Whole seedlings were recovered from flasks, shoot tissue dissected using microscissors and collected in a 2-mL microfuge tube. Tissue was immediately frozen in liquid nitrogen, and ground to a fine powder with a bead mill. RNA was extracted using the Spectrum Plant Total RNA kit (Sigma-Aldrich). Paired-end libraries were prepared with the NEBNext Ultra II RNA Library Prep Kit for Illumina® (New England Biolabs) according to the manufacturers' protocol.