Comparison between tracheary element lignin formation and extracellular lignin-like substance formation during the culture of isolated Zinnia elegans mesophyll cells

Lignin is synthesized not only during morphogenesis of vascular plants but also in response to various stresses. Isolated Zinnia elegans mesophyll cells can differentiate into tracheary elements (TEs), and deposit lignin into cell walls in TE-inductive medium (D medium). Meanwhile isolated mesophyll cells cultured in hormone-free medium (Co medium) accumulate stress lignin-like substance during culture. Therefore this culture system is suitable for study of lignin and lignin-like substance formation.In D medium lignin was deposited in TEs, but in Co medium, extracellular lignin-like substance accumulated. Analysis of the culture media indicated the presence of dilignols in D culture, but not in Co culture. To investigate the fate of lignin precursors, we added coniferyl alcohol (CA) in each culture. In Co medium, CA was polymerized into dilignols rapidly but they were present only temporarily, and in D medium CA was polymerized into dilignols relatively slowly but their content increased continually.Meanwhile, in Co culture, peroxidase activity in the medium was much higher than the peroxidase activity bound ionically to the cell walls. In D culture, ionically bound peroxidase activity was higher than that in the medium. These results may suggest that lignin deposition in TEs is related to ionically bound peroxidases in D culture, and lignin-like substance deposition in the medium is related to peroxidases in the medium in Co culture.