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Reaction tissue formation and stem tensile modulus properties in wild-type and p-coumarate-3-hydroxylase downregulated lines of alfalfa, Medicago sativa (Fabaceae)¹

The Institute of Biological Chemistry, Washington State University, Pullman, Washington 99164-6340 USA; Wood Materials and Engineering Laboratory, Washington State University, Pullman, Washington 99164-1806 USA

ABSTRACT

To our knowledge, xylary reaction tissue has never been reported in a forage crop species. Here we report the discovery of reaction tissue in a transgenic line of Medicago sativa (pC3H, for the gene for p-coumarate-3-hydroxylase) with reduced lignin content and in the wild-type (WT) line. Based on microscopy and biomechanical testing of internodal alfalfa branch sections, the transgenic (pC3H-I) line, relative to the WT (1) apparently formed more reaction tissue containing gelatinous fibers with adjacent thick-walled fibers (presumed to be "intermediate" tissue) more rapidly, (2) had more xylem tissue, and (3) had comparable tensile dynamic modulus properties. These findings thus establish the (limited) ability of this perennial angiosperm to form (inducible) reaction tissue in a manner somewhat analogous to that of woody arborescent angiosperms. The potential of effectuating reductions in lignin amounts in (woody) angiosperms with increased formation of reaction (tension wood) tissue is discussed because reaction tissues are often viewed as a deleterious trait in processing for many agronomic/industrial applications, especially with the current interest in biofuels.

Key Words: alfalfa • gelatinous fiber • lignin • Medicago sativa • p-coumarate-3-hydroxylase (pC3H) • reaction tissue • tensile modulus • tension wood • zinc chloro-iodide

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