Abstract:
Arabidopsis is a model plant used to study disease resistance; Solanum tuberosum or
potato is a crop species. Both plants possess inducible defense mechanisms that are
deployed upon recognition of pathogen invasion. Transcriptional reprogramming is
crucial to the activation of defense responses. The Pathogenesis-Related (PR) genes are
activated in these defense programs. Expression of Arabidopsis PR-l and potato PR-10a
serve as markers for the deployment of defense responses in these plants.
PR-l expression indicates induction of systemic acquired resistance (SAR).
Activation of SAR requires accumulation of salicylic acid (SA), in addition to the
interaction of the non-expressor of pathogenesis-related genes I (NPRI), with the TGA
transcription factors.
The PR-10a is activated in response to pathogen invasion, wounding and elicitor
treatment. PR-10a induction requires recruitment of the Whirly I (Whyl) activator to the
promoter. This locus is also negatively regulated by the silencer element binding factor
(SEBF).
We established that both the PR-l and PR-10a are occupied by repressors under
non-inducing conditions. TGA2 was found to be a constitutive resident and repressor of
PR-l, which mediates repression by forming an oligomeric complex on the promoter.
The DNA-binding activity of this oligomer required the TGA2 N-terminus (NT).
Under resting conditions we determined that the PR-10a is bound by a
repressosome containing SEBF and curiously the activator Pto interacting protein 4
(Pti4). In the context of this repressosome, SEBF is responsible for PR-10a binding, yet
rWe also showed that PR-l and PR-10a are activated by different means. In PR-l
activation the NPRI NT domain alleviates TGA2-mediated repression by interacting with
the TGA2 NT. TGA2 remains at the PR-l but adopts a dimeric conformation and forms
an enhanceosome with NPRl. In contrast, the PR-10a is activated by evicting the
repressosome and recruiting Why! to the promoter.
These results advance our understanding of the mechanisms regulating PR-l and
PR-10a expression under resting and inducing conditions. This study also revealed that
the means of regulation for related genes can differ greatly between model and crop
s
