Transcriptomic and metabolomic profiling reveal the close implication of alkaloid biosynthesis and ureide metabolism to drought stress tolerance in maize
Addressing sustainable agriculture production and food security requires a thorough understanding of crop drought response. Although the importance of plant secondary metabolites (SMs) in mediating drought tolerance has been extensively reported, the implication of alkaloids in maize drought resistance remains elusive. In this study, the physiological, transcriptome, and metabolome responses of two contrasting maize varieties, CIMMYT PAK (C, drought-tolerant) and Haq Nawaz (H, drought-sensitive), were investigated. We identified 1,043 and 4,485 drought-responsive differentially expressed metabolites and genes, respectively, in C. Alkaloid biosynthesis was particularly significantly induced under drought stress (DS) in C. Notably, lupanine was 20.87-fold up-regulated under DS in C, while it exhibited no accumulation difference in H. (S)-ureidoglycolate was over 37-fold significantly up-regulated in both varieties under DS, indicating the importance of ureide metabolism to maize drought tolerance. Three potential candidate alkaloid biosynthesis-related genes (gene-LOC100279491, primary amine oxidase precursor; gene-LOC100279999, nicotianamine aminotransferase 1; and gene-LOC100273650, carboxylesterase 1) and two related to purine metabolism (gene-LOC103647994/ZmAPY3, apyrase 3 and gene-LOC100272659, ureide permease), were uncovered. In addition, other critical pathways and candidate genes in response to DS in C were revealed, highlighting key drought-responsive mechanisms in maize. Our results provide new insights into the drought tolerance mechanisms of maize and valuable resources for genomics-assisted breeding of drought-resilient maize varieties. The molecular contribution of alkaloids and ureide metabolism to maize drought tolerance deserves further in-depth investigations.