Abstract
Under experimental conditions, mature living conifer needles on isolated stem segments induce cells at the junction of the needle trace and the cambium to differentiate into tracheary elements (TEs). The response is an all-or-none event in terms of secondary-wall deposition, but the extent of deposition varies between cells, and hence may serve as a model for developmental plasticity at the cellular level. Attempts to reproduce needle-induced TE differentiation using plant growth regulators such as auxin and cytokinin with or without carbohydrates have been unsuccessful but nevertheless have provided evidence for interactions between the needle-produced tracheid-differentiation factor (tdf) and phytohormones in the regulation of cellular differentiation. Treatment of needle-pair stumps with D-myoinositol 1,4,5-trisphosphate, the second messenger from bovine brain, has induced the tdf response at the needle tracecambium junction; however, unequivocal characterization of the endogenous tdf remains to be ach...
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