Other factors than apoplastic ascorbate contribute to the differential ozone tolerance of two clones of Trifolium repens L.

Abstract

Apoplastic reactive oxygen intermediates, which are formed during the exposure of a higher plant to ozone (O 3 ), have been proposed to be detoxified by apoplastic ascorbate (ASC). An investigation to determine whether the differential sensitivity of two white clover clones ( Trifolium repens L. cv Regal) to O 3 was related with their levels of ASC, glutathione derivatives or with the total antioxidative capacity. In contrast to what might be expected, the sensitive clone of white clover (NC-S) constitutively showed a 72% higher concentration of apoplastic ASC compared to the O 3 -tolerant clone (NC-R). Furthermore, NC-S also showed a higher redox status of apoplastic ASC. These results indicate that higher ASC levels in the apoplast of NC-S are not sufficient to induce a higher O 3 tolerance. The redox status, but not the absolute concentration of homoglutathione in the symplast was found to be constitutively higher in NC-R than in NC-S. It is not clear, however, whether homoglutathione is a direct cause of the differential O 3 detoxification capacity of both clones. Total antioxidative capacity measurements ruled out the contribution of other low-molecular antioxidants to the relative tolerance of NC-R. It was concluded that elevated apoplastic ASC levels can not always be sufficient to render a plant O 3 tolerant.

AMR1, an Arabidopsis Gene That Coordinately and Negatively Regulates the Mannose/l-Galactose Ascorbic Acid Biosynthetic Pathway

“…However, in two white clover (Trifolium repens) clones, NC-S and NC-R, neither the amount of extracellular or symplastic AsA nor its oxidation state correlated with ozone tolerance, indicating that additional ROS-neutralizing strategies exist in some species (D'Haese et al, 2005). Ozone treatment revealed that the two Arabidopsis insertion mutants, amr1-1 and amr1-2, are indeed more tolerant to this oxidative stress compared with wild-type plants.…”

Involvement of AsA/DHA and GSH/GSSG Ratios in Gene and Protein Expression and in the Activation of Defence Mechanisms Under Abiotic Stress Conditions

“…Low AsA decreases the threshold for sensing stress and can trigger cell death and systemic acquired resistance (SAR), as seen in AsA-deficient mutants (Pastori et al 2003;Foyer and Noctor 2005a). However, in others works it was suggested that apoplastic AsA alone could not explain differential ozone tolerance in Trifolium clones and soybean leaves (D'Haese et al 2005;Cheng et al 2007). In support to this hypothesis, Kollist et al (2000) provide experimental evidence that the direct removal of O 3 by apoplastic AsA is limited in various plant species because mesophyll cell walls are thin and the effective path length for the reaction is short.…”

Is Ozone Flux Inside Leaves Only a Damage Indicator? Clues from Volatile Isoprenoid Studies

“…In a recent experiment, ascorbate levels of Catharanthus roseus explained part of the variation in ozone flux that stomatal conductance did not account for (Eller and Sparks, 2006). Ascorbate was detected at a level 50% to 70% higher in the ozone resistant clone of T. repens cv Regal with respect to the sensitive one (Ferreira Severino et al, 2007), although this finding is still controversial (D'Haese et al, 2005). Ozone may apparently also induce biosynthesis and emission of volatile isoprenoids, mainly sesquiterpenes, even in plants that do not naturally emit these compounds (Heiden et al, 1999).…”