Re-evaluating the role of phenolic glycosides and ascorbic acid in ozone scavenging in the leaf apoplast of Arabidopsis thaliana L

Authors: Booker, Fitzgerald; Burkey, Kent; JONES, ALAN - University Of North Carolina

Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2012
Publication Date: 8/1/2012
Citation: Booker, F.L., Burkey, K.O., Jones, A. 2012. Re-evaluating the role of phenolic glycosides and ascorbic acid in ozone scavenging in the leaf apoplast of Arabidopsis thaliana L. Plant Cell and Environment. 35:1456-1466.

Interpretive Summary: In plants, the first line of defense against the air pollutant ozone are protective chemicals located outside cells in the leaves. This region, known as the apoplast, contains antioxidants such as ascorbic acid (vitamin C) and phenolic glycosides in many plants. It is unknown how this chemistry is regulated in response to ozone. To determine if membrane-bound G-proteins are involved in the regulation of these defense responses in the leaf apoplast, the apoplastic concentrations of ascorbic acid and phenolic glycosides in mutants lacking specific G-protein components of the model plant, Arabidopsis thaliana, were compared with wild-type plants following exposure to ozone. Plants were treated with a range of ozone concentrations (5, 125 or 175 ppb) in controlled environment chambers. In contrast to the current understanding of early plant responses to ozone, our results showed that almost no ascorbate was available for initial ozone detoxification reactions in this plant species. However, ozone increased in a dose-dependent manner the apoplastic levels of the phenolic glycoside, sinapoyl malate. For sinapoyl malate, plants lacking the G-protein alpha component were significantly less responsive to ozone, possibly a reflection of the role played by the alpha G-protein in cellular processes that sense oxidative stress from ozone. Phenolic glycosides are effective reactive oxygen scavengers, suggesting that compounds in addition to ascorbate in the apoplast may be important for protecting plants against oxidative stress. If a protective function is confirmed in other species, then phenolic glycoside concentration in the leaf apoplast becomes a potential target for manipulation to improve the ozone tolerance of crops.

Technical Abstract: To determine if membrane-bound G-proteins are involved in the regulation of defense responses against ozone in the leaf apoplast, the apoplastic concentrations of ascorbic acid and phenolic glycosides in Arabidopsis thaliana L. lines with null mutations in the alpha- and beta-subunits were compared with wild-type plants following exposure to ozone. Plants were treated with a range of ozone concentrations (5, 125 or 175 ppb) in controlled environment chambers. Results showed that ascorbate in leaf apoplast extracts was almost entirely oxidized in all genotypes and ozone concentrations tested, suggesting that little ascorbate is available for initial ozone detoxification reactions. However, ozone increased in a dose-dependent manner the apoplastic levels of sinapoyl malate. For sinapoyl malate, plants lacking the G alpha subunit were significantly less responsive to ozone. Phenolic glycosides are effective reactive oxygen scavengers, suggesting that compounds in addition to ascorbate in the apoplast may have functional importance. Current understanding of the role of extracellular ascorbate in ozone response mechanisms deserves reassessment as other metabolites may predominate in some species. Signal transduction by the alpha G-protein modulated the response of sinapoyl malate in the leaf apoplast during ozone exposure, possibly as an indirect effect of regulating ozone-induced secondary oxidative bursts by the plant.