Ribosome biogenesis,which takes place mainly in the nucleolus,involves coordinated expression of preribosomal RNAs (pre-rRNAs) and ribosomal proteins,pre-rRNA processing,and subunit assembly with the aid of numerous assembly factors.Our previous study showed that the Arabidopsis thaliana protein arginine methyltransferase AtPRMT3 regulates pre-rRNA processing;however,the underlying molecular mechanism remains unknown.Here,we report that AtPRMT3 interacts with Ribosomal Protein S2 (RPS2),facilitating processing of the 90S/Small Subunit (SSU) processome and repressing nucleolar stress.We isolated an intragenic suppressor of atprmt3-2,which rescues the developmental defects of atprmt3-2 while produces a putative truncated AtPRMT3 protein bearing the entire N-terminus but lacking an intact enzymatic activity domain We further identified RPS2 as an interacting partner of AtPRMT3,and found that loss-of-function rps2a2b mutants were phenotypically reminiscent of atprmt3,showing pleiotropic developmental defects and aberrant pre-rRNA processing.RPS2B binds directly to pre-rRNAs in the nucleus,and such binding is enhanced in atprmt3-2.Consistently,multiple components of the 90S/SSU processome were more enriched by RPS2B in atprmt3-2,which accounts for early pre-rRNA processing defects and results in nucleolar stress.Collectively,our study uncovered a novel mechanism by which AtPRMT3 cooperates with RPS2B to facilitate the dynamic assembly/disassembly of the 90S/SSU processome during ribosome biogenesis and repress nucleolar stress.