We have discovered that deoxynivalenol (DON) induces the chloroplast unfolded protein response (cpUPR) in Chlamydomonas and that the cpUPR signaling mutant affected in chloroplast-to-nucleus retrograde signaling (MARS1) mutant is highly sensitive to this class of fungal toxins. This mutant, along with others, were identified by screening the indexed and mapped Chlamydomonas reinhardtii deletion library with 500 nM trichothecin (Tcin) by deep sequencing of mutant bar codes from DNA isolated from bulk flask cultures grown in duplicate. This mars1 deletion mutant plus additional mutant alleles were screened on Tcin and DON and a high degree of sensitivity was confirmed for both. Previous work in the lab has documented chloroplast damage after exposure to DON via confocal microscopy in Chlamydomonas, Arabidopsis, and wheat. These new findings suggest a greater involvement of the chloroplast in the perception/response to DON than previously realized. This is critical to understand because the chloroplast can serve as a hub for ROS and programmed cell death (PCD) signaling impacting the plant response to the fungus and trichothecenes. A recent report indicates that cpUPR is critical for chloroplasts to mitigate photodamage. We also found that overexpression of the cpUPR gene MARS1 significantly enhances resistance to the virulence factor DON. Our results suggest that cpUPR is involved in increasing the tolerance to chloroplast stress due to DON exposure. The Chlamydomonas knockout library, a “green yeast” mutant library for genome-wide screening, represents a unique and valuable resource to discover plant genes which are impacted by trichothecenes. Additional screening is likely to identify other mutants that play a role in how photosynthetic cells respond to DON. We will present our efforts to screen the B-Series CLiP Collection of ~13,000 high quality Chlamydomonas gene knockouts available from the Chlamydomonas Resource Center. Expanding these findings to targeted research in wheat and barley would enhance our understanding of the Fusarium graminearum pathosystem. Understanding how the chloroplast protein quality control system in higher plants is impacted by trichothecenes may provide unique methods to increase resistance to the mycotoxin and the fungus.

ACKNOWLEDGEMENT 

We thank Silvia Ramundo, now at the Vienna BioCenter, Austria and previously at University of California, San Francisco (Peter Walter's laboratory) for providing the cpUPR reporter, the MARS1 OE strain, and other resources used in this research.