Trichothecenes as Chemical Probes: Impact on Photosynthesis, Chloroplast Morphology, and Plant Stress Response

Introduction: Deoxynivalenol (DON), a trichothecene mycotoxin produced by Fusarium graminearum, is a potent virulence factor and a critical determinant of Fusarium Head Blight (FHB) in small grain cereals like wheat and barley. Previous research, utilizing a pooled chemical–genetic screen of Chlamydomonas reinhardtii mutants, identified that DON activates the chloroplast unfolded protein response (cpUPR), a chloroplast retrograde signaling pathway. This demonstrated that trichothecenes induce chloroplast stress. The present research leveraged trichothecenes as chemical probes to elucidate the chloroplast-to-nucleus communication during stress. Translating insights from algal systems to higher plants offers crucial clues to ways of mitigating crop diseases like FHB.

Methods: We investigated the impact of trichothecenes on chloroplasts in Chlamydomonas, Arabidopsis, wheat, and barley. The chloroplast damage was assessed via changes in the autofluorescence and general chloroplast morphology. Using a miniaturized Fluorescence Induction and Relaxation (FIRe) system developed by Max Gorbunov at Rutgers University, we measured the photosynthetic rates and photooxidation in Arabidopsis leaf tissues exposed to vacuum-infused trichothecenes.

Results: Trichothecenes consistently caused significant chloroplast damage in higher plants. The exposure of Arabidopsis leaves to 50 μM DON for 1 hour reduced the quantum efficiency of PSII (Fv/Fm) by approximately 20%. DON exposure also significantly reduced the chloroplasts' autofluorescence. In Chlamydomonas, the disruption of Mars1, a key cpUPR kinase, inhibited the induction of proteotoxic stress proteins, including heat shock protein CrHSP22F. Testing orthologous cpUPR-related nuclear genes in wheat showed the robust induction of TaHSP22E. The screen identified additional mycotoxin-sensitive Chlamydomonas mutants exhibiting defective cpUPR activation, providing further insights for higher plant research.

Conclusions: Our findings underscore that trichothecenes are valuable tools for investigating the chloroplast retrograde signaling pathway and identifying novel players, including heat shock proteins, in the cpUPR. These comprehensive analyses, from detailed chloroplast damage assessments to genetic screens, have deepened our understanding of plant stress responses and the mechanisms by which these toxins contribute to causing cereal crop diseases.