Lignification and related parameters in copper-exposed Matricaria chamomilla roots: Role of H2O2 and NO in this process

Despite many investigations of the role of ROS and NO in plant metabolism, their involvement in lignification is still poorly understood. We tested the regulatory role of hydrogen peroxide (scavenged by dithiothreitol, DTT) and nitric oxide (scavenged by 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxy1-3-oxide, PTIO) using chamomile roots exposed to Cu for 24 h. The application of DTT and PTIO led to a compensatory increase in NO and H2O2, respectively. DTT prevented Cu-induced lignin accumulation concomitant with a decrease in H2O2 content and the activities of guaiacol- and ascorbate peroxidases. Cinnamyl alcohol dehydrogenase (CAD) and polyphenol oxidase (PPO) also decreased. DTT stimulated accumulation of total soluble phenols and phenolic acids in all three fractions investigated. The opposite trend was recorded in PTIO-treated roots. Within the free amino acids, proline and phenylalanine increased in response to DTT application. In a subsequent experiment an NO donor (sodium nitroprusside) stimulated the accumulation of phenols and PAL activity as well as prevented a Cu-induced decrease in proteins. The present study has revealed that an increase in total soluble phenols and phenolic acids, including lignin precursors, in DTT-treated roots may be caused by enhancement of PAL activity (stimulated by an increase in NO) and at the same time, by a reduced lignin accumulation (owing to a decrease in ROS level). This study also showed that a compensatory increase in either ROS or NO (depending on which scavenger is used) may sustain PAL activity for antioxidative protection, but that H2O2 is essential for complete lignin deposition during Cu excess. (C) 2010 Elsevier Ireland Ltd. All rights reserved.