Several recent studies have shown that citric acid/citrate (CA) can confer abiotic stress tolerance to plants. Exogenous CA application leads to improved growth and yield in crop plants under various abiotic stress conditions. Improved physiological outcomes are associated with higher photosynthetic rates, reduced reactive oxygen species, and better osmoregulation. Application of CA also induces antioxidant defense systems, promotes increased chlorophyll content, and affects secondary metabolism to limit plant growth restrictions under stress. In particular, CA has a major impact on relieving heavy metal stress by promoting precipitation, chelation, and sequestration of metal ions. This review summarizes the mechanisms that mediate CA-regulated changes in plants, primarily CA's involvement in the control of physiological and molecular processes in plants under abiotic stress conditions. We also review genetic engineering strategies for CA-mediated abiotic stress tolerance. Finally, we propose a model to explain how CA's position in complex metabolic networks involving the biosynthesis of phytohormones, amino acids, signaling molecules, and other secondary metabolites could explain some of its abiotic stress-ameliorating properties. This review summarizes our current understanding of CA-mediated abiotic stress tolerance and highlights areas where additional research is needed.

Chemistry Research Unit United States Department of Agriculture Agricultural Research Service Gainesville FL 32608 USA

Department of Agronomy Khulna Agricultural University Khulna 9100 Bangladesh

Department of Biochemistry and Molecular Biology Bangladesh Agricultural University Mymensingh 2202 Bangladesh

Department of Botany and Plant Physiology Faculty of Agrobiology Food and Natural Resources Czech University of Life Sciences Prague 16500 Prague Czech Republic

Department of Crop Botany Bangladesh Agricultural University Mymensingh 2202 Bangladesh

Department of Fisheries Bangamata Sheikh Fojilatunnesa Mujib Science and Technology University Melandah Jamalpur 2012 Bangladesh

Department of Plant Physiology Slovak University of Agriculture 94976 Nitra Slovakia

Department of Seed Science and Technology Bangladesh Agricultural University Mymensingh 2202 Bangladesh

Graduate School of Environmental and Life Science Okayama University Okayama 700 8530 Japan

Plant Breeding Division Bangladesh Rice Research Institute Gazipur 1701 Bangladesh

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