Final Report Summary - KINGDOM CELL DEATH (Autophagic cell death across Kingdoms; using the plant genetic model system Arabidopsis to characterize cell death regulators in animals, with applications for human cancer)
Unlike vertebrates, plants lack a somatic, adaptive immune system and immunological memory. Therefore, plants rely on a large repertoire of pre-existing immune receptors, encoded by hypervariable Resistance (R) genes, which recognize specific pathogens and activate strong defense responses. These responses include the programmed cell death (PCD) of host cells at infection sites to restrict pathogen access in a process called the hypersensitive response (HR). Members of the animal NOD-like receptor (NLR) family exhibit similar domain architecture to many plant R proteins, and NLRs are likewise involved in immunity.
In plants, there are numerous examples of mutants with autoimmunity-related phenotypes. These so-called “lesion-mimics” are, in many cases, caused by mutations in genes hypothesized to be negative regulators of the HR. Other examples include point mutations in R proteins. Since R proteins have the potential to trigger host PCD, their activity is tightly regulated. R genes are typically constitutively expressed at low levels and some are up-regulated in response to pathogen-derived peptides or to the accumulation of the phytohormone salicylic acid (SA). The lethal, recessive accelerated cell death 11 (acd11) mutant of Arabidopsis is characterized by constitutive activation of immune responses and PCD in the absence of pathogen attack. ACD11 encodes a putative sphingosine transfer protein, but the biochemical significance of this remains unknown. acd11 mutants develop normally until the 2-4 leaf stage, and PCD involves SA such that expression of a bacterial SA hydroxylase (NahG) strongly suppresses cell death. Application of SA agonists, such as benzothiadiazol-S-methyl ester (BTH), restores autoimmunity in acd11. Interestingly, the genetic requirements for acd11 cell death are similar to those for the HR triggered by one class of R protein immune receptors.
Using a screen for genetic suppressors of death in acd11 (lazarus mutants), we discovered that acd11 is suppressed by mutations in genes encoding a histone methyltransferase (LAZARUS2, LAZ2 also known as SET DOMAIN GROUP 8, SDG8) and an R protein (LAZ5; Palma et al., 2010, PLoS Pathog 6(10): e1001137). In addition, the expression of LAZ5 is dependent on the activity of LAZ2. We proposed that the R receptor is triggered by the absence of ACD11, implying that ACD11 (or a complex containing ACD11) may be a guarded pathogen effector target. Our study provides strong evidence that a specific type of histone modification is directly involved in chromatin remodeling and transcriptional control of a subset of R genes including LAZ5.
The major breakthrough during the Marie Curie-funded postdoctoral research was the discovery that chromatin remodeling is a key process determining expression of pathogen-responsive genes, particularly genes encoding R receptors. Chromatin remodeling has emerged as a complex regulator of transcription and an epigenetic mechanism to maintain lasting changes in gene activity states. Dynamic post-translational modifications of various residues of histones tails, including methylation, phosphorylation, acetylation, and ubiquitination, play important roles in both promoting and repressing gene expression by recruiting histone binding proteins and chromatin remodeling enzymes. The combinatorial nature of histone modifications results in a complex “histone code” that adds an important level of control to fine-tune gene-specific responses to broader transcriptional inputs. Changes in chromatin state may therefore modulate gene expression in a context-dependent manner to maintain a flexible response to pathogen attack.
In plants, there are numerous examples of mutants with autoimmunity-related phenotypes. These so-called “lesion-mimics” are, in many cases, caused by mutations in genes hypothesized to be negative regulators of the HR. Other examples include point mutations in R proteins. Since R proteins have the potential to trigger host PCD, their activity is tightly regulated. R genes are typically constitutively expressed at low levels and some are up-regulated in response to pathogen-derived peptides or to the accumulation of the phytohormone salicylic acid (SA). The lethal, recessive accelerated cell death 11 (acd11) mutant of Arabidopsis is characterized by constitutive activation of immune responses and PCD in the absence of pathogen attack. ACD11 encodes a putative sphingosine transfer protein, but the biochemical significance of this remains unknown. acd11 mutants develop normally until the 2-4 leaf stage, and PCD involves SA such that expression of a bacterial SA hydroxylase (NahG) strongly suppresses cell death. Application of SA agonists, such as benzothiadiazol-S-methyl ester (BTH), restores autoimmunity in acd11. Interestingly, the genetic requirements for acd11 cell death are similar to those for the HR triggered by one class of R protein immune receptors.
Using a screen for genetic suppressors of death in acd11 (lazarus mutants), we discovered that acd11 is suppressed by mutations in genes encoding a histone methyltransferase (LAZARUS2, LAZ2 also known as SET DOMAIN GROUP 8, SDG8) and an R protein (LAZ5; Palma et al., 2010, PLoS Pathog 6(10): e1001137). In addition, the expression of LAZ5 is dependent on the activity of LAZ2. We proposed that the R receptor is triggered by the absence of ACD11, implying that ACD11 (or a complex containing ACD11) may be a guarded pathogen effector target. Our study provides strong evidence that a specific type of histone modification is directly involved in chromatin remodeling and transcriptional control of a subset of R genes including LAZ5.
The major breakthrough during the Marie Curie-funded postdoctoral research was the discovery that chromatin remodeling is a key process determining expression of pathogen-responsive genes, particularly genes encoding R receptors. Chromatin remodeling has emerged as a complex regulator of transcription and an epigenetic mechanism to maintain lasting changes in gene activity states. Dynamic post-translational modifications of various residues of histones tails, including methylation, phosphorylation, acetylation, and ubiquitination, play important roles in both promoting and repressing gene expression by recruiting histone binding proteins and chromatin remodeling enzymes. The combinatorial nature of histone modifications results in a complex “histone code” that adds an important level of control to fine-tune gene-specific responses to broader transcriptional inputs. Changes in chromatin state may therefore modulate gene expression in a context-dependent manner to maintain a flexible response to pathogen attack.