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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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.
O-Acetylserine (thiol) lyase (OASTL), Plant immunity, Arabidopsis, Plant metabolism, Arabidopsis genetics