A xylem-specific cellulose synthase gene from aspen (Populus tremuloides) is responsive to mechanical stress

Angiosperm trees accumulate an elevated amount of highly crystalline cellul ose with a concomitant decrease in lignin in the cell walls of tension-stre ssed tissues. To investigate the molecular basis of this tree stress respon se, we cloned a full-length cellulose synthase (PtCesA) cDNA from developin g xylem of aspen (Populus tremuloides). About 90% sequence similarity was f ound between the predicted PtCesA and cotton GhCesA proteins. Northern blot and in situ hybridization analyses of PtCesA gene transcripts in various a spen tissues, and PtCesA gene promoter-beta-glucuronidase (GUS) fusion anal ysis in transgenic tobacco, demonstrated conclusively that PtCesA expressio n is confined to developing xylem cells during normal plant growth. During mechanical stress induced by stem bending, GUS expression remained in xylem and was induced in developing phloem fibers undergoing tension stress, but was turned off in tissues undergoing compression on the opposite side of t he bend. Our results suggest a unique role for PtCesA in cellulose biosynth esis in both tension-stressed and normal tissues in aspen, and that the on/ off control of PtCesA expression may be a part of a signaling mechanism tri ggering a stress-related compensatory deposition of cellulose and lignin th at is crucial to growth and development in trees.