Plant 22-nt siRNAs mediate translational repression and stress adaptation

Small interfering RNAs (siRNAs) are essential for proper development and immunity in eukaryotes 1 . Plants produce siRNAs with lengths of 21, 22 or 24 nucleotides. The 21- and 24-nucleotide species mediate cleavage of messenger RNAs and DNA methylation 2 , 3 , respectively, but the biological functions of the 22-nucleotide siRNAs remain unknown. Here we report the identification and characterization of a group of endogenous 22-nucleotide siRNAs that are generated by the DICER-LIKE 2 (DCL2) protein in plants. When cytoplasmic RNA decay and DCL4 are deficient, the resulting massive accumulation of 22-nucleotide siRNAs causes pleiotropic growth disorders, including severe dwarfism, meristem defects and pigmentation. Notably, two genes that encode nitrate reductases— NIA1 and NIA2 —produce nearly half of the 22-nucleotide siRNAs. Production of 22-nucleotide siRNAs triggers the amplification of gene silencing and induces translational repression both gene specifically and globally. Moreover, these 22-nucleotide siRNAs preferentially accumulate upon environmental stress, especially those siRNAs derived from NIA1/2 , which act to restrain translation, inhibit plant growth and enhance stress responses. Thus, our research uncovers the unique properties of 22-nucleotide siRNAs, and reveals their importance in plant adaptation to environmental stresses. Characterization of 22-nucleotide short interfering RNAs in plants finds that they accumulate in response to environmental stress, causing translational repression, inhibition of plant growth and enhanced stress responses.