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Patterns of Tracheary Differentiation in Lettuce Pith Expiants: Positional Control and Temperature Effects

J. WARREN WILSON, P. M. WARREN WILSON and E. S. WALKER
Annals of Botany
Vol. 68, No. 2 (August 1991), pp. 109-128
Published by: Oxford University Press
Stable URL: http://www.jstor.org/stable/42764381
Page Count: 20
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Patterns of Tracheary Differentiation in Lettuce Pith Expiants: Positional Control and Temperature Effects
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Abstract

When lettuce pith expiants were cultured for 14 d on a xylogenic medium, tracheary elements differentiated in greatest numbers between 25 and 30 °C. Numbers were depressed at lower temperatures by slower development and at higher temperatures by adverse processes. The data did not support previous suggestions of a great stimulation of xylogenesis above 30 °C and of a special sensitivity to low temperatures. Tracheary elements differentiated in various spatial patterns: as clumps in the peripheral callus, as strands which extended radially and longitudinally from some of these clumps, as individual large tracheids especially at the more extreme temperatures, and as short strands associated with nodules and roots that formed at favourable temperatures. We suggest that indoleacetic acid (IAA) has various roles in the positional control of these tracheary patterns: (1) IAA destruction at the expiant surface leads to concentration gradients that inhibit tracheary induction close to the surface; (2) IAA transport from the source in the culture medium to sinks especially at the expiant surface, coupled with autocatalytic flow facilitation, leads to canalization along pathways that become meristematic and then tracheary strands; (3) the IAA flux (and associated proton flux) along these pathways tend to orient cortical microtubules at right angles to the flow, by some mechanism as yet unknown, and hence to control the orientation of tracheary element elongation and secondary wall banding. These suggestions, supported by morphometric studies of tracheary element dimensions and orientations, and by experiments with localized IAA application, lead to a general interpretation of the development of polarity in plants.

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