Secondary metabolites in the drought stress tolerance of crop plants: A review

Plants encounter various abiotic stresses under field conditions. Among them, drought stress adversely affects crop productivity and threatens global food security. Drought stress triggers downstream pathways such as phytohormone homeostasis and their signaling pathways. Consequently, this initiates the biosynthesis of different types of protective secondary metabolites (SMs). SMs provide multistress tolerance, including abiotic and biotic stresses. In drought conditions, soil become dry, and drought stress is perceived through roots as a stress signal through cell to cell signaling networks. These stress signals subsequently travel towards leaves through root to shoot signaling via xylem to induce the systemic phytohormone signaling and SMs biosynthesis. Consequently, this triggers stomatal closure through vascular to guard cell signaling, which prevents water loss. Different regulatory pathways work together to regulate downstream plant responses to drought stress. These responses further minimize oxidative stress, excessive water loss, and other adverse effects of drought in plants. SMs scavenge reactive oxygen species (ROS) to protect plants from lipid peroxidation and other oxidative damages under drought stress. Additionally, drought-induced volatile SMs alerts plant tissue to attain defensive drought stress mitigating processes via systemic induction of drought signaling. Modulation of the biosynthetic pathway genes of SMs could provide drought resistance. Expression analyses have revealed that genes encoding transcription factors such as MAT, MYB, ERF, CBL, CCR, and NAC play a crucial role in SM mediated drought response. Thus, SMs could perform a vital role in the drought stress condition in crop plants.