title: Polyphenol oxidase-mediated protection against oxidative stress is not associated with enhanced photosynthetic efficiency.
abstract: OBJECTIVE
  Polyphenol oxidases (PPOs) catalyse the oxidation of monophenols and/or o-diphenols to highly reactive o-quinones, which in turn interact with oxygen and proteins to form reactive oxygen species (ROS) and typical brown-pigmented complexes. Hence PPOs can affect local levels of oxygen and ROS. Although the currently known substrates are located in the vacuole, the enzyme is targeted to the thylakoid lumen, suggesting a role for PPOs in photosynthesis. The current study was designed to investigate the potential involvement of PPOs in the photosynthetic response to oxidative stress.
  
  
  METHODS
  Photosynthesis (A, Fv/Fm, ΦPSII, qN, qP, NPQ) was measured in leaves of a wild-type and a low-PPO mutant of red clover (Trifolium pratense 'Milvus') under control conditions and under a stress treatment designed to induce photooxidative stress: cold/high light (2 °C/580 µmol m(2 )s(-1)) or 0-10 µm methyl viologen. Foliar protein content and oxidation state were also determined.
  
  
  RESULTS
  Photosynthetic performance, and chlorophyll and protein content during 4 d of cold/high light stress and 3 d of subsequent recovery under control growth conditions showed similar susceptibility to stress in both lines. However, more extensive oxidative damage to protein in mutants than wild-types was observed after treatment of attached leaves with methyl viologen. In addition, PPO activity could be associated with an increased capacity to dissipate excess energy, but only at relatively low methyl viologen doses.
  
  
  CONCLUSIONS
  The presence of PPO activity in leaves did not correspond to a direct role for the enzyme in the regulation or protection of photosynthesis under cold stress. However, an indication that PPO could be involved in cellular protection against low-level oxidative stress requires further investigation.
authors.affiliation: Institute of Biological, Environmental and Rural Sciences (IBERS), Aberystwyth University, Aberystwyth, Ceredigion SY23 3FG, UK.
authors.forename: Tinne
authors.initials: T
authors.lastname: Boeckx
authors.affiliation: Institute of Biological, Environmental and Rural Sciences (IBERS), Aberystwyth University, Aberystwyth, Ceredigion SY23 3FG, UK.
authors.forename: Richard
authors.initials: R
authors.lastname: Webster
authors.affiliation: Institute of Biological, Environmental and Rural Sciences (IBERS), Aberystwyth University, Aberystwyth, Ceredigion SY23 3FG, UK.
authors.forename: Ana L
authors.initials: AL
authors.lastname: Winters
authors.affiliation: Institute of Biological, Environmental and Rural Sciences (IBERS), Aberystwyth University, Aberystwyth, Ceredigion SY23 3FG, UK.
authors.forename: K Judith
authors.initials: KJ
authors.lastname: Webb
authors.affiliation: Institute of Biological, Environmental and Rural Sciences (IBERS), Aberystwyth University, Aberystwyth, Ceredigion SY23 3FG, UK.
authors.forename: Alan
authors.initials: A
authors.lastname: Gay
authors.affiliation: Institute of Biological, Environmental and Rural Sciences (IBERS), Aberystwyth University, Aberystwyth, Ceredigion SY23 3FG, UK ahk@aber.ac.uk.
authors.forename: Alison H
authors.initials: AH
authors.lastname: Kingston-Smith
mesh_terms: D004156:Catechol Oxidase / Q000378:metabolism*; D004579:Electron Transport; D018384:Oxidative Stress; D010788:Photosynthesis; D010940:Plant Proteins / Q000378:metabolism*; D013312:Stress, Physiological; D029921:Trifolium / Q000201:enzymology / Q000378:metabolism*
chemical_list: D010940:Plant Proteins; D004156:Catechol Oxidase
keywords: PPO; ROS; Trifolium pratense ‘Milvus’; chlorophyll fluorescence; cold; methyl viologen; photosynthesis; polyphenol oxidase; protein oxidation; reactive oxygen species; red clover