Role of Aquaporins in Brassica napus Responses to Root Hypoxia and Re-aeration

• Author / Creator

  Liu, Mengmeng

• Root oxygen deprivation (root hypoxia) in flooding-prone areas is detrimental to most terrestrial plants and results in growth reductions and mortality. Although plants widely vary in their waterlogging tolerance, our understanding of these variations is limited to the ability of some plants to improve root aeration through morphological and structural features. My thesis research focused on the importance of aquaporins in plant waterlogging tolerance. Aquaporins play multifunctional roles in plant stress responses including root hypoxia. However, their role in gas and water transport as well as their regulation mechanisms during and following hypoxic events have been rarely addressed. In this thesis, I studied the responses of canola (Brassica napus cv Westar) plants to root hypoxia and re-aeration to determine the role of aquaporins in these processes. To reveal the significance of oxygen transport through aquaporins, NtPIP1;3 from tobacco (Nicotiana tabacum) was overexpressed in canola and plant responses to hypoxia were examined.
  In Chapter 2, I examined the effects of root hypoxia (waterlogging) on canola (Brassica napus) plants at the seedling, flowering and podding growth stages. The plants were waterlogged for up to eight days and their growth, gas exchange, leaf water potentials, and root hydraulic conductance (Kr) were examined. In addition, relative contributions of the aquaporin-mediated and apoplastic root water transport, gene expression levels of the plasma membrane intrinsic proteins (BnPIPs), as well as the lignin and suberin deposition in roots were detected. Waterlogging decreased dry weights, gas exchange, leaf water potentials Kr and the effect was greater in plants at the seedling stage compared with other growth stages. Waterlogging accelerated root suberization and lignification resulting in an increased contribution of aquaporin-mediated water transport. The transcript levels of BnPIPs in roots increased with the increasing waterlogging duration. The study demonstrated that maintaining functional aquaporins is critical to the survival of waterlogged plants.
  In the second study (Chapter 3), hydroponically-grown canola plants were exposed to three days of root hypoxia (waterlogging) followed by re-aeration. Hypoxia impacted gas exchange and plant water relations. However, the most severe reduction of Lpr was on one day following re-aeration and it was accompanied by an increase in apoplastic transport. After one day of re-aeration, a sharp increase of root reactive oxygen species was measured together with a decrease in transcript abundance of most of the BnPIP2s in roots and leaves. A water permeability assay of these PIP2s overexpressed in yeast confirmed that they are fast water transporters. The yeast H2O2 survival assay demonstrated that BnPIP1;2 and BnPIP1;3 facilitate H2O2 transport and, therefore, the upregulations of these aquaporins likely contributed to plant waterlogging recovery. A gradual recovery of Lpr following re-aeration was accompanied by upregulations of BnPIPs in roots and leaves and the activation of antioxidant enzymes in roots. Net photosynthesis, transpiration rates, and shoot water contents remained depressed one day after re-aeration but recovered over time. Collectively, the results indicate that oxidative burst had a decisive impact on modulating the hydraulic recovery of plants upon re-aeration.
  To determine the significance of oxygen-transporting aquaporins in waterlogging tolerance, in the third study (Chapter 4), I overexpressed NtPIP1;3 aquaporin from tobacco (Nicotiana tabacum) in canola (Brassica napus cv Westar) plants and examined its effects on growth, physiological parameters, and energy metabolism in plants subjected to three and eight days of root hypoxia. The overexpression of NtPIP1;3 did not affect physiological parameters in well-aerated plants, but maintained greater dry biomass and higher gas exchange rates, root hydraulic conductivity, leaf water potentials, root respiration rates, as well as root ATP concentrations under root hypoxia stress. Moreover, metabolic profiling revealed that overexpression of NtPIP1;3 enabled plants to respond to a decreased oxygen environment by altering the glycolysis, pyruvate metabolism, and TCA cycle in roots. The results collectively demonstrate that the overexpression of NtPIP1;3 improved the waterlogging tolerance of canola plants by increasing root oxygen availability, which impacted the energy metabolism, improved the plant growth and physiological performance. The results provide direct evidence for the functional importance of aquaporin-mediated oxygen transport in plants.

• Subjects / Keywords

  • Aquaporins

• Graduation date

  Fall 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-zf8k-5477

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.