Coordinated regulation of sucrose and lignin metabolism for arrested silk elongation under drought stress in maize

The delay in silking due to drought stress generates a greater anthesis-silking interval in maize, causing compromised kernel set and thus catastrophic loss of grain yield, which is primarily attributed to the lower silk elongation. However, few studies are done in the understanding of the physiochemical and molecular mechanism that regulate silk elongation in response to drought stress. Thus, a field experiment was conducted to examine silk transcriptomic and metabolomic responses to drought stress using the maize cultivar of AN591. A total of 757 differently expressed genes (DEGs) and 80 differently expressed metabolites (DEMs) were identified in maize silk grown in the drought stress treatment compared to the control treatment. These DEGs and DEMs were enriched in 191 and 36 Kyoto Encyclopedia of genes and genomes (KEGG) pathways, respectively, most being associated with primary and secondary metabolism. Starch and sucrose metabolism (SSM) pathway in primary metabolism and phenylpropanoid biosynthesis (PB) pathway in secondary metabolism were identified in common in both transcriptome and metabolome. Most of DEGs in the SSM pathway were increased in expression, while the DEM were decreased in expression. Conversely, most of DEGs in the PB pathway were decreased in expression, while DEMs were increased in expression. This could be explained by the negative correlation between DEGs and DEMs. In the SSM pathway, there was a significant increase in sucrose content, but a significant decrease in fructose and glucose in response to drought stress, which was attributed to the decreased activities in cell wall invertase (CWIN), vacuolar invertase (VIN), and sucrose synthase (SuSy). Silk lignin content in PB pathway was in a significant decrease in drought treatment compared to the control, due to the decreased activity of peroxidase (POD) that is the key enzyme in the final stage of lignin biosynthesis. In conclusion, this study identified key genes or metabolites involved in sucrose and lignin metabolism which appears to be regulating silk elongation response to drought stress.