The Tracheid-Differentiation Factor of Conifer Needles

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 achieved. Induction of TE differentiation using pine needle pairs is an attractive experimental approach to understanding regulation of cellular differentiation for a number of reasons: (1) A tissue that normally never differentiates, the vascular cambium, can be induced to differentiate into TEs. (2) The location of differentiation can be accurately predicted, and the response can be compared with that of adjoining, nondifferentiating cells. (3) In vitro research is possible using stem segments in test tubes in the absence of complex media, and interactions between the tdf and other regulatory molecules, or other chemical or physical factors, can readily be investigated. (4) The differentiating tissue is heterotrophic; hence, interpretations about regulation are not confounded by autotrophic considerations. (5) Differentiation occurs in "dormant" cells in the absence of preceding vacuolation, cell division, or cell expansion; therefore, TE differentiation per se can be investigated independently of those processes normally accompanying but not essential for differentiation. (6) Polyribosomes, endoplasmic reticulum, dictyosomes, exocytotic blebs, and other organelles clearly fulfill roles related to TE differentiation and appear worthy of further investigation. (7) The system is well suited for performance of in situ hybridization or immunolocalization research.