# Nanoparticles--Plant Interaction: What We Know, Where We Are?
## 2 Apoplast
### Postulated Mechanism That Allows NP to Cross the Cell Wall  
Chemical composition of cell wall influences its structure, including porosity. This parameter in turn determines the pore size of walls and controls the apoplastic exchange of macromolecules. Cell wall is composed of cellulose, hemicelluloses, pectins, structural proteins and phenolic compounds (such as lignin in specific walls). It is postulated that pectins and networks they form in muro are the main players in regulation of cell wall porosity [21; 50; 51; 52; 53]. Moreover, it was shown that low pH, cell-wall degrading enzymes such as PG (polygalacturonase) and EGase (endo-1,4-b-glucanase) or EDTA (ethylenediaminetetraacetic acid), all affect pectin properties and increase the porosity of a cell wall in different plant species [29; 54]. Therefore, research on the penetration of NP through the wall should consider possible modifications of pectins and other wall components that may result in a changes of cell wall physical properties (including pore size) and lead to the penetration through the cell wall by the NP with a diameter larger than the specified pore size. So far only a few reports indicate that NP may influence the physical properties of the cell wall causing enlargement of wall pores [42; 43]. In the _Arabidopsis_ root it was shown that copper oxide NP (CuO-NP) decreased the cell wall xyloglucan and esterified pectin contents [55]. Studies on radish taproot treated with lanthanum oxide NP (La\({}_{2}\)O\({}_{3}\)-NP) showed decrease of pectin content in rhizodermis and parenchyma cell walls [56]. In _Oryzasativa_ roots treated with Ag-NP changes in pectins and hemicellulose were described [57]. In _Brassica_ seedlings the ZnO-NP caused cell wall modifications including lignification, pectin accumulation and lignin-suberin deposition [40]. In alfalfa roots the nZVI-NP (nano zero-valent iron) mediated hydroxyl radicals, which induced cell wall loosening [58]. Salicylic acid-chitosan NP (SA-CS-NP) applied to maize triggered (among others) cell wall reinforcement by lignin deposition [59]. The potential impact of NP on the changes in the wall are indicated also by proteomic studies [60]. It is postulated that the influence of NP on the chemistry and structure of the wall may be a consequence of induced oxidative stress. It has been proved that NP in the form of quantum dots can transmit optical energy to nearby oxygen atoms resulting in the formation of free radicals and peroxides [61]. Depending on the level of the resulting oxidative stress (oxidative stress appears when reactive oxygen species (ROS) lead to imbalance between oxidative pressure and antioxidant defense system and can trigger various reactions like damage to proteins, lipids and DNA), which is closely related to the generated amount of ROS, plant can exhibit various reactions, including changes in structure and chemical composition of cell walls [40]. For example, NP may induce ROS, which can modulate the xyloglucan polymers in cell wall what leads to wall loosening (i.e., hydroxyl radicals in cell walls could cause non-enzymatic wall loosening) and in such conditions large NP can cross the cell wall [62]. Tobacco BY-2 cells treated with 30-40 nm in diameter CuO-NP showed that some NP were attached to the surface of plant cells, while some particles were transported into cytoplasm and mitochondria [63]. In grapevine treated with poly(d,l-lactide-co-glycolide) NP (PLGA-NP) the cell wall prevented the uptake of NP with diameter over 50 nm [64]. It is worth emphasizing that NP do not have to get inside plant cells to trigger a response. In case of _Panicum virgatum_ TiO\({}_{2}\)-NP in size of 21 nm did not penetrate to the root, however, they influenced the plant growth [65]. Primary root of maize seedlings treated with the industrially produced TiO\({}_{2}\)-NP 30 nm in size also did not enter the root, but decreased the cell wall pore size from 6.6 to 3 nm and influenced plant growth [66].