Apoplastic expression of the xylanase and ?(1?3, 1?4) glucanase domains of the xyn D gene from Ruminococcus flavefaciens leads to functional polypeptides in transgenic tobacco plants

We are investigating the possibilities of transgenic plants as bioreactors for the production of industrial enzymes using cell wall-hydrolysing enzymes as first examples. Within the frame work of this work two distinct domains of the xynD gene from Ruminococcus flavefaciens encoding a xylanase (XYLD-A) and a β(1–3, 1–4)glucanase (XYLD-C) were separately cloned into a plant expression vector which would target the proteins into the apoplast. Transgenic tobacco plants were obtained expressing xylan-hydrolysing as well as lichenan-hydrolysing activities. Despite similar steady-state levels of the respective mRNAs xylan hydrolysis rates were between 40 and 170 μmol min−1 m−2 leaf area depending on the transgenic plant while β(1–3, 1–4)glucan degradation was much more effective ranging between 200 and 2000 μmol min−1 m−2. The high activity levels of the XYLD-C expressing plants were reflected on the protein level. XYLD-C accumulated in the intercellular space and was one of the most prominent bands in protein gels. Despite their apoplastic location as confirmed by activity measurements using intercellular fluids the transgenic plants had not undergone any phenotypic alteration.