Mediator subunit MED19 modulates elf18-induced <i>PR1</i> expression via TGA transcription factors

The Mediator complex bridges transcription factors and RNA polymerase II to regulate gene expression during plant immune responses. We previously demonstrated that the Mediator subunit MED19a modulates <i>PATHOGEN-RELATED 1</i> (<i>PR1</i>) expression through interactions with Fibrillarin 2 (FIB2) and the long non-coding RNA ELENA1 upon elf18 treatment. Because TGA transcription factors are known to play important roles in regulating <i>PR</i> genes, we sought to determine whether specific TGAs cooperate with MED19a. To identify TGA transcription factors cooperating with MED19a, we performed yeast two-hybrid assays and found that MED19a interacts with TGA1 and TGA5. These interactions were confirmed by in vitro pull-down assays and bimolecular fluorescence complementation in planta. Expression analysis revealed that <i>PR1</i>, <i>PR2</i>, <i>PR3</i>, and <i>PR5</i> were significantly upregulated in <i>tga1</i> and <i>tga5</i> mutants, while <i>PR1</i> expression remained unchanged in TGA1 and TGA5 overexpression lines after elf18 treatment. Consistent with prior observations, <i>PR1</i> expression was slightly reduced in <i>med19a</i> mutants. We generated <i>med19a med19b</i> double mutants using RNAi and verified reduced <i>MED19b</i> expression. In triple mutants (<i>med19a med19b tga1</i> and <i>med19a med19b tga5</i>), <i>PR1</i> expression levels were intermediate between those of the respective <i>tga</i> single mutants and the <i>med19a med19b</i> double mutant. These results suggest that MED19a modulates <i>PR1</i> expression through physical and functional interactions with TGA1 and TGA5, contributing to fine-tuned transcriptional control of immune responses triggered by elf18 in Arabidopsis.