Download Synopsis - Shodhganga

Synopsis
Meiosis represents an important event in the life cycle of all sexually
reproducing organisms. It is a specialized cell division comprising of a single
round of DNA replication followed by two rounds of nuclear division resulting in
four haploid daughter cells from a single diploid cell. Meiosis involves a series of
complex
chromosomal
events
such
as
homologue
pairing,
synapsis,
recombination and bivalent formation. Several genes have been identified in
different organisms including plants that play critical roles at various stages of
meiosis. Nevertheless, certain key aspects of meiosis such as its regulation and
meiotic chromatin organization remain poorly understood especially in plants.
Therefore, it would be of great interest to elucidate function of such genes and
the molecular mechanisms governing these aspects of plant meiosis owing to the
importance it harbors particularly in the field of agriculture. In this thesis, I present
functional characterization of the AML gene family and the AtSCC2 gene, both of
which are critical for chromatin organization and meiotic progression in plants.
This thesis comprises of three chapters. In the first chapter, a brief
account of different kinds of reproduction has been presented. This is followed by
a point wise listing of features that makes Arabidopsis a good model organism. A
short description of sporogenesis and gametogenesis in Arabidopsis has also
been described. In the second half of Chapter 1, I discuss a powerful tool
adopted in plants to study gene function called RNAi. Also, some characterized
Arabidopsis genes involved in meiosis are discussed.
In Chapter 2, I describe functional characterization of the AML gene family
in Arabidopsis comprising of 5 members AML 1-AML5. The AML genes represent
the Arabidopsis homologues of mei2, a master regulator of meiosis in yeast. I
have used an RNAi approach in combination with T-DNA insertion mutants, to
dissect out likely functions of this important class of genes during meiosis in
Arabidopsis. Expression analysis of AML4 revealed strong expression in male
and female meiocytes, consistent with other members of the AML gene family.
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This suggested a possible role of AMLs in plant meiosis. Subsequent meiotic
analysis in the AML5-RNAi and amI triple mutant lines demonstrated a range of
meiotic abnormalities. These results provide support for a level of conservation in
the basic molecular mechanisms involved in the regulation of meiosis between
yeast and plants. Abnormalities were also observed during seedling development
in multiple AML5-RNAi and amI triple mutant lines which are consistent with the
AML expression pattern (strong expression in the shoot and root meristems) in
plants. The role of AMLs during vegetative development suggests an additional
role that the AMLs have acquired during its evolution. Findings from this study
suggest a likely role of AMLs in chromatin organization in plants and also support
a role for nutrition mediated signaling in plant development. Implications of these
results are discussed in the latter part of Chapter two.
In Chapter three, I describe identification and functional characterization of
an Arabidopsis adherin homologue, AtSCC2. I also discuss role of this key gene
in various aspects of plant growth and development. The remarkable feature of
AtSCC2 is the presence of a PHD-finger domain, which differentiates this adherin
protein from other characterized adherin family of proteins. Analysis of three TDNA insertion alleles of Atscc2, all of which caused embryo lethality suggested
that AtSCC2 is an essential gene in Arabidopsis. I detected relatively higher
levels of AtSCC2 transcripts during multiple stages of embryogenesis. A
conditional RNAi mediated down regulation of AtSCC2 during post-embryonic
development resulted in reduced fertility. This was subsequently traced back to
defective meiosis in both male and female meiocytes. Evidence is also provided
to suggest the role of AtSCC2 in maintenance of chromosome structure and
architecture in plants besides its role in cohesion establishment. One of the
interesting observations from this work is the demonstration of a likely functional
overlap between cohesin and condensin complexes in plants. In the latter part of
Chapter 3, I have discussed the implications of my findings especially regarding
the role of AtSCC2 in plant meiosis.
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In conclusion, this thesis describes an attempt towards enhancement of
our understanding of plant meiosis. I have employed a candidate gene approach
(reverse genetics) involving RNAi mediated post transcriptional gene silencing
and T-DNA insertion mutants to study the selected genes. The work presented
provides a very clear understanding of the role of two important groups of genes,
AML gene family and AtSCC2 during meiosis in Arabidopsis. This would be
beneficial and certainly add on to our current knowledge of plant meiosis.
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