The coordination of phytoglobin-nitric oxide cycle and alternative oxidase in plant adaptation to hypoxia

Zafari, Somaieh (2022) The coordination of phytoglobin-nitric oxide cycle and alternative oxidase in plant adaptation to hypoxia. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The non-energy conserving protein alternative oxidase (AOX) of the mitochondrial respiratory electron transport chain (ETC) is hypothesized to regulate nitric oxide (NO), reactive oxygen species (ROS), and ethylene levels in plants under stress. The purpose of this research is to provide direct evidence in favor of this hypothesis, as well as to investigate the implications of this regulation during plant adaptation to hypoxic stress. We studied NO metabolism and the involvement of key components of the phytoglobin (Pgb1) -NO cycle in imbibed transgenic barley seeds with altered Pgb1 levels during germination process as well as in transgenic tobacco seedlings with altered AOX levels exposed to nitrogen atmosphere. NO emission increased more in overexpressing lines of tobacco under hypoxia, although the quantity of nitrosylated proteins was higher in AOX knockdown plants. There was a significant increase in Pgb1 expression which upregulates the turnover of the Pgb1-NO cycle in imbibed barley seeds and tobacco leaves under hypoxic condition. The cycle's operation not only controls NO metabolism and redox homeostasis, but it improves the efficiency of energy metabolism. The current study demonstrates that AOX which contributes positively to the operation of the Pgb1-NO cycle, regulates NO generation under hypoxia and leads to a shift towards biosynthesis of amino acids. It demonstrates that hypoxia results in the upregulation of fermentation pathways in the plants expressing AOX. The plants lacking AOX exhibited the increased levels of ROS as compared to wild type and AOX overexpression plants during hypoxia, suggesting that AOX induces a fine-tuned balance in ROS levels by the regulation of ROS production and scavenging. I found that ethylene biosynthesis genes are induced during hypoxia and correlate with AOX and NO levels. I conclude that AOX is involved in NO turnover and plays a protective role by reducing ROS, regulating the ethylene levels, and sustaining energy requirements during hypoxia.

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