Systemic translocation of RNA exerts non-cell-autonomous epigenetic control over plant development and defense. The present invention provides a polynucleotide sequence encoding a component of the protein machinery involved in small RNA trafficking, Cucurbita maxima PHLOEM SMALL RNA-BINDING PROTEIN1 (CmPSRB 1), and the corresponding polypeptide sequence. Hornologs of the disclosed sequences exist in other plant species. The invention also provides polynucleotide sequences of phloem small RNAs involved in signaling. Methods are also provided for using these PSRB I and small RNA sequences and genetic constructs comprising these sequences to alter (e.g., prevent, reduce or elevate) non-cell autonomous signaling events in plants involving small RNA metabolism. These signaling events are involved in a broad spectrum of plant physiological and biochemical processes, including, for example, flowering, coordination of plant growth and development, systemic resistance to pathogens, responses to environmental stresses, e.g., heat, drought, salinity, systemic gene silencing (e.g., viral infections), biomass distribution, regulation of carbon metabolism and control of plant size and developmental timing and patterning.
