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

Abstract

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.

Characteristics of turion development in two aquatic carnivorous plants: Hormonal profiles, gas exchange and mineral nutrient content

“…As turions represent quite contrasting organs (both morphologically and physiologically) in comparison to summer growing shoot apices, it can be expected that the transition between both “stable” states shall include profound changes in the profiles of phytohormones such as CKs, auxins, and ABA in the developing shoot apices towards future turions, but also in the remaining, dying annual shoots (Adamec, 2018a; Šimura et al, 2016 ; Winston & Gorham, 1979b). The same conclusion can also be drawn from external applications of ABA or cytokinin kinetin to some aquatic plants (mainly Spirodela polyrhiza ) to induce turion formation or to revert it (Chaloupková & Smart, 1994; Smart et al, 1995; Smart & Trewavas, 1983; Weber & Noodén, 1976) and also from first genomic or transcriptomic studies on turions (Li et al, 2022; Pasaribu et al, 2023; Wang et al, 2014).…”

“…In turions, a total of 208 genes exhibited four times more increased expression as compared to growing fronds, while 154 genes exhibited markedly reduced expression. In this species, turion formation thus represents very complex developmental changes on the genomic level comparable, e.g., with flowering (see also Pasaribu et al, 2023).…”

The genomes and epigenomes of aquatic plants (Lemnaceae) promote triploid hybridization and clonal reproduction

“…Turions are dormant buds induced by cold temperatures and low phosphate, and are found in many duckweed species including S. polyrhiza and L. turionifera (Figures 1A, 2A) 42 and W. australiana . In contrast, L. minor does not form turions, prompting us to examine whether this trait was retained in hybrids.…”