symplastic flow (Anjum et al. 2019; Marslin et al. 2017). Despite its potential of
toxicity, studies have reported positive effects on plant development and physiology, which are dependent on the nature of the nanomaterial, dose and time of
exposure, the plant species, and growth conditions (Cox et al. 2016).
Positive physiological effects using carbon-based NPs include increased water
uptake, and enhanced assimilation of CO 2 in broccoli (Martínez-Ballesta et al.
2016), promotion of seed germination and root growth in rice (Jiang et al. 2014),
enhanced germination and seedling growth in sweet corn, barley, rice soybean,
switchgrass, and tomato (Lahiani et al. 2015; Tiwari et al. 2014), increase fruit yield
in tomato and bitter melon (Khodakovskaya et al. 2013; Kole et al. 2013), among
many others. Studies using metallic nanoparticles have reported similar effects. In
wheat, CeO 2 particles improved plant growth, shoot biomass, and grain yield (Rico
et al. 2014). Au nanoparticles in chinese mustard (Brassica juncea) had positive
effects on growth parameters and seed yield (Arora et al. 2012). The response of
maize exposed to ZnO nanoparticles showed enhanced germination, seedling vigor,
and zinc biofortification of grains (Subbaiah et al. 2016).
The effect of NPs on secondary plant metabolism is still largely unknown
compared to physiological and phenotypic responses. However, studies have shown
that a constant response between species is the induction of reactive oxygen species
(ROS) (Marslin et al. 2017). Studies reporting NPs-elicitation of specialized
metabolites often report a reduction in the photosynthetic rate and inhibition of
growth. These phytotoxic effects have been linked to the inhibition of
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity and decreased
photo-protective capacity of PSII (Jiang et al. 2017a; Wang et al. 2016). When NPs
permeate the cells, damage on the photosynthetic apparatus is done because of its
accumulation in chloroplasts, at the same time, when they cross the plasma
membrane they probably dissociate into ions (rather than stay as intact particles)
and bound to NADPH oxidases, causing the production of ROS at the apoplast
(Jiang et al. 2017a; Sosan et al. 2016). Besides oxidative burst, it has been reported
that NPs also induce reactive nitrogen species (*NO, nitric oxide) (Marslin et al.
2017).
Initial responses also include calcium ion (Ca
2+ ) spikes, Ca
2+
flux movements,
and upregulation/phosphorylation of mitogen-activated protein kinase (MAPK)
cascades that together with ROS production, ultimately lead to the activation of the
pathways of specialized metabolites biosynthesis (Anjum et al. 2019; Marslin et al.
2017). As expected, plants exposed to stressful concentrations of NPs have shown
to cope with the oxidative stress and lipid peroxidation through the upregulation of
enzymatic antioxidants such as superoxide dismutase (SOD), ascorbate peroxidase
(APX), glutathione-S-transferase (GST), and catalase (CAT) (Dimkpa et al. 2012;
Fu et al. 2014; Mirzajani et al. 2014; Zhao et al. 2012).
Through these findings, the concept of “nano-elicitors” have recently emerged as
a novel alternative to stimulate the production of valuable bioactive compounds that
might be used as additives in food, cosmetics, and pharmaceutical products.
170
H. Aguirre-Becerra et al.
toxicity, studies have reported positive effects on plant development and physiology, which are dependent on the nature of the nanomaterial, dose and time of
exposure, the plant species, and growth conditions (Cox et al. 2016).
Positive physiological effects using carbon-based NPs include increased water
uptake, and enhanced assimilation of CO 2 in broccoli (Martínez-Ballesta et al.
2016), promotion of seed germination and root growth in rice (Jiang et al. 2014),
enhanced germination and seedling growth in sweet corn, barley, rice soybean,
switchgrass, and tomato (Lahiani et al. 2015; Tiwari et al. 2014), increase fruit yield
in tomato and bitter melon (Khodakovskaya et al. 2013; Kole et al. 2013), among
many others. Studies using metallic nanoparticles have reported similar effects. In
wheat, CeO 2 particles improved plant growth, shoot biomass, and grain yield (Rico
et al. 2014). Au nanoparticles in chinese mustard (Brassica juncea) had positive
effects on growth parameters and seed yield (Arora et al. 2012). The response of
maize exposed to ZnO nanoparticles showed enhanced germination, seedling vigor,
and zinc biofortification of grains (Subbaiah et al. 2016).
The effect of NPs on secondary plant metabolism is still largely unknown
compared to physiological and phenotypic responses. However, studies have shown
that a constant response between species is the induction of reactive oxygen species
(ROS) (Marslin et al. 2017). Studies reporting NPs-elicitation of specialized
metabolites often report a reduction in the photosynthetic rate and inhibition of
growth. These phytotoxic effects have been linked to the inhibition of
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity and decreased
photo-protective capacity of PSII (Jiang et al. 2017a; Wang et al. 2016). When NPs
permeate the cells, damage on the photosynthetic apparatus is done because of its
accumulation in chloroplasts, at the same time, when they cross the plasma
membrane they probably dissociate into ions (rather than stay as intact particles)
and bound to NADPH oxidases, causing the production of ROS at the apoplast
(Jiang et al. 2017a; Sosan et al. 2016). Besides oxidative burst, it has been reported
that NPs also induce reactive nitrogen species (*NO, nitric oxide) (Marslin et al.
2017).
Initial responses also include calcium ion (Ca
2+ ) spikes, Ca
2+
flux movements,
and upregulation/phosphorylation of mitogen-activated protein kinase (MAPK)
cascades that together with ROS production, ultimately lead to the activation of the
pathways of specialized metabolites biosynthesis (Anjum et al. 2019; Marslin et al.
2017). As expected, plants exposed to stressful concentrations of NPs have shown
to cope with the oxidative stress and lipid peroxidation through the upregulation of
enzymatic antioxidants such as superoxide dismutase (SOD), ascorbate peroxidase
(APX), glutathione-S-transferase (GST), and catalase (CAT) (Dimkpa et al. 2012;
Fu et al. 2014; Mirzajani et al. 2014; Zhao et al. 2012).
Through these findings, the concept of “nano-elicitors” have recently emerged as
a novel alternative to stimulate the production of valuable bioactive compounds that
might be used as additives in food, cosmetics, and pharmaceutical products.
170
H. Aguirre-Becerra et al.