The hormonal control of the redifferentiation of tracheids from parenchyma cells was studied in the hypocotyl of young Pinus pinea L. seedlings. Auxin alone induced short regenerative tracheids usually in discontinuous patterns across the hypocotyl. Combinations of both auxin and gibberellin promoted the differentiation of relatively long tracheids in continuous strands parallel to the stem axis. While combinations of ethrel (which releases ethylene) with either auxin, or both auxin and gibberellin, induced more regenerative tracheids many of them with various horizontal, or radial orientations. Neither gibberellin, nor ethrel could induce regenerative tracheids in the absence of an auxin stimulation. These results show that tracheids are induced and controlled by combinations of various morphogenetic stimuli.
From the evolution perspective the vessels and fibers in the xylem of angiosperms have originated from the tracheids of conifers. Therefore, it is suggested that during evolution, the hormonal mechanism that controls tracheid formation has been modified in the higher plants. From a mechanism based on combinations of auxin, gibberellin and ethylene for the differentiation of tracheids, the mechanisms in angiosperms become more specific for each xylem element. Auxin alone is enough to induce vessels, while gibberellin in the presence of auxin induces fibers. Ethylene which promotes tracheid differentiation might repress vessel and fiber differentiation. The ethylene, which interrupts the longitudinal orientation of the developing regenerative tracheids, is probably involved in the differentiation of radially oriented tracheids in the vascular rays of conifers.
Key words: auxin, xylem differentiation, ethylene, gibberellin, regenerative tracheids.
Correspondence: Roni Aloni, Department of Botany, Tel Aviv University, Tel Aviv 69978, Israel
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