Role of Aquaporin-Mediated Water Transport in Salinity Tolerance of Northern Grasses
Creators: Vaziriyeganeh, Maryamsadat
Publisher: name: University of Alberta Library
Year: 2023

<p>Although many aspects of salinity tolerance in plants have been thoroughly researched, some of the fundamental aspects of plant water relations remain unclear. The main objective of my thesis research was to examine the role of aquaporin-mediated water transport in salinity tolerance of three northern grass species varying in salt tolerance: salt-loving halophytic grass Puccinellia nuttalliana, salt tolerant Poa juncifolia, and relatively salt intolerant Poa pratensis. To achieve this objective, three studies were conducted under controlled-environment conditions.<br><br>
In Chapter 2, I tested the hypothesis that Na+ is the principal salt factor responsible for the enhancement of root water transport in halophytic plants, which enables the plants to maintain water balance, gas exchange, and growth under salinity conditions. I subjected plants to 150 mM NaCl, KCl, and Na2SO4 treatments to separate different salt factors that affect physiological and growth responses in P. pratensis, P. juncifolia, and P. nuttalliana. The study confirmed the hypothesis and demonstrated that sustained growth, chlorophyll concentrations, gas exchange, and enhanced root cell hydraulic conductivity in P. nuttalliana were associated with the presence of Na+ in the salt treatments. The enhancement of cell hydraulic conductivity was abolished by 50 µM HgCl2, confirming that the effect of Na+ was on the aquaporin-mediated water transport. The observed increases in root Ca2+ and K+ concentrations also likely played some role in the transcriptional and (or) posttranslational regulation of aquaporins that enhanced root water transport in P. nuttalliana. <br>
To examine the molecular basis contributing to water balance maintenance in P. nuttalliana under salinity conditions, high-throughput RNA-sequencing and metabolome analyses were carried out in plants subjected to 0 (control) and 150 mM NaCl treatments (Chapter 3). In RNAseq, a total of 31 Gb clean bases were generated and de novo assembled into 941,894 transcripts. The PIP2;2 and HKT1;5 transcript levels increased in response to the NaCl treatment implying their roles in water and ion homeostasis. The expression of several transcription factors, including WRKY39, DEK3, HY5, and ABF2, also increased in response to NaCl treatment. The metabolomic analysis revealed that the concentrations of proline and dopamine significantly increased under NaCl stress. Several phosphatidylcholines increased in roots suggesting their possible role in maintaining the functionality of cell membranes under salt stress conditions. In leaves, the TCA cycle was enriched pointing to enhanced energy metabolism to cope with salt stress. Microscopic examination of leaf and root structures revealed the presence of epidermal salt glands and enhanced cell wall lignification in the root cells in response to NaCl, which likely contribute to the salt tolerance strategy of P. nuttalliana. <br>
In Chapter 4, I cloned PIP1;1, PIP1;3, PIP2;1, PIP2;2, PIP2;3, and TIP1;1 aquaporins from P. nuttalliana, P. juncifolia, and P. pratensis to better understand their structure and function in relation to salt tolerance. I analyzed mRNA expression changes of the aquaporins in roots by exposing the plants to 0 (control) and 150 mM NaCl in hydroponic culture. The NaCl treatment upregulated several PIP transcripts and decreased the PnuTIP1;1 expression in P. nuttalliana. The PnuPIP2;2 transcripts increased by about six-fold in P. nuttalliana and by about two-fold in P. juncifolia, while no changes were observed in P. pratensis. I then expressed PnuPIP2;2 in yeast (Saccharomyces cerevisiae) and examined the water fluxes in the absence and presence of 150 mM NaCl using a stopped-flow light-scattering spectrometer. The results demonstrated that the rate of water transport in yeast expressing PnuPIP2;2 was enhanced by 150 mM NaCl compared to an empty vector control. PnuPIP2,2 expression also resulted in a higher Na+ uptake by yeast cells, suggesting its ion transporting function. Structural analysis revealed relatively high hydrophilicity of PnuPIP2;2 and the unique pore characteristics that likely affect its transporting properties. <br>
The results of my PhD research clearly point to the importance of aquaporins in water balance maintenance of salt tolerant grasses under salinity conditions. A model based on the study results is proposed to explain the complex responses of P. nuttalliana to salt stress and the processes, which likely contribute to water homeostasis in these plants under salinity conditions.<br>
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