Role of Class III Peroxidases in Stem Lignification of Zinnia elegans Jacq.

Class III peroxidases (EC 1.11.1.7) have an affinity for a wide range of substrates and perform numerous functions, including the formation of lignin precursors - monolignol radicals using hydrogen peroxide. The activity and tissue localization of guaiacol (GPOX) and benzidine peroxidases (BPOX), which differ in the optimum pH (7.0 and 5.0, respectively) were determined in the first internode (relative to the hypocotyl) of zinnia plants of different age. Its length increased during 40 days from seed germination, and did not change then. The Klason lignin content increased linearly up to 60 days. Histochemical analysis revealed that in juvenile plants (20 days) lignin was found mainly in protoxylem, and in adult plants (60 days) – in sclerenchyma, protoxylem and metaxylem.

Hydrogen peroxide is a marker of lignification; together with phenolic compounds, it is used by class III peroxidases to form monolignol radicals. H₂O₂ content increased in internode tissues for 40 days, and then did not change up to 60 days. According to Ros Barcelo (2004), H₂O₂ is localized mainly in non-lignified cells of stem parenchyma and protoxylem in juvenile plants, and in mechanical tissues, metaxylem, and phloem in adult.

Histochemistry of enzymes revealed that BPOX was localized in endoderm, phloem, and protoxylem, while GPOX – in the metaxylem and sclerenchyma. A moderate increase of GPOX activity during internode growth was shown, and it correlated with lignin content. In contrast, BPOX activity was high at the initial growth stage, and declining to 60 days.

So the active lignification in mechanical tissue and xylem occurs during the period from 20 to 40 days. BPOX is likely involved in the processes at the early stages of growth, while GPOX is responsible for sclerenchyma and metaxylem lignification at the later stages.