Degree

Doctor of Philosophy (PhD)

Department

Biological Sciences

Document Type

Dissertation

Abstract

Copper (Cu) is a transition metal that is an essential nutrient for plants and is required for a number of critical processes. Therefore, proper homeostasis of copper is vital for plant life and must be tightly regulated. Metallothioneins are small molecular weight proteins, rich in cysteine residues and have the capability of binding to several different transition metals such as copper. Cu induces the expression of MTs, therefore they are believed to be involved in Cu transport, however their true functions have yet to be resolved. T-DNA insertions were identified in four Arabidopsis MT genes expressed in vegetative tissues, and these quadruple mutant plants deficient in four MTs (mt1a/mt2a/mt2b/mt3), (quad Δmt), were used to study the effects on copper homeostasis during leaf senescence of Arabidopsis. Loss of MTs in the quadruple mutant led to over-accumulation of Cu in vegetative leaves, lower relative chlorophyll content, and reduction of PSII activity. Additionally, MT-deficient plants also accumulated more reactive oxygen species compared to wild-type plants. A transcriptomic analysis was performed on quad Δmt to determine the effects of metallothionein deficiency on Arabidopsis through three developmental stages, pre-flowering, flowering, and senescence. Evaluation of transcriptomic profiles revealed divergence of MT-deficient plants in key processes during leaf senescence, including cell cycle regulation, development, and cellular component organization. Furthermore, WGCNA revealed distinct modules of co-expressed genes that relate to metallothionein deficiency, such as defense response, mis-regulation of the cell cycle, and photosynthetic regulation. Expression patterns of known copper transport and homeostatic genes revealed that COPT2, HMA5, CSD1, and YSL2 were among the most affected and all expression patterns observed were consistent with copper excess in leaves of quad Δmt. These findings suggest that metallothioneins are critical for proper copper homeostasis and transition through developmental phases during the plant life cycle.

Date

7-10-2023

Committee Chair

Smith, Aaron

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