Sub-lethal concentrations of silver nanoparticles mediate a phytostimulatory response in tobacco via the suppression of ethylene biosynthetic genes and the ethylene signaling pathway

While the antimicrobial activity of silver nanoparticles (AgNPs) is well established, the phytostimulatory and/or phytotoxic influences of AgNPs in closed tissue culture vessels is more controversial, to some extent. This is because numerous research papers have been published that ultimately conclude apparently contradictory results. In this paper, the physiological responses which AgNPs induce are studied in both tobacco micro-shoots and tobacco seedlings via the utilization of a number of morphological, biochemical, and molecular approaches. This report provides direct evidence that AgNPs positively regulate growth response in tobacco via a curtailing of ethylene production and an inhibition of the general ethylene signaling pathway. Tobacco seedlings exposed to media supplemented with high concentrations of AgNPs (150 μg mL −1 ) were shown to exhibit a hypersensitivity response. However, when compared to the control, tobacco seedlings exposed to sub-lethal concentrations of AgNPs (50 μg mL −1 ) displayed increased leaf fresh weight and plant height, as well as a 6-fold increase to root length. Inductively coupled plasma mass spectrometry (ICP-MS) confirmed the presence of silver ions (Ag + ) in AgNP-treated tobacco leaves, and given that silver is known to be a potent inhibitor of ethylene action, this result led us to question whether AgNPs might be inhibiting ethylene receptor expression. Subsequent qRT-PCR testing in tobacco tissue treated with 50 μg mL −1 AgNP revealed a downregulation of ETR1 , ERS1 , and CTR1 , which are all key ethylene signaling genes, as well as a downregulation of an important downstream ethylene-synthesizing gene, ACS2. This clearly established that the increase to root length in response to AgNPs might be occurring via an AgNP-dependent suppression of ethylene signaling genes. Further, gas chromatography analysis confirmed that tobacco planted on media supplemented with sub-lethal concentrations of AgNPs exhibited a reduction of gaseous ethylene production in closed in vitro vessels. Plants exposed to a higher concentration of AgNPs in their media did not show any additional inhibitory effects, with regard to ethylene production. Nonetheless, ultimately, the collective data implies that AgNPs inhibit various aspects of the ethylene signaling pathway, in addition to inhibiting the production of ethylene itself, and both of these inhibitions occur in a dose-dependent manner.