Physiological and Transcriptomic Analysis Reveals Distorted Ion Homeostasis and Responses in the Freshwater Plant Spirodela polyrhiza L. under Salt Stress

Abstract

Duckweeds are a family of freshwater angiosperms with morphology reduced to fronds and propagation by vegetative budding. Unlike other angiosperm plants such as Arabidopsis and rice that have physical barriers between their photosynthetic organs and soils, the photosynthetic organs of duckweeds face directly to their nutrient suppliers (waters), therefore, their responses to salinity may be distinct. In this research, we found that the duckweed Spirodela polyrhiza L. accumulated high content of sodium and reduced potassium and calcium contents in large amounts under salt stress. Fresh weight, Rubisco and AGPase activities, and starch content were significantly decreaseded in the first day but recovered gradually in the following days and accumulated more starch than control from Day 3 to Day 5 when treated with 100 mM and 150 mM NaCl. A total of 2156 differentially expressed genes were identified. Overall, the genes related to ethylene metabolism, major CHO degradation, lipid degradation, N-metabolism, secondary metabolism of flavonoids, and abiotic stress were significantly increased, while those involved in cell cycle and organization, cell wall, mitochondrial electron transport of ATP synthesis, light reaction of photosynthesis, auxin metabolism, and tetrapyrrole synthesis were greatly inhibited. Moreover, salt stress also significantly influenced the expression of transcription factors that are mainly involved in abiotic stress and cell differentiation. However, most of the osmosensing calcium antiporters (OSCA) and the potassium inward channels were downregulated, Na+/H+ antiporters (SOS1 and NHX) and a Na+/Ca2+ exchanger were slightly upregulated, but most of them did not respond significantly to salt stress. These results indicated that the ion homeostasis was strongly disturbed. Finally, the shared and distinct regulatory networks of salt stress responses between duckweeds and other plants were intensively discussed. Taken together, these findings provide novel insights into the underlying mechanisms of salt stress response in duckweeds, and can be served as a useful foundation for salt tolerance improvement of duckweeds for the application in salinity conditions.

Accumulation of starch in duckweeds (Lemnaceae), potential energy plants

“…In the study performed by Fu et al (2019) using S. polyrhiza 7498, it was reported that in the first few hours of treatment until 12 h, the starch content decreased and this correlated positively with the activity of AGP, the initial and rate-limiting enzyme of starch synthesis, which is involved in starch biosynthesis. A set of transcriptomic data with differentially expressing gene patterns, related to the time points within a day of treatment have also been presented.…”

“…and Ca 2? content (Fu et al 2019). Of interest is the increase of ABA levels by 150% with the treatment of duckweeds with 85 mM NaCl (Huber and Sankhla 1979) as it is known that ABA induces, very effectively, starch accumulation in duckweeds.…”

Identification, Phylogeny, and Comparative Expression of the Lipoxygenase Gene Family of the Aquatic Duckweed, Spirodela polyrhiza, during Growth and in Response to Methyl Jasmonate and Salt

“…This is particularly evident in land plants as JAs can alleviate salt stress by increasing the endogenous hormones and the antioxidative system [ 57 , 58 , 59 , 60 ]. Downregulation of three LOXs has been shown in a previous transcriptome analysis of duckweed exposed to salt stress [ 61 ]. However, our studies demonstrated that salt stress of duckweeds causes a dynamic pattern of the LOX gene family, with induction of several of them within 6 h of salt exposure.…”

“…Relative gene expression was quantified according to the 2 −∆∆ C T method [ 73 ]. The S. polyrhiza actin (Spipo17G0011400) and 18S rRNA (Spipo23G0000600) genes were used as standard housekeeping genes to normalize the expression of target genes [ 61 ]. qRT-PCR data represent the average ± standard deviation from a minimum of three independent biological replicates for each gene.…”

Identification and Expression Analysis of GARP Superfamily Genes in Response to Nitrogen and Phosphorus Stress in Spirodela Polyrhiza

“…The temporal and spatial expression pro les of SpGARPs in different tissues/organs (leaves, roots, and stipule, Bio-Project PRJNA557001), under various nutrient stresses (supply with P and N (+ P/+N), N deprivation (+ P/−N), P deprivation (− P/+N), P and N deprivation (− P/−N), and none nutrient supply (H 2 O) for seven days, PRJNA724886), under salt stress (salt-0 h, salt-6 h, salt-12 h, salt-24 h, PRJNA563960) were obtained using the publicly available transcriptome data from NCBI [23,73]. The heat map showing the correlations between the expression patterns of SpGARP genes was generated with the gplots package in R (version 4.1.2, https://www.R-project.org/).…”