Lignin Synthesis, Affected by Sucrose in Lotus (Nelumbo nucifera) Seedlings, Was Involved in Regulation of Root Formation in the Arabidopsis thanliana

Adventitious roots (ARs) have an unmatched status in plant growth and metabolism due to the degeneration of primary roots in lotuses. In the present study, we sought to assess the effect of sucrose on ARs formation and observed that lignin synthesis was involved in ARs development. We found that the lignification degree of the ARs primordium was weaker in plants treated with 20 g/L sucrose than in 50 g/L sucrose treatment and control plants. The contents of lignin were lower in plants treated with 20 g/L sucrose and higher in plants treated with 50 g/L sucrose. The precursors of monomer lignin, including p-coumaric acid, caffeate, sinapinal aldehyde, and ferulic acid, were lower in the GL50 library than in the GL20 library. Further analysis revealed that the gene expression of these four metabolites had no novel difference in the GL50/GL20 libraries. However, a laccase17 gene (NnLAC17), involved in polymer lignin synthesis, had a higher expression in the GL50 library than in the GL20 library. Therefore, NnLAC17 was cloned and the overexpression of NnLAC17 was found to directly result in a decrease in the root number in transgenic Arabidopsis plants. These findings suggest that lignin synthesis is probably involved in ARs formation in lotus seedlings.