Understanding of the tolerance mechanisms in resistant and tolerant plants can be useful for improving the stress tolerance of crop plants. In this research, lignin biosynthetic changes under salt stress and during internodal maturation were studied in halophyte Aeluropus littoralis that is considered as a genetic resource for identification of the mechanisms related to salt, drought, and heat stresses in monocotyledons. Lignin histochemistry and total lignin CAD gene expression (Cinnamyle Alcohol Dehydrogenas, one of the most important enzyme in lignin biosynthesis) were studied by two phloroglucinol and Maule reactions, acetyl bromide and real-time PCR methods, respectively. Both of the two coloring methods in lignin histochemistry showed enhancement of lignin deposition with increasing of NaCl concentration and with internode age increment, uniformly. Maule coloring indicated that lignin G deposited in apical internodes, whereas deposition of lignin S increased with salt enhancing and with progressing of internode development. According to histochemistry studies of lignin, total lignin assay showed enhancement of lignin content with increasing of salinity levels and internode maturation. CAD gene expression is induced by salinity stress, but it did not follow the regular pattern. CAD mRNA levels in middle internodes were more than apical and basal internodes. Coordinated results obtained from histochemical and metabolical studies indicated role of lignification and S lignin monomer in salinity tolerance in this halophyte and closed relation between lignification and stem development. According to the present results, it seems that immature apical internode is appropriate to study the effects of different stresses. However, this study showed that lignin content and composition and gene expression of lignin biosynthetic enzymes are controlled exactly in different internode positions.
