Changes in phenylalanine ammonia-lyase activity during xylem differentiation in Coleus and soybean

Xylem differentiation was induced in cultured Coleus internode slices when grown in the light on a simple agar/sucrose/IAA medium and in darkgrown soybean callus tissue when cultured on a complex defined medium containing 5×10(-7) M kinetin. In the Coleus system, the activity of phenylalanine ammonialyase followed the same time course as the formation of lignified wound vessel members. The specific activity of PAL was higher in the soybean callus tissues grown on 5×10(-7) M kinetin, which produced tracheary elements, than in the soybean tissue grown on 10(-8) M kinetin, which did not produce tracheids. These observations suggest that PAL is a "marker enzyme" for xylogenesis and that PAL activity may be a rate limiting step in lignification.