Lignin is the second most abundant biopolymer on earth. It gives the cell wall strength and hydrophobicity. The lignified cell wall is, therefore, resistant to the fungal attacks and plays an important role for water flow. Biosynthesis of lignin has been extensively studied, and the pathway from phenylalanine to lignin monomers is well known. Phenylalanine ammonia-lyase (PAL) and cinnamyl alcohol dehydrogenase (CAD) are involved in the biosynthesis of lignin monomers. The former catalyzes the conversion of phenylalanine to cinnamic acid, and the latter involves in the reduction of cinnamyl aldehydes to their corresponding alcohols. If the activities of these enzymes are suppressed by specific inhibitors or gene manipulation, it is possible to control the lignification in the cell walls.
Treatment of 2-aminoindan-2-phosphonic acid (AIP), a specific inhibitor of PAL, caused a striking effect on lignification. Though the developing secondary walls showed strong UV-absorption in the control specimen, it showed very weak absorption after treatment with AIP. Developing secondary walls in the control specimen showed fibrillar texture, and then were paced with electron opaque materials. On the contrary, the fibrillar texture of the developing secondary walls were observed even after the final stage of cell wall formation in the AIP treated specimen. These results indicated that the treatment of AIP did not affect the cell wall thickening, but inhibited effectively the lignification.
Transgenic tobacco plants with antisense CAD gene showed 2055% depression of CAD activity. Interestingly, VIS-microspectrometry after phloroglucinol reaction showed strong absorption at 570 nm in fiber secondary wall of transgenic plants. Because the aldehyde groups in lignins showed red-purple color in the presence of phloroglucinol and HCl, the fiber secondary walls of transgenic plants contain more aldehyde groups in lignins than those of control plants.
Key words: lignification, cell wall, phenylalanine ammonialyase (PAL), cinnamyl alcohol dehydrogenase (CAD).
Correspondence: Keiji Takabe, Department of Wood Science and Technology, Faculty of Agriculture, Kyoto University, Kyoto, 606-01, Japan
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