In Arabidopsis thaliana, the MOS4-ASSOCIATED COMPLEX (MAC) is required for defense and development. The evolutionarily conserved putative RNA helicase MAC7 is a component of the Arabidopsis MAC, and the human MAC7 homolog, Aquarius, is implicated in precursor messenger RNA (pre-mRNA) splicing. In my thesis research, I studied the functions of MAC in RNA metabolism and other aspects of Arabidopsis development and defense. First, we show that the partial loss-of-function mutant mac7-1 and two other MAC subunit mutants, mac3a mac3b and pleiotropic regulatory locus 1 pleiotropic regulatory locus 2 (prl1 prl2), exhibit reduced microRNA (miRNA) levels, indicating that MAC promotes miRNA biogenesis. The mac7-1 mutant shows reduced primary miRNA (pri-miRNA) levels without affecting miRNA gene (MIR) promoter activity or the half-life of pri-miRNA transcripts. As a nuclear protein, MAC7 is not concentrated in dicing bodies, but it affects the localization of HYPONASTIC LEAVES1 (HYL1), a key protein in pri-miRNA processing, to dicing bodies. Immunoprecipitation of HYL1 retrieved eleven known MAC subunits, including MAC7, indicating association between HYL1 and MAC. We propose that MAC7 links MIR transcription to pri-miRNA processing. mRNA-seq analysis showed that down-regulated genes in MAC subunit mutants are mostly involved in plant defense and stimulus responses, confirming a role of MAC in biotic and abiotic stress responses. We also discovered global intron retention defects in mutants of three subunits of MAC, thus linking MAC function to mRNA splicing in Arabidopsis. Second, since previous studies showed that MAC has multiple functions in eukaryotes, including roles in transcription elongation, mRNA splicing, DNA repair, RNA export and the ubiquitin-proteasome pathway, we explored other potential functions of Arabidopsis MAC and the MAC7 subunit in particular beyond miRNA biogenesis and pre-mRNA splicing. We found that MAC7 has a potential role in photosynthetic pathways, as RNA-seq analysis showed that many photosynthesis-related genes are down-regulated in this mutant. In addition, MAC7 and PRL proteins are very likely to be involved in ribosomal RNA (rRNA) biogenesis, as many rRNA precursors accumulated in the mac7-1 and prl1 prl2 mutants.