Transfer of phenylpropanoids via the medium between xylem cells in Zinnia xylogenic culture

Abstract

In order to understand cell-cell interactions involved in xylem differentiation, we studied intercellular molecules in an in vitro Zinnia xylogenic culture system, where single mesophyll cells transdifferentiate into tracheary elements (TEs) and xylem parenchyma cells. We found that UV-absorbing substances accumulated predominantly in xylogenesis-inducing medium and kept increasing even after the TEs died. This accumulation was inhibited by L-α-aminooxy-β-phenylpropionic acid (AOPP), an inhibitor of phenylalanine ammonia-lyase, and also by brefeldin A, an inhibitor of vesicle transport. These results indicated that living non-TE cells, probably xylem parenchyma cells, secrete some kinds of phenylpropanoids via a vesicle transport system. Further experiment showed that inhibition of brassinosteroid biosynthesis by uniconazole suppressed TE differentiation, but not the secretion of UV-absorbing substances into the medium, implying that differentiation of xylem parenchyma cells might not be strongly affected by the depletion of endogenous brassinosteroids.