Phenylpropanoid metabolism and phenolic composition of soybean [Glycine max (L.) Merr.] leaves following exposure to ozone

Abstract

Plants treated with the air pollutant, ozone (O3), often respond with increased transcript levels and activities of enzymes in the general phenylpropanoid and lignin pathways. This suggests that increased biosynthesis of lignin and related products also occurs. The purpose of this study was to determine whether O3 stimulated enzyme activities in these pathways in soybean [Glycine max (L.) Merr.] leaves, and if so, were hydroxycinnamic acids, lignin and suberin also produced. Plants were grown for 6 weeks in charcoal-filtered (CF) air and then treated with either CF air or CF air plus 100 nmol O3 mol−-1 7 h daily for up to 13 d in chambers in the greenhouse or in open-top chambers in the field. In greenhouse experiments, the activities of general phenylpropanoid pathway enzymes (phenylalanine ammonia-lyase and 4-coumarate:CoA ligase) were stimulated by O3 after 6 h. The activity of an enzyme in the lignin pathway (cinnamyl alcohol dehydrogenase) increased in O3-treated plants after 27 h. In greenhouse and field experiments, levels of cell-wall-bound total phenolics, acid-insoluble lignin and lignothioglycolic acid (LTGA) extracted from leaf tissue from O3-treated plants increased on average by 65%. However, histochemistry, UV and IR spectra, radiolabelling and a nitrobenzene oxidation assay all indicated that lignin and suberin did not increase with O3 treatment. Acidinsoluble lignin and LTGA extracted from O3-treated plants probably contained phenolic polymers that form in wounded or senescent tissues, thereby causing overestimates of the changes. Ozone-induced increases in phenolic metabolism, resembling certain elicited defence responses, thus occurred in concert with effects characteristic of the browning reaction and wound responses.