The Trihelix Transcription factor GT2-like 1 (GTL1) was previously shown to be a key regulator of ploidy-dependent trichome growth and drought tolerance. Here, we report that GTL1 plays an important role in coordinating plant immunity. We show that gtl1 mutants are compromised in the regulation of basal immunity, microbial pattern-triggered immunity (PTI) and effector-triggered RIN4-mediated immunity. Transcriptome analysis revealed that GTL1 positively regulates defense genes and inhibits factors that mediate growth and development. By performing hormonal measurements and chromatin-immunoprecipitation studies, we found GTL1 to coordinate genes involved in salicylic acid metabolism, transport and response. Interaction studies and comparative transcriptomics to known data sets revealed that GTL1 is part of the MPK4 pathway and regulates oppositely the expression of differentially expressed genes in mpk4 plants. We introduced the gtl1 mutation in the mpk4 mutant and thereby partially suppressed its dwarfism and the high resistance against a bacterial invader. Our data show that GTL1 is part of the MPK4 pathway and acts as a positive regulator of bacterial-triggered immunity and SA homeostasis.