Heterogenous expression of Pyrus pyrifolia PpCAD2 and PpEXP2 in tobacco impacts lignin accumulation in transgenic plants

Lignin, a natural macromolecular compound, plays an important role in the texture and taste of fruit. Hard end is a physiological disorder of pear fruit, in which the level of lignification in fruit tissues is dramatically elevated. Cinnamyl alcohol dehydrogenase and expansin genes (PpCAD2 and PpEXP2, respectively) exhibit higher levels of expression in 'Whangkeumbae' (Pyrus pyrifolia) pear fruit exhibiting this physiological disorder, relative to control fruit without symptoms. These genes were isolated from pear fruit and subsequently expressed in tobacco (Nicotiana tabacum) to investigate their function. Histochemical staining for lignin revealed that the degree of lignification in leaf veins and stem tissues increased in plants transformed with sense constructs and decreased in plants transformed with antisense constructs of PpCAD2. The expression of native NtCADs was also inhibited in the antisense PpCAD2 transgenic tobacco. Sense and antisense PpCAD2 transgenic tobacco exhibited an 86.7% increase and a 60% decrease in CAD activity, respectively, accompanied by a complementary response in lignin content in root tissues. The basal portion of the stem in PpEXP2 transgenic tobacco was bent and highly lignified. Additionally, the level of cellulose also increased in the stem of PpEXP2 transgenic tobacco. Collectively, these results suggested that PpCAD2 and PpEXP2 genes play a significant role in lignin accumulation in transgenic tobacco plants, and it is inferred that these two genes may also participate in the increased lignification observed in hard end pear fruit.