Roles of a major O-acetylserine (thiol) lyase (OASTL) in cysteine biosynthesis, innate immunity and disease resistance in Arabidopsis : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Biology at Massey University, Palmerston North, New Zealand
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2012
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Massey University
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Abstract
O-Acetylserine (thiol) lyases (OASTLs) are evolutionary conserved proteins among many
prokaryotes and eukaryotes that carry out sulphur acquisition and synthesis of cysteine. OASTL
catalyses cysteine biosynthesis using O-acetylserine (OAS) and sulfide as substrates. OASTL also
interact with another enzyme Serine acetyltransferase (SERAT) to facilitate the production of OAS.
Cysteine-derived thiols and metabolites play an important function in regulating cellular redox
conditions and modulate abiotic and biotic stress responses. The Arabidopsis thaliana genome
encodes multiple OASTL isoforms that are targeted to different sub-cellular compartments. The
cytosolic OASTL-A1 or known as ONSET OF LEAF DEATH3 (OLD3) is the major OASTL
isoform due to its high OASTL activity. The old3-1 mutation causes a dysfunctional oastl-a1/old3-1
protein in vitro and was previously shown to cause autonecrosis in specific Arabidopsis accessions.
To investigate why a mutation in a major OASTL isoform causes cell death and necrosis in some
but not other accessions different mutations in OASTL-A1 were characterised in Arabidopsis
accessions. Here it is shown that the old3-1 mutation causes an autoimmune syndrome and
metabolic disorder, in the Ler-0 accession (parent) genetic background, but not in the reference
accession Col-0. This is not the result of lack of functional OASTL-A1 or impaired cysteine
biosynthesis. A Recognition of Peronospora Parasitica 1 (RPP1)-like disease resistance R gene,
from an evolutionary divergent R gene cluster in Ler-0, shows a negative epistatic interaction to
old3-1 and activates autonecrosis. The severity of autonecrosis was found to be dependent upon
variations in temperature and day length. Next, the role of OASTL-A1 was also identified in
resistance against infection with virulent and non-virulent Pseudomonas syringae pv. tomato
DC3000 strains. Since OASTL also interacts with SERAT, old3-1 was found to negatively affect
the interaction with SERAT in vivo, highlighting that the release of R-mediated immunity is
associated with the loss of key functions associated with OASTL. Finally various mutations were
generated in OASTL-A1 isoforms to identify the relevance between the loss of OASTL functions
and R-mediated immunity. These results indicated that motifs in close proximity of old3-1 mutation
play an important role in cysteine biosynthesis and therefore likely an important interface to affect
R-mediated immunity. The study indicates a novel cross-talk between cysteine procuring a major
OASTL isoform and components of plant immunity and further support emerging evidence that
cysteine-derived metabolites function in immune signalling across kingdoms.
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Keywords
O-Acetylserine (thiol) lyase (OASTL), Plant immunity, Arabidopsis, Plant metabolism, Arabidopsis genetics