Table 1.

Humic substance (HS)-triggered mode of action and alterations that regulate specific activities in plants to mitigate stresses.

Mechanisms	Plants	Alterations	Functions	Reference
Photosynthesis and energy metabolism	Agrostis tenuis, Arabis paniculata, Lonicera japonica, Pteris vittata	Abundance of photosynthetic proteins of the cytochrome b6/f complex, OEC, RuBisCO, ATPase in green tissues	Detoxification of heavy metals	Visioli and Marmiroli, 2013
Protein and nitrogen metabolism	Alyssum lesbiacum, A. paniculata, B. juncea, P. americana	Modulation and induction of specific proteins	Detoxification of heavy metals and prevention against biotic stress	Bona et al., 2011; García et al., 2016b
Sulfur sequestration	Alyssum lesbiacum, A. thaliana, Brassica napus	Modulation of proteins	Detoxification of heavy metals and ROS scavenging	Schneider et al., 2013
Defense protein triggering	Alyssum bertolonii, N. caerulescens, P. vittata	Disruption of the cellular redox status	Detoxification of heavy metals, ROS scavenging, abiotic and biotic stress tolerance	DalCorso et al., 2013
Membrane trafficking	A. thaliana, A. halleri, N. caerulescens	Activation of ATPase and ionic pumps residing in membrane	Sequestration of metal ions into vacuoles and other sub-cellular compartments, and their export across the membranes	DalCorso et al., 2013; Schneider et al., 2013
Metal ligation	A. lesbiacum, N. caerulescen, P. americana	Abundance of small cysteine-rich metal-binding proteins and constitutive expression of respective genes	Detoxification of heavy metals	Bona et al., 2011; Ramos et al., 2015