Transcriptome and Metabolome Analyses Reveal the Molecular Mechanisms of <i>Albizia odoratissima</i>’s Response to Drought Stress

<i>Albizia odoratissima</i> is a deciduous tree species belonging to the family <i>Leguminosae</i>. It is widely distributed in the southern subtropical and tropical areas of China and has important ecological and economic value. The growth and metabolic processes of <i>A. odoratissima</i> are affected by drought stress, but the molecular mechanisms remain unknown. Therefore, this study investigated the physicochemical properties, gene expression, and metabolites of <i>A. odoratissima</i> seedlings under drought stress. The results show that, in leaves of <i>A. odoratissima</i> seedlings, drought stress reduced the moisture content, chlorophyll content, photosynthetic efficiency, superoxide dismutase (SOD) activity, and gibberellin (GA) and indoleacetic acid (IAA) contents while increasing the catalase (CAT) and peroxidase (POD) activities and malondialdehyde (MDA), proline, soluble sugar, and soluble protein contents. Within the CK5 (Day 5 of control group) vs. T5 (Day 5 of drought treatment), CK10 vs. T10, CK15 vs. T15, and CK20 vs. T20 groups (CK: control group; T: drought treatment), a total of 676 differentially expressed genes (DEGs) were upregulated and 518 DEGs were downregulated, and a total of 228 and 143 differential accumulation metabolites (DAMs) were identified in the CK10 vs. T10 and CK20 vs. T20 groups. These were mainly involved in the amino acid and alkaloid metabolism pathways in the leaves of the <i>A. odoratissima</i> seedlings. In the amino acid and alkaloid biosynthesis pathways, the relative expression levels of the <i>AoproA</i> (<i>Aod04G002740</i>, <i>ORTHODONTIC APPLIANCE</i>), <i>AoOAT</i> (<i>Aod07G015970</i>, <i>ORNITHINE-OXO-ACID TRANSAMINASE</i>), and <i>AoAOC3</i> (<i>Aod12G005010/08G003360/05G023920/08G003000/08G003010</i>, <i>AMINE OXIDASE COPPER CONTAINING 3</i>) genes increased, which concurrently promoted the accumulation of arginine, proline, piperine, cadaverine, and lysine. Furthermore, some key transcription factors in the response to drought were identified in the leaves using the weighted gene co-expression network analyses (WGCNA) method. These findings reveal that <i>A. odoratissima</i> seedlings respond to drought stress by improving the capacities of the antioxidant system and secondary metabolism.