Small RNA pathways in plants, including microRNAs (miRNAs) and short interfering RNAs (siRNAs), play critical roles in gene regulation, chromatin modification, and antiviral defense. Plants have multiple DICER-like (DCL) and RNA-dependent RNA polymerase (RDR) proteins, which are essential for the biogenesis of miRNAs and siRNAs. This study identifies distinct functions for three DCL proteins in Arabidopsis: DCL1 for miRNA, DCL3 for endogenous siRNA, and DCL2 for viral siRNA. RDR2 is required for all endogenous siRNAs analyzed. Loss of endogenous siRNA in dcl3 and rdr2 mutants leads to loss of heterochromatic marks and increased transcript accumulation. Defects in siRNA generation in dcl2 mutants correlate with increased virus susceptibility. These findings suggest that the diversification of DCL and RDR genes in plants has contributed to the specialization of small RNA pathways for development, chromatin structure, and defense. The study also highlights the functional roles of DCL3 and RDR2 in endogenous siRNA generation and their involvement in chromatin modification. Additionally, DCL2 is shown to function in antiviral defense, as dcl2 mutants exhibit delayed viral siRNA accumulation and increased susceptibility to turnip crinkle virus. The results indicate that plants have evolved distinct small RNA pathways with specialized functions, reflecting their adaptation to various biological processes and environmental challenges.Small RNA pathways in plants, including microRNAs (miRNAs) and short interfering RNAs (siRNAs), play critical roles in gene regulation, chromatin modification, and antiviral defense. Plants have multiple DICER-like (DCL) and RNA-dependent RNA polymerase (RDR) proteins, which are essential for the biogenesis of miRNAs and siRNAs. This study identifies distinct functions for three DCL proteins in Arabidopsis: DCL1 for miRNA, DCL3 for endogenous siRNA, and DCL2 for viral siRNA. RDR2 is required for all endogenous siRNAs analyzed. Loss of endogenous siRNA in dcl3 and rdr2 mutants leads to loss of heterochromatic marks and increased transcript accumulation. Defects in siRNA generation in dcl2 mutants correlate with increased virus susceptibility. These findings suggest that the diversification of DCL and RDR genes in plants has contributed to the specialization of small RNA pathways for development, chromatin structure, and defense. The study also highlights the functional roles of DCL3 and RDR2 in endogenous siRNA generation and their involvement in chromatin modification. Additionally, DCL2 is shown to function in antiviral defense, as dcl2 mutants exhibit delayed viral siRNA accumulation and increased susceptibility to turnip crinkle virus. The results indicate that plants have evolved distinct small RNA pathways with specialized functions, reflecting their adaptation to various biological processes and environmental challenges.