Role of Aquaporins in Plant Responses to Oxygen Deprivation

  • Author / Creator
    Tan, Xiangfeng
  • Oxygen deprivation (hypoxia and anoxia) is an environmental stress affecting plants worldwide in many natural and man-made ecosystems. Aquaporins exhibit multiple physiological functions in challenging environments. The central role of aquaporins in plant water relations has been widely acknowledged, but the role of aquaporins under O2 deficiency remains elusive. Growing evidence has shown that aquaporins are involved in pore-mediated gas trans-membrane movement. In this thesis, Arabidopsis thaliana, tobacco (Nicotiana tabacum L.) and trembling aspen (Populus tremuloides Michx.) seedlings were studied to shed more light on the responses of aquaporins to root hypoxia. Oxygen transport through aquaporins was also examined in yeast heterologously expressing aquaporins from plant, human and fungal organisms. In tobacco, after one week of root hypoxia treatment, plants produced twice as many adventitious roots as the aerated plants, but their maximum length was reduced. Hypoxia severely reduced net photosynthesis, transpiration rates, and photosynthetic light responses. Relative transcript abundance of the examined aquaporins in lateral roots was reduced by hypoxia, but in adventitious roots it remained unchanged. This apparent lack of an effect of root hypoxia on the aquaporin expression likely contributed to maintenance of high hydraulic conductance in adventitious roots. Lateral roots had lower porosity compared with adventitious roots and the expression of the ACS (1-aminocyclopropane-1-carboxylate synthase) gene was induced in hypoxic lateral roots, but not in adventitious roots, providing additional evidence that lateral roots were more affected by hypoxia compared with adventitious roots. ATP concentrations were markedly lower in both hypoxic lateral and adventitious roots compared with aerated roots, while the expression of fermentation-related genes, ADH1 (alcohol dehydrogenase 1) and PDC1 (pyruvate decarboxylase 1), was higher in lateral roots compared with adventitious roots. Since root porosity was greater in adventitious compared with lateral roots, the results suggest that the improved O2 delivery and stable root aquaporin expression in adventitious roots were likely the key factors helping flooded tobacco plants maintain high rates of root hydraulic conductance and, consequently, shoot gas exchange. Effects of ethylene on hypoxia tolerance in plants are little understood. In this thesis research, I examined the physiology and gene expression of several aquaporins in aspen (Populus tremuloides) seedlings exposed to root hypoxia and treated with exogenous ethylene. Ethylene enhanced net photosynthesis, light saturation level for net photosynthesis, transpiration rates, and root hydraulic conductance in hypoxic plants. Root O2 concentrations increased in the hypoxic plants treated with ethylene compared to the hypoxic plants that were not subjected to ethylene treatment. The effect of ethylene on root O2 content could be either due to the enhanced net photosynthesis or facilitated O2 transport from shoots. Ethylene triggered the expression of PIP2;4 aquaporin in hypoxic aspen which could potentially help facilitate root water transport. The enhanced root water transport by ethylene could be also partly responsible for the increase in net photosynthesis of the hypoxic plants which, in turn, could enhance root O2 supply. Arabidopsis seedlings were subjected to waterlogging and compared with well-aerated plants. Photosynthesis and transpiration were inhibited by waterlogging. Waterlogging increased the relative transcript abundance of ADH, PDC-1, and NIP2;1 in plants. After 24 and 48 hours of waterlogging, relative transcript abundance of most of the PIPs decreased, while PIP1;1 and PIP1;4 in waterlogged roots exhibited significantly higher relative transcript abundance compared with aerated roots suggesting that they may play a role in plant resistance mechanisms under hypoxic conditions. O2 permeability of various human, plant, and fungal aquaporins was examined by co-expressing heterologous aquaporin and myoglobin in yeast. Two of the most promising O2-transporters (Homo sapiens AQP1 and N. tabacum PIP1;3) were confirmed to facilitate O2 transport in further spectrophotometric assay using yeast protoplasts. The over-expression of NtPIP1;3 in yeasts significantly increased their O2 uptake rates in suspension culture. In N. tabacum roots subjected to hypoxic conditions in hydroponic culture, it demonstrated a sharp increase in the relative transcript abundance of the O2-transporting aquaporin NtPIP1;3 after seven-day hypoxia treatment, accompanied by the increase in root ATP levels, particularly in the apical root segments. The results show that the functional significance of aquaporin-mediated O2 transport should be considered in further studies and that the possibility of controlling the rate of transmembrane O2 transport should be further explored.

  • Subjects / Keywords
  • Graduation date
    Spring 2019
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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