5-Aminolevulinic Acid (5-ALA)-Induced Drought Resistance in Maize Seedling Root at Physiological and Transcriptomic Levels

Drought stress seriously affects the growth, development, yield, and quality of maize. This study aimed to investigate the effects of exogenous 5-ALA on root morphology and physiological changes in maize seedlings and to detect its regulatory network. The results showed that adding 25 mg/L 5-ALA accelerated root morphogenesis (root average diameter, main root length, total root length, and root surface area) and promoted dry matter accumulation and free radical removal. Transcriptome analysis showed that after applying exogenous 5-ALA, differently expressed genes (DEGs) were mainly involved in histidine metabolism, amino acid biosynthesis, plasma membrane components, secondary active sulfate transmembrane transporter activity, and anion reverse transporter activity. Two inbred lines specifically responded to organelle and structural molecular activity, and 5-ALA may regulate maize roots to achieve drought tolerance through these two pathways. In addition, candidate genes that may regulate maize root growth were screened by weighted gene co-expression network analysis (WGCNA). These genes may play important roles in alleviating drought stress through lignin synthesis, heat shock proteins, iron storage and transport, calcium binding proteins, and plasma membrane regulation of exogenous regulator 5-ALA. Our results may provide a theoretical basis for clarifying the response of maize seedling roots to drought and the mechanism of exogenous hormones in alleviating drought.