4:00 pm - 5:00 pm
Event details: A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by Mengmeng Liu. This seminar is open to the general public to attend.
Role of Aquaporins in Brassica napus Responses to Root Hypoxia and Re-aeration
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 largely limited to the ability of some plants to improve root aeration through morphological and structural features such as the aerenchyma and adventitious roots. My thesis research focused on the importance of aquaporins in waterlogging tolerance of plants. Aquaporins play multifunctional roles in plant stress responses including root hypoxia. However, their role in gas and water transport processes as well as their regulation mechanisms during and following hypoxic events have been rarely addressed. In this thesis research, 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 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 and root hydraulic conductivity and the effect was greater in plants at the seedling stage compared with the other growth stages. Root water transport was an important determinant of plant sensitivity to waterlogging. 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 duration of waterlogging. 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 decreased root hydraulic conductivity (Lpr) and the apoplastic contribution to water transport, which 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 the contribution of apoplast to water flow. After one day of re-aeration, a sharp increase in ROS in roots was measured together with a decrease in transcript abundance of most of the BnPIP2 aquaporins 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 increase in gene expression of these aquaporins during re-aeration likely contributed to plant waterlogging recovery. A gradual recovery of Lpr following re-aeration was accompanied by up-regulation 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. The results demonstrate that plant injury from root hypoxia involved both the hypoxic event and the ROS burst that occurred soon after root aeration. Collectively, the results indicate that oxidative burst had a decisive impact on modulating the hydraulic recovery of plants upon re-aeration. Therefore, the ability of plants to recover from waterlogging is partly determined by their ability to handle oxidative stress.
Earlier studies have demonstrated that some of the plant aquaporins may be involved in oxygen transport. To determine the functional 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 dry biomass or any of the examined physiological parameters in well-aerated plants. However, compared with the wild-type plants, the plants overexpressing NtPIP1;3 maintained greater dry biomass and had higher gas exchange rates, root hydraulic conductivity, leaf water potentials, root respiration rates, as well as root ATP concentrations when exposed to root hypoxia. Moreover, metabolic profiling revealed that the 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 and improved the growth and physiological performance of plants. The results provide direct evidence for the functional importance of aquaporin-mediated oxygen transport in plants.