Transcriptomic and physiological analysis revealed the ammonium tolerance mechanisms of Myriophyllum aquaticum

Abstract

The macrophyte Myriophyllum aquaticum was found to have strong resistance to the high ammonium (NH4+) in the aquatic environment and high effectiveness to treat swine wastewater. In order to understand the NH4+ tolerance and toxicity mechanisms, we conducted on the investigations of transcriptomic and physiological response of M. aquaticum to NH4+ stress. The plant was grown in Hoagland’s solutions with high (10 mM; N2) and extra-high (70 mM; N3) NH4+ concentrations, using 0.2 mM NH4+ as the control (N1) in a greenhouse, and then proceeded with Illumina next-generation sequencing and PacBio single-molecule real-time sequencing. The N3 plants appeared obvious NH4+ toxicity symptoms, while the N1 and N2 plants showed vigorous growth after two-weeks treatment. A total of 31,678 and 35,072 differentially expressed genes were identified in M. aquaticum under the N2 and N3 treatments, respectively. Transcriptional up-regulation of the light-harvesting chlorophyll a/b binding proteins genes in the leaves contributed to high NH4+ resistance in M. aquaticum. Transcriptional regulation of catalase genes likely balanced the reactive oxygen species in plants under high NH4+ conditions. Transcriptional down- or/and up- regulation of nitrogen metabolism genes, including NH4+ transporter, glutamine synthetase, glutamate synthetase, glutamate dehydrogenase, and asparagine synthetase genes were indispensable for NH4+ homeostasis through the NH4+ uptake, transport, and assimilation under high NH4+ concentrations. Our study provides insights into the molecular and physiological mechanisms of M. aquaticum response to NH4+ stress, and our multi-transcriptome database is beneficial to future investigations on this species. These results lay a theoretical foundation for the treatment of high-strength NH4+ in swine wastewater by M. aquaticum.