The Salicylic Acid-Mediated Growth-Immunity Tradeoff in Arabidopsis

Plants have a dynamic immune system to protect themselves against pathogens. Recognition of a pathogen leads to the activation of this immune system, steered by different phytohormones. One of these, salicylic acid, regulates plant responses to biotrophic pathogens that thrive on living host tissue. Typical SA-mediated responses are the production of antimicrobial proteins and metabolites and the strengthening of the cell wall. These responses can occur both locally and systemically. Due to their nature, these responses cost energy and resources that cannot be invested in plant growth and development. This suppression also works the other way around: active growth and developmental processes suppress immunity. This mutual inhibition is called the growth-immunity tradeoff and is actively regulated by the plant. The research in this thesis is focused on the model plant Arabidopsis thaliana (thale cress), in which the SA content is controlled at the level of synthesis and catabolism. Two enzymes that catabolize SA through hydroxylation are the paralogous 2-oxoglutarate Fe(II)-dependent oxygenases DMR6/S5H (DOWNY MILDEW RESISTANT 6/SA 5-HYDROXYLASE) and DLO1/S3H (DMR6-LIKE OXYGENASE 1/SA 3-HYDROXYLASE). dmr6/dlo1 mutants have enhanced SA levels and therefore a constitutively active immune system and enhanced disease resistance. The high SA levels of double mutant plants cause hyperresistance but also dwarfism. On the other hand, overexpression of DMR6 or DLO1 leads to depletion of SA and thus enhanced susceptibility. We use the perturbed SA catabolism of the dmr6/dlo1 mutants and DMR6/DLO1 overexpression lines to investigate the SA-mediated growth-immunity tradeoff. We found that altered expression of DMR6 and/or DLO1 had major effects on the transcriptome that correlated to the plant’s SA content. We found that enhanced SA levels in dmr6/dlo1 mutants caused reduced photosynthetic efficiency and a sped up development and reduced SA levels in DMR6/DLO1 overexpression lines led to reduced pathogen-associated molecular pattern (PAMP)-triggered immune responses. We further identified a significant effect of the genetic background to perturbations in DMR6/DLO1 expression in different Arabidopsis accessions: several Arabidopsis accessions had a reduced growth tradeoff but retained high disease resistance. To systematically investigate the genetics behind regulation of the growth-immunity tradeoff, we further employed a forward genetics screen for restored growth and high resistance in dmr6-3 dlo1 double mutants and identified seven unique zund mutants. These plants carried mutations in known regulators of SA synthesis (ICS1, ISOCHORISMATE SYNTHASE 1), perception (NPR1, NONEXPRESSOR OF PR GENES 1), or signaling (PAD4, PHYTOALEXIN DEFICIENT 4), indicating that a suppression of SA synthesis or SA responses reduces the growth tradeoff in high immunity dmr6-3 dlo1 mutants. Three other zund mutants carried a mutation in the same gene: MED15a (MEDIATOR 15a), a component of the MEDIATOR complex for transcription initiation. We investigate the function of MED15a in the growth-immunity tradeoff in detail, and find that around 80% of dmr6-3 dlo1-induced gene expression changes are genetically dependent on the integrity of two amino acids in the MED15a kinase-inducible (KIX) domain. I discuss the implications of this research for applications to breed for sustainable disease resistance in crops.