# Nanoparticles--Plant Interaction: What We Know, Where We Are?
## 3. Symplast  
The symplast is a system consisting of protoplasts interconnected by PD, which are plasma membrane-lined pores that traverse the walls of adjacent cells thus connecting their cytoplasm [69; 70]. In most cases, the diameter of the PD ranges from 25 to 40 nm [71], but there are also described cases where the diameter is 50 nm and even 80 nm [72].  
Desmotubule (10-15 nm in diameter), a cylindrical structure formed from an expressed ER membrane in continuity with the ER of the connected cells, is located in the centre of PD [73]. Between the desmotubule and the plasma membrane there are cytoplasmic microchannels (cytoplasmic sleeve) with the diameter between 2 and 4 nm. The diffusion of molecules through PD takes place through these microchannels and is called symplasmic transport [74; 75]. Changes in the number, structure and capacity of PD and PD modifications regulate the movement of substances between cells. These changes take place during growth and development of plant or due to operating biotic and abiotic factors [73]. The transport efficiency of molecules dissolved in the cytoplasm (diffusion) depends on the electrochemical gradient between adjacent cells, the cross-section of PD (understood as the diameter of the channel PD and length) and the number/density of PD at the interface of certain cells. In turn, each of the above features are subjected to physiological and/or mechanical regulating factors originating from the external environment [70]. Transport via PD is under complex and multistage control.  
PD may exist in several conformations: closed/blocked, open-resting, dilated and gated [70]. During open/resting stage PD allows free exchange of substances between cells, such as ions, sugars, amino acids and proteins (on the condition that their molecular dimensions are smaller than the diameter of the transport channels of PD). In contrast, when PD is in stage called dilated, substances with larger diameter can move through PD [75]. Last conformational state of PD, termed gated, concerns the dilation of PD accompanying the movement of a targeted macromolecules such as proteins and mRNA molecules having a specific signal sequences that direct them to the PD so they can pass through them.  
A detailed analysis of PD and their participation in the NP movement within the plant body is essential to understand the mechanism of NP translocation by symplast. If we assume that NP move through cytoplasm by diffusion, they should pass PD by cytoplasmic sleeve. This means that NP larger than a few nanometers cannot move through PD, at least on the mechanism of simple diffusion. Studies on Brassica with the use of ZnO-NP of 8 nm diameter led to the hypothesis that this NP can use the PD as a route within the plant symplast [40]. In the case of maize plants, PbS-NP translocation by the symplasmic route has also been suggested [33]. Studies with the fluorescently labeled mesoporous silica NP pointed that NP may translocate through PD, however such supposition was indirect as no PD were shown [76]. The results of studies on the ZnO-NP translocation in _S. tubernaententant_ indicate that this process occurs with the participation of symplasmic pathway, however without strong evidence of PD involvement [32].  
Though it is possible that in described above cases, there is a facilitated transport through PD, which was demonstrated for many proteins, including transcription factors [75] it would be extremely important to find out if this mechanism is also involved in NP translocation.