title: Suppression of the auxin response pathway enhances susceptibility to Phytophthora cinnamomi while phosphite-mediated resistance stimulates the auxin signalling pathway.
abstract: BACKGROUND
  Phytophthora cinnamomi is a devastating pathogen worldwide and phosphite (Phi), an analogue of phosphate (Pi) is highly effective in the control of this pathogen. Phi also interferes with Pi starvation responses (PSR), of which auxin signalling is an integral component. In the current study, the involvement of Pi and the auxin signalling pathways in host and Phi-mediated resistance to P. cinnamomi was investigated by screening the Arabidopsis thaliana ecotype Col-0 and several mutants defective in PSR and the auxin response pathway for their susceptibility to this pathogen. The response to Phi treatment was also studied by monitoring its effect on Pi- and the auxin response pathways.
  
  
  RESULTS
  Here we demonstrate that phr1-1 (phosphate starvation response 1), a mutant defective in response to Pi starvation was highly susceptible to P. cinnamomi compared to the parental background Col-0. Furthermore, the analysis of the Arabidopsis tir1-1 (transport inhibitor response 1) mutant, deficient in the auxin-stimulated SCF (Skp1 - Cullin - F-Box) ubiquitination pathway was also highly susceptible to P. cinnamomi and the susceptibility of the mutants rpn10 and pbe1 further supported a role for the 26S proteasome in resistance to P. cinnamomi. The role of auxin was also supported by a significant (P < 0.001) increase in susceptibility of blue lupin (Lupinus angustifolius) to P. cinnamomi following treatment with the inhibitor of auxin transport, TIBA (2,3,5-triiodobenzoic acid). Given the apparent involvement of auxin and PSR signalling in the resistance to P. cinnamomi, the possible involvement of these pathways in Phi mediated resistance was also investigated. Phi (especially at high concentrations) attenuates the response of some Pi starvation inducible genes such as AT4, AtACP5 and AtPT2 in Pi starved plants. However, Phi enhanced the transcript levels of PHR1 and the auxin responsive genes (AUX1, AXR1and AXR2), suppressed the primary root elongation, and increased root hair formation in plants with sufficient Pi.
  
  
  CONCLUSIONS
  The auxin response pathway, particularly auxin sensitivity and transport, plays an important role in resistance to P. cinnamomi in Arabidopsis, and phosphite-mediated resistance may in some part be through its effect on the stimulation of the PSR and auxin response pathways.
authors.affiliation: Centre for Phytophthora Science and Management, School of Veterinary and Life Sciences, Murdoch University, South Street, Murdoch, WA 6150, Australia. L.Eshraghi@murdoch.edu.au.
authors.forename: Leila
authors.initials: L
authors.lastname: Eshraghi
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Anderson
authors.forename: Nader
authors.initials: N
authors.lastname: Aryamanesh
authors.forename: Jen A
authors.initials: JA
authors.lastname: McComb
authors.forename: Bryan
authors.initials: B
authors.lastname: Shearer
authors.forename: Giles St J E
authors.initials: GS
authors.lastname: Hardy
mesh_terms: D017360:Arabidopsis / Q000187:drug effects / Q000235:genetics / Q000378:metabolism / Q000382:microbiology*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D001692:Biological Transport / Q000187:drug effects; D060467:Disease Resistance / Q000276:immunology*; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D017343:Genes, Plant; D007210:Indoleacetic Acids / Q000378:metabolism*; D031297:Lupinus / Q000187:drug effects / Q000378:metabolism / Q000382:microbiology*; D009154:Mutation / Q000235:genetics; D010710:Phosphates / Q000172:deficiency / Q000494:pharmacology; D017905:Phosphites / Q000494:pharmacology*; D010838:Phytophthora / Q000187:drug effects / Q000502:physiology*; D010935:Plant Diseases / Q000235:genetics / Q000382:microbiology; D018517:Plant Roots / Q000187:drug effects / Q000235:genetics; D030821:Plants, Genetically Modified; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D021122:Protein Subunits / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects* / Q000235:genetics; D014283:Triiodobenzoic Acids
chemical_list: D029681:Arabidopsis Proteins; D007210:Indoleacetic Acids; D010710:Phosphates; D017905:Phosphites; D021122:Protein Subunits; D014283:Triiodobenzoic Acids; D046988:Proteasome Endopeptidase Complex; C052968:2,3,5-triiodobenzoic acid