Humic substances that can be obtained from coal resources such as leonardite in a bulk scale have been employed as crop stimulators and soil conditioners. The polymeric organics containing a variety of aromatic and aliphatic structures are known to activate plants in a multifunctional way, thus resulting in enhanced germination rate and abiotic stress resistance concomitant with induction of numerous genes and proteins. Although detailed structural-functional relationship of humic substances for plant stimulations has not been deciphered yet, cutting-edge analytical tools have unraveled critical features of humic architectures that could be linked to the action mechanisms of their plant stimulations. In this review article, we introduce key findings of humic structures and related biological functions that boost plant growth and abiotic stress resistance. Oxygen-based functional groups and plant hormone-like structures combined with labile and recalcitrant carbon backbones are believed to be critical moieties to induce plant stimulations. Some proteins such as HIGH-AFFINITY K+ TRANSPORTER 1, phospholipase A2 and H+-ATPase have been also recognized as key players that could be critically involved in humic substance-driven changes in plant physiology.

ABSTRACT
 Ⅰ. Humic substances in Agricultures
 Ⅱ. Structure-function relationship of humic substances
 Ⅲ. Molecular evidences on humic substance-induced plant stimulations
 Ⅳ. ACKNOWLEDGEMENTS
 Ⅴ. REFERENCES

• Jong-Rok Jeon(Department of Agricultural Chemistry and Food Science & Technology, Institute of Agriculture and Life Science (IALS), Gyeongsang National University)
• Ho Young Yoon(Department of Agricultural Chemistry and Food Science & Technology, Institute of Agriculture and Life Science (IALS), Gyeongsang National University)
• Gyeong-Im Shin(Division of Applied Life Science (BK21Plus), Plant Molecular Biology and Biotechnology Research Center (PMBBRC), Research Institute of Life Sciences (RILS), Gyeongsang National University)
• Song Yi Jeong(Division of Applied Life Science (BK21Plus), Plant Molecular Biology and Biotechnology Research Center (PMBBRC), Research Institute of Life Sciences (RILS), Gyeongsang National University)
• Joon-Yung Cha(Division of Applied Life Science (BK21Plus), Plant Molecular Biology and Biotechnology Research Center (PMBBRC), Research Institute of Life Sciences (RILS), Gyeongsang National University)
• Woe-Yeon Kim(Department of Agricultural Chemistry and Food Science & Technology, Institute of Agriculture and Life Science (IALS), Gyeongsang National University, Division of Applied Life Science (BK21Plus), Plant Molecular Biology and Biotechnology Research Center (PMBBRC), Research Institute of Life Sciences (RILS), Gyeongsang National University) Corresponding author