MED9, a mediator complex component, and its interacting protein MORC1 balance growth and defense in Arabidopsis
MetadataShow full metadata
Arabidopsis MORC1 (Microrchidia) is required for multiple levels of immunity, including effector-triggered immunity and basal resistance. MORC1 is also a topoisomerase that tightens chromatin/DNA supercoiling. I identified 14 MORC1-interacting proteins (MIPs) via yeast 2-hybrid screening, eight of which have putative nuclear-associated functions. While a few loss-of-function mutants for these MIPs displayed altered bacterial resistance in Arabidopsis, MIP13 was highly unusual. MIP13’s single mutant was susceptible to Pseudomonas syringae, but when combined with morc1/2 in which MORC1 and its closest homolog were knocked-out, it regained wild-type level resistance; note that morc1/2 is also susceptible to the same pathogen. MIP13 encodes MED9, a mediator complex component that interfaces with RNA polymerase II and transcriptional factors to reprogram transcriptional activities. Transcription dynamics assessments at multiple-time points of defense genes, up- and down-regulated, reacting to avirulent P. syringae revealed that med9 morc1/2 expressed some defense genes in a slow but sustained manner, unlike its low-ordered mutants. This expression pattern may explain the restored resistance but raised concerns that the absence of MORC1/2 and MED9 might incur fitness costs from the prolonged defense responses. Indeed, repeated challenges with avirulent P. syringae triggered significant growth and reproduction decline in med9 morc1/2, suggesting that an optimum growth-defense balance requires MED9 and its interacting partner MORC1. Interestingly, down-regulated defense genes were substantially associated with photosynthesis, and many of them had the canonical G-box in their promoter, linking photosynthesis to defense responses. PIF3, as a G-box-binding factor and a known negative photosynthetic regulator, was positioned to be a strong candidate for negatively regulating photosynthesis. Overexpression of PIF3, consistent with the prospect, led to enhanced resistance to avirulent P. syringae, perhaps by channeling energy from growth to defense responses. Moreover, nuclear PIF3 was significantly increased under ETI, suggesting its suppressive function in photosynthesis aiding defense responses. In summary, my thesis research i) established MORC1, MED9, and PIF3 as important regulators in balancing growth and defense, and ii) laid an insightful mechanical groundwork to achieve optimum resistance for maximum yield in plants.