Effect of shade stress on lignin biosynthesis in soybean stems

To clarify how shade stress affects lignin biosynthesis in soybean stem, two varieties, Nandou 12 (shade tolerant) and Nan 032-4 (shade susceptible) grew under normal light and shade conditions (the photosynthetically active radiation and the ratio of red:far-red were lower than normal light condition). Lignin accumulation, transcripts of genes involved in lignin biosynthesis, and intermediates content of lignin biosynthesis were analyzed. Both soybean varieties suffered shade stress had increased plant heights and internode lengths, and reduced stem diameters and lignin accumulation in stems. The expression levels of lignin-related genes were significantly influenced by shade stress, with interactions between the light environment and variety. The gene of 3-hydroxylase (C3H), cinnamoyl-CoA reductase (OCR), caffeoyl-CoA O-methyltransferase (CCoAOMT), and peroxidase (POD) attributed to lignin biosynthesis under shade stress, and the down-regulation of these genes resulted in lower caffeic, sinapic, and ferulic acid levels, which caused a further decrease in lignin biosynthesis. Under shade stress, the shade tolerant soybean variety (Nandou 12) showed stiffer stems, higher lignin content, and greater gene expression level and higher metabolite contents than shade susceptible one. So these characteristics could be used for screening the shade-tolerant soybean for intercropping.