THE PLANT CELL, Vol 2, Issue 12 1239-1248, Copyright © 1990 by American Society of Plant Biologists
Analysis of Tomato Polygalacturonase Expression in Transgenic Tobacco
K. W. Osteryoung, K. Toenjes, B. Hall, V. Winkler and A. B. Bennett
Mann Laboratory, Department of Vegetable Crops, University of California, Davis, California 95616
Tomato polygalacturonase is a cell wall enzyme secreted in large amounts
during tomato fruit ripening. Polygalacturonase is synthesized as a
glycoprotein precursor that undergoes numerous cotranslational and
post-translational processing steps during its maturation, yielding three
isozymes in tomato fruit, PG1, PG2A, and PG2B. To investigate the
physiological roles of the three isozymes and the functional significance
of the polygalacturonase processing domains in its intracellular transport
and activity, we have examined polygalacturonase expression in transgenic
tobacco plants. A full-length polygalacturonase cDNA was placed under
control of the cauliflower mosaic virus 35S promoter and introduced into
tobacco by way of Agrobacterium-mediated transformation. Analysis of
transgenic tobacco plants indicated that (1) immunologically detectable
polygalacturonase can be extracted from leaves, roots, and stems of
transgenic tobacco plants; (2) only PG2A and PG2B were detectable in
transgenic tobacco; (3) the polygalacturonase isozymes present in
transgenic tobacco were electrophoretically indistinguishable from the
tomato isozymes; (4) the N-terminal sequence, degree of N-linked
glycosylation, and extent of oligosaccharide processing were similar in
polygalacturonase from transgenic tobacco and tomato; (5) polygalacturonase
was properly localized in cell walls of transgenic tissue; (6) the protein
was enzymically active in vitro; however, (7) accumulation of PG2A and PG2B
in cell walls of transgenic tobacco did not result in pectin degradation in
vivo. These results indicated that tomato polygalacturonase was properly
processed and transported to the cell wall of tobacco. However,
accumulation of the two polygalacturonase isozymes expressed in this
heterologous host was insufficient to promote polyuronide degradation in
tobacco leaf tissue.
