Plant Interactions with Other Organisms
Braam, Janet; Bonnie, Bartel
Doctor of Philosophy
With pressures on food availability, improved plant crop productivity and nutritional value are important. To increase crop yield, research into optimizing plant defenses against pests and pathogens can inform new strategies. In addition, efforts to optimize nutrition and health promoting content of plant crops may further contribute to the benefits of plant-rich diets. Arabidopsis thaliana is a model host to investigate defense against fungi. Leaves have often been used to evaluate plant defenses. However, I have found that, unlike rosette leaves, the rosette core, where the leaf petioles join the stem, can prevent fungal infiltration and therefore can display robust fungal resistance. Rosette core fungal resistance is age- and jasmonate-dependent, and may involve formation of an abscission-like zone. Broccoli and cabbage also display rosette core resistance to Botrytis cinerea. Therefore, spatial and developmental aspects of the plant host can play critical roles in fungal resistance and are important to our understanding of plant defense mechanisms. The jasmonate phytohormone is critical to plant biotic resistance. Jasmonate intermediate product (9S,13S)-12-oxo-phytodienoic acid (OPDA) has been implicated as a signaling molecule in fungal defense. To verify this role for OPDA, mutants that block jasmonate biosynthesis at OPDA would be valuable tools. Mutations in OPDA REDUCTASE (OPR3), encoding the only known OPDA reductase, were generated using CRISPR-Cas9. New opr3 mutants were generated, however, these mutants remained capable of jasmonate synthesis. This unexpected finding is consistent with a recent report that Arabidopsis has an OPR3-independent jasmonate biosynthesis pathway. Mutations that block all jasmonate synthesis pathways will be required to verify direct functional roles of OPDA. The plant circadian clock impacts biotic defense. To test whether the time-dependent differential accumulation of plant metabolites is sufficient to confer significant biological effects in feeding organisms, I compared biological effects of broccoli harvested at different times of a day using both the insect herbivore model Trichoplusia ni and a mammalian model rat. Insect choice experiments suggest that time-of-day harvesting may impact feeding behaviors; whereas the rat experiments remain inconclusive. Additional experimentation is required to elucidate how the plant circadian clock may impact host organisms.