Activity Range of Arabidopsis Small RNAs Derived from Different Biogenesis Pathways

Several classes of small RNAs are known in plants and accumulating evidence indicates that different classes of small RNAs may function either cell auton- omously or noncell autonomously to regulate gene expression. Here, a simple visual assay used to com- pare the biological activity of small RNAs produced via different biogenesis pathways suggests that trans- acting small interfering RNAs (tasiRNAs) are mobile while confirming that microRNAs (miRNAs) are not. Small RNAs have several functions in plant cells, with small interfering RNAs (siRNAs) involved in immunity against invading nucleic acids (RNAviruses andtransposons)andinsilencingofrepetitiveDNA.In contrast, miRNAs modulate physiological and develop- mental gene expression, a role also played by a limited number of siRNAs produced from endogenous genes. Transgene-bornorviral-inducedsiRNAscanmovefrom cell to cell. Current evidence suggests that miRNAs are not mobile, while it is unknown if endogenously encoded siRNAs such as tasiRNAs act noncell autono- mously. The question of small RNA mobility is biolog- ically critical since these molecules may act as signals. During RNA silencing, Dicer ribonuclease enzymes produce 21- to 24-nucleotide RNAs from double- stranded RNA (dsRNA) substrates (Baulcombe, 2004;