Plants are sessile organisms adapted to cope with dynamic changes in their environment. Abiotic stresses, such as heat, drought, or nutrient deficiency must be overcome simultaneously with biotic threats such as pathogens and herbivores. Oomycete pathogens represent a significant threat to global food production and natural ecosystems. Novel modes of oomycete disease control could increase crop yield and reduce pesticide application. Overlaps between the plant response to iron deficiency and pathogens have been documented, but the impact of simultaneous imposition of both stresses on the plant have not been studied. Additionally, nothing is known about the impact of iron deficiency on oomycete infection, or mechanisms of oomycete iron uptake. We adapted a hydroponic system to simultaneously impose iron deficiency and monitor pathogen infection. The oomycete pathogens Hyaloperonospora arabidopsidis, and Phytophthora capsici grew less well on iron-deficient Arabidopsis thaliana, at least in part because of observed activation of immunity due to iron stress. We screened A. thaliana T-DNA insertion mutants defective in iron metabolism and transport and identified potential mechanisms of H. arabidopsidis iron acquisition. We conducted RNA sequencing to understand how A. thaliana responds to iron deficiency and root infection of P. capsici. 323 genes were differentially upregulated in iron-starved plants over three days, irrespective of pathogen infection, representing a core iron deficiency response. This group of core genes included the primary A. thaliana iron uptake pathway and genes for coumarin biosynthesis. Salicylic acid responsive genes were observed in both treatments consistent with this defense hormone's previously identified role in iron deficiency. Genes related to glucosinolate production – shown to be important in defense against P. capsici – were down regulated during infection, potentially due to the activity of virulence effectors. Our work demonstrates crosstalk between the iron deficiency response and plant immunity, and that iron acquisition remains important to the plant even after pathogen invasion. These new insights provide a first step in developing novel resistance strategies to control oomycetes in agronomically important crops. / Doctor of Philosophy / Oomycetes can cause diseases in plants resulting in loss of crops and requiring application of chemical pesticides. Better understanding of how oomycetes interact with plants will lead to new strategies to control them and more efficient agriculture. In this study, we investigated the role of iron in plant-oomycete interactions, to see what this important metal nutrient might be doing to help or hurt the plants response to infection. We developed a growth system to limit iron to the plant and simultaneously observe oomycete infection. We studied the leaf pathogen Hyaloperonospora arabidopsidis or downy mildew, and Phytophthora capsici, a root pathogen that infects many types of vegetable crops. In rice, iron restriction hurt the plant's ability to fight off disease, but we observed the opposite: iron limitation caused the plant to be more resistant to both oomycete pathogens. Microscopic observation revealed that the plants ability to fight off downy mildew was not compromised by iron deficiency. Our results suggest that iron limitation triggers an immune response in the plant, which limits pathogen growth. We performed RNA sequencing on iron-deficient roots also infected with the root pathogen. This allowed us to observe how the plant responded to both stresses. The plant balances the response to iron deficiency and infection. Again, we found that iron deficiency triggers immune activation, and observed that iron-deficient plants are more resistant to infection.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/97571 |
| Date | 08 April 2020 |
| Creators | Herlihy, John H. |
| Contributors | Plant Pathology, Physiology and Weed Science, McDowell, John M., Pilot, Guillaume, Long, Terri A., Bombarely Gomez, Aureliano |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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