The evolutionarily conserved, putative RNA helicase MAC7 exists in both animals and plants. The human MAC7 homolog, Aquarius, is part of the spliceosome and plays a role in pre-mRNA splicing in vitro. In Arabidopsis, MAC7 was shown to be part of the MOS4-associated complex (MAC), which is required for plant defense and development. Here through RNA-seq analysis we discover that down-regulated genes in MAC subunit mutants are mostly involved in plant defense and stimulus response, confirming a role of MAC in the regulation of biotic and abiotic stress responses. We also discover global intron retention defects in mutants in three members of MAC, thus linking the functions of MAC to splicing in Arabidopsis. In addition, we show that mac7-1, a partial loss-of-function mutant in MAC7, and two other MAC subunit mutants, mac3a mac3b and prl1 prl2, exhibit reduced microRNA levels in general, indicating that MAC promotes microRNA biogenesis. The mac7-1 mutant shows reduced primary miRNA (pri-miRNA) levels without affecting MIR promoter activity or the degradation of pri-miRNA transcripts, implicating functions of MAC7 during transcription elongation or maturation of pri-miRNAs. As a nuclear protein, MAC7 is not localized in dicing bodies, but it affects the localization of HYL1 to dicing bodies. We propose that MAC acts to link MIR transcription to pri-miRNA processing. Overall design: Polyadenylated RNA was isolated from total RNA extracted from 12-day-old seedlings of Col-0, mac7-1, mac3a mac3b, prl1 prl2,with 2 biological replicates for each genotype.