Morpho-Physiological and Transcriptional Regulation of Root System under Saline Conditions in Nymphaea Plants

Water lilies (Nymphaea L.) are ancient angiosperms that can be cultivated in both fresh and brackish water. Water lily plants have adapted morphologically and physiologically to the aqueous environment. Nonetheless, little is known about the regulatory mechanisms that enable water lily to acclimate to saline conditions, restricting its production and distribution. To illustrate the role of roots in water lily salinity tolerance, we investigated the adaptive regulation of the water lily root system under high salinity. Aspects of its root architecture, including root length, surface area, volume, and tip number, were significantly reduced by salt stress. Transcriptome sequencing showed that 120 genes were upregulated and 1214 genes were downregulated under salt stress. The differentially expressed genes were mainly enriched in oxidoreductase activity, structural molecule activity, and transmembrane transporter activity. Most ion transporter genes were downregulated, suggesting that water lily may partially close ion channels and/or transporters to avoid excessive ion accumulation or ion imbalance under long-term salt stress. Genes related to NO3- transport were both up- and downregulated, whereas genes related to ammonium transport were uniformly downregulated, suggesting that transcriptional changes may play a role in balancing nitrogen metabolism under long-term saline conditions. The roots showed relatively high concentrations of Na+ and had the ability to hyper-accumulate Na+ under salt stress. These findings provide insight into the regulatory mechanisms that enable water lily roots to tolerate salinity and lay a foundation for the breeding of salt-tolerant cultivars.