Author

Shuying Pan

Year of Award

5-27-2019

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Biology.

Principal Supervisor

Xia, Yiji

Keywords

Arabidopsis ; Exonucleases ; Growth (Plants) ; RNA

Language

English

Abstract

RNA decay plays an essential role in the regulation of gene expression during plant development and response to environmental stimuli. The protein DXO is a 5' to 3' exonuclease that functions in RNA degradation and RNA quality control that has been studied in animals. It has not yet been identified in plants. The gene locus At4g17620 in Arabidopsis thaliana encodes a protein homolog of the mammalian DXO, termed AtDXO. Recombinantly expressed AtDXO possesses a 5'-3' RNA exonuclease activity in vitro. Loss-of-function of AtDXO in Arabidopsis generates multiple growth defects, including curled and yellowish leaves, growth retardation and limited fertility, whereas overexpression show no obvious growth phenotype. The development defect of atdxo might be attributed to aberrant RNAs, which are not degraded when AtDXO is dysfunctioning. From the RNA-Seq analysis, the transcriptome pattern of atdxo mutants shows significant disparity from wild-type. Among the differences, the defense response genes are elevated in atdxo while photosynthesis-related and plastid genesis-related genes are downregulated. The constitutive expression of defense response genes causes the autoimmune phenotypes of atdxo. This could be modulated by temperature and is partially dependent on the master immunity regulators EDS1 or NPR1. Reactive oxygen species (ROS) accumulation was also detected in the atdxo mutant, and atdxo showed insensitivity to oxidative stress imposed by paraquat. Moreover, the atdxo mutant is hypersensitive to salt stress but not sensitive to general osmotic stress. In Arabidopsis, the 5'-3' RNA decay pathway could act as a repressor of endogenous post-transcriptional gene silencing (PTGS), which is regulated by small RNAs (sRNA). The mutation of AtDXO caused productions of 24- and 25-nucleotide endogenous sRNAs. The growth defect phenotype of atdxo could not be repressed by dysfunction of the RDR6 (RNA-DEPENDENT RNA POLYMERASE 6)-dependent sRNA biogenesis pathway. These findings demonstrate that AtDXO functions as a 5'-3' exoribonuclease both in vitro and in vivo to regulate plant development and to mediate the response to environmental stresses.

Comments

Principal supervisor: Professor Xia Yiji ; Thesis submitted to the Department of Biology ; Thesis (Ph.D.)--Hong Kong Baptist University, 2019.

Bibliography

Includes bibliographical references (pages 77-90)

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