MOLECULAR GENETIC ANALYSIS OF CELL DEATH AND DEFENSE SIGNALING IN ARABIDOPSIS hrl1 MUTANT

Open Access
Author:
Devadas, Sendil Kumaran
Graduate Program:
Plant Physiology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 12, 2001
Committee Members:
  • Teh Hui Kao, Committee Member
  • Ramesh Raina, Committee Chair
  • Timothy W Mcnellis, Committee Member
  • Hong Ma, Committee Member
  • Richard Cyr, Committee Member
Keywords:
  • epistasis
  • genetic analysis
  • microarray
  • lesion
  • arabidopsis
  • defense signaling
  • cell death
Abstract:
Defense against pathogens in Arabidopsis is orchestrated by at least three signaling molecules: salicylic acid (SA), jasmonic acid (JA), and ethylene (ET). The hrl1 (for hypersensitive response-like lesions 1) mutant of Arabidopsis is characterized by spontaneous necrotic lesions, accumulation of reactive oxygen species (ROS), constitutive expression of SA- and ET/JA-responsive defense genes, and enhanced resistance to virulent bacterial and oomycete pathogens. Epistasis analyses of hrl1 with npr1, etr1, coi1, and SA-depleted nahG plants revealed novel interactions between SA and ET/JA signaling pathways in regulating PR gene expression and cell death. Northern analysis of the RNA isolated from the lesionƒy (local) and the lesionƒ{ (systemic) leaves of double mutants of hrl1 uncovered different signaling requirements for the expression of defense genes in these tissues. Expression of ET/JA-responsive PDF1.2 gene was markedly reduced in hrl1 npr1 and in the SA-depleted hrl1 nahG plants. In hrl1 nahG plants, expression of PDF1.2 was regulated by benzathiadiazole (BTH) in a concentration dependent manner: induced at low concentration and suppressed at high concentration. The hrl1 etr1 plants lacked systemic PR-1 expression, and exhibited compromised resistance to virulent Pseudomonas syringae. Inhibiting JA responses in hrl1 coi1 plants lead to exaggerated cell death and severe stunting of plants. Finally, the hrl1 mutation led to elevated expression of AtrbohD, which encodes a major sub-unit of the NADPH oxidase complex. These results indicate that cell death and defense induction in hrl1 is regulated synergistically by SA and ET/JA defense pathways. The preexisting defense responses enable hrl1 plants to resist virulent pathogen infection. However, the same preexisting defense responses in hrl1 suppress HR cell death against virulent pathogens. Furthermore, the high PR-1 expression observed in hrl1 remains unaltered following avirulent and virulent pathogen infections. The suppressed HR phenotype in hrl1 is observed even when an elicitor is expressed endogenously from an inducible promoter, suggesting that an impaired transfer of avirulent factors is not the reason. Interestingly, the lack of HR phenotype in hrl1 is reversed if the constitutive defense responses are compromised either by a mutation in NPR1 or by depleting salicylic acid (SA) due to the expression of the nahG gene. The rescue of HR cell death in hrl1 npr1 and in hrl1 nahG depends on the extent to which the constitutive systemic acquired response (SAR) is compromised. Pre-treating Arabidopsis wild-type plants with SAR-inducers, prior to pathogen infection resulted in a significant decrease in HR cell death. The suppressed HR cell death and the failure to enhance PR gene expression following avirulent pathogen infection in hrl1 suggest that the preexisting defense responses serve as negative feedback loops to regulate extensive cell death and defense-related gene expression. In hrl1, the chronic stress response due to PR gene expression may alter the cellular physiology at various nodes that lead to insensitivity to subsequent pathogen attack. Down-regulation of HR cell death in the presence of an already existing systemic resistance response may be a way for plants to prevent excessive cell death and further defense induction Microarray analyses of plant-pathogen interactions have revealed a complex interplay of multiple signaling pathways (Schenk et al., 2000; Maleck et al 2000). The changes in the transcript levels of approximately 1200 Arabidopsis ESTs were examined in hrl1 and the various double mutants of hrl1. Consistent with the earlier Northern results, genes like PR-1, PR-2, GST1 and PDF1.2 were induced both in the lesionƒy and in the lesion- tissue of hrl1. However several GST1-like and PR-1-like genes were not induced in the lesion- tissue suggesting that they are regulated differently in the lesion+ and lesion- tissue. The lesionƒy tissue had a stronger overall response than the lesionƒ{ tissue suggesting that the lesionƒy tissue mimic local HR cell death and the lesionƒ{ tissue resemble the distant SAR response. Approximately 50 genes (22%) that have altered expression pattern in hrl1 encode proteins that are involved in metabolism. This suggests that cell death and defense signaling in hrl1 may interact with components involved in cellular homeostasis. These studies further highlight the complexity involved in the regulation of the defense response in plants against pathogens.