plant development have been widely investigated. However, the role of the miR156/SPLs regulatory network in plant immune systems remains obscure. Here, we found that the accumulation of reactive oxygen species (ROS) and the transcripts of basal salicylic acid (SA) signaling pathway genes were lower in Arabidopsis Pro35S:MIR156 seedlings (miR156 overexpression mutants) but higher in Pro35S:MIM156 (miR156 repression mutants) and ProSPL9:rSPL9 (SPL9 overexpression mutants) seedlings compared with wild-type Col-0 plants (WT). As a result, Pro35S:MIR156 mutants induced greater susceptibility to Pseudomonas syringae pv. tomato DC3000 following syringe infiltration than WT, while Pro35S:MIM156 and ProSPL9:rSPL9 mutants showed enhanced resistance. In addition, foliar H2O2 application resulted in activation of SA-mediated defense response and ablation of miR156-induced susceptibility to P. syringae pv. tomato DC3000 infection. Collectively, our results provide new insights into the function of the miR156/SPL network in Arabidopsis immune response by regulating ROS accumulation and activating the SA signaling pathway.