Document Type


Degree Name

Master of Science (MSc)



Program Name/Specialization

Integrative Biology


Faculty of Science

First Advisor

Dr. Allison McDonald

Advisor Role

Mentor and reviewer of work


The photosynthetic electron transport (PET) chain relies on the chlorophyll and carotenoid biosynthetic pathways for pigments that harness light, and protect the photosynthetic apparatus from damage by light. The pigment pathways also connect to the PET chain through electron transfer where the plastoquinol (PQ) pool accepts electrons from phytoene desaturase (PDS) of the carotenoid pathway and the aerobic cyclase (ACS) of the chlorophyll biosynthetic pathway. Genetic studies have shown a plastoquinol terminal oxidase (PTOX) protein functions to regulate the redox state of the PQ pool. Without PTOX to maintain balance between the two redox states of the PQ pool it is at high risk of becoming hyper-reduced. The same studies have shown mutated PTOX causes variegation (abnormal white sectors within the normal green tissue on a plant’s leaves). The lack of PTOX due to mutation allows the PQ pool to become hyper-reduced. Hyper-reduction prevents PDS and ACS from using the PQ pool as a source to deposit electrons from their respected pathways. This stops pigment production and variegation becomes apparent on the plants’ leaves. The current study explores the hypothesis that in addition to being caused by genetic mutations, variegated plants can also be generated using a direct chemical inhibitor of PTOX. The study also explores variegation under the condition of drought stress. This study is the first to apply chemicals as a means of inhibiting PTOX activity and in a whole plant system of tobacco (Nicotiana tabacum). The observations of the treated tobacco leaves showed variegation had occurred and pigment analysis of the abnormal sections showed there were significant changes in the chlorophyll and carotenoid concentrations of tobacco leaves compared to the control (normal leaf). These findings support the literature in that the PTOX interacts with the pigment pathways since the chemical inhibitors altered pigment concentrations. Using whole tobacco plants, we provided findings that further show the complexity and integration of the carotenoid and chlorophyll biosynthetic pathways with the photosynthetic electron transport chain.

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