Nanotechnology is the technology for studying the properties and applications of materials in the nanoscale (mainly 1-100 nm) range. Since the discovery of fullerene, carbon nanomaterials (CNMs) including carbon nanotubes (CNTs), graphenes and other CNMs with different structural features have been discovered and applied. With the development of nanotechnology, the application of some CNMs attracted the attention of plant biologists, especially for some beneficial regulation on plant growth [1–3]. Some CNMs have been applied to agricultural purpose and demonstrated their potential to promote agricultural production [4–7]. Among carbon-based nanomaterials, such as single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), single-walled carbon nanohorns (SWCNHs), water-soluble carbon nano-onions and carbon nano-dots, have been investigated for their effects on plant growth. Although some researches has shown the adverse reaction on plants from exposure to a range of CNMs, the effects exerted by the CNMs always involve a high exposure concentration specific to plant species and growth environment [4]. Plenty of studies reported the positive effects of low-dose CNMs on plant growth and development [8]. The positive morphological effects include the promotion of the seed germination process, root elongation, drought stress, and the accumulation of biomass and so on [9–12]. However, the mechanism behind this is still not clarified well. More studies would be necessary to reveal the mechanism of CNMs on plants.
To explore the effects of CNMs on plant growth and development, a lot of researches has been conducted. Among them, the reports of carbon nanotubes appeared relatively early, and reports on the regulation of plant growth were relatively abundant. Khodakovskaya et al. [13] found that SWCNTs can significantly promote the growth of tobacco callus, the highest increase of fresh weight reached 64%. Three genes were up-regulated in cells after callus was exposed to MWCNTs. These genes are involved in water transport, cell wall extension and cell division respectively. In the rhizobium-plant interaction, MWCNTs could improve the activity of nitrogenase and increase the level of the key gene (NIN) which regulates the development of nodules [14]. In some commercial crop, oxidized MWCNTs could penetrate the cell wall and enter the cytoplasm in roots, which promote cell elongation and dehydrogenase activity in the root system [15]. At low concentrations, MWCNTs show a strong positive effect on the growth of maize seedlings by enhancing water absorption and nutrient transport [8]. There are also reports about the changes of plant hormones in rice seedlings under carbon nanotube treatment, which indicated some possible relationships with reactive oxygen species [16]. Some raw CNPs isolated from biochar also can enhance the growth rate of wheat plants, and the optimum growth occurs at concentration of 50 mg/L in seeds [10].
The current studies mainly focused on the physiological changes in plants, activity of enzymes and some gene expression levels in plants. However, they are far from being able to reveal the detailed physiological and molecular mechanism of plants in response to CNMs. Proteomics is a useful tool for investigating some unknown changes in proteomes and achieves an evaluation of proteins on a global scale [17]. Given the crucial role of proteins in almost all cellular functions, it is necessary to study plants exposed to CNMs at the proteomic level. Proteomic profile analysis of mammalian cells exposed in SWCNTs was previously reported [18]. By studying changes in plants at the level of protein combinations, it will help to reveal the mechanism of action of CNMs on plants.
CNPs is a promising carbonaceous nanosized material. Electrolytic graphite method is an efficient way to prepare CNPs[19]. The CNPs prepared by this method can promote plant growth and improve plant nutrient utilization efficiency. There have been reports on improving the utilization rate of nutrient and improving root vitality and crop quality[20]. The aim of this study is to assess the promoting effect of CNPs on tobacco callus and using iTRAQ coupled with 2D-LC MS/MS to identify the proteins differentially expressed after exposed to CNPs.