Genomics of turions from the Greater Duckweed reveal its pathways for dormancy and reemergence strategy

Over 15 families of aquatic plants are known to use a strategy of developmental switching upon environmental stress to produce dormant propagules called turions. However, few molecular details for turion biology have been elucidated due to the difficulties in isolating high-quality nucleic acids from this tissue. We successfully developed a new protocol to isolate high-quality transcripts and carried out RNA-seq analysis of mature turions from the Greater Duckweed Spirodela polyrhiza. Comparison of turion transcriptome to that of fronds, the actively growing leaf-like tissue, were carried out. Bioinformatic analysis of high confidence, differentially expressed transcripts between frond and mature turion tissues revealed major pathways related to stress tolerance, starch and lipid metabolism, and dormancy that are mobilized to reprogram frond meristems for turion differentiation. We identified the key genes that are likely to drive starch and lipid accumulation during turion formation, as well as in pathways for starch and lipid utilization upon turion germination. Comparison of genome-wide cytosine methylation levels also revealed evidence for epigenetic changes in the formation of turion tissues. Similarities between turions and seeds provided evidence that key regulators for seed maturation and germination have been retooled for their function in turion biology.