The extremely low in-planta expression (<0.1% efficiency compared to Agrobacterium) of the functional DNA unit delivered via PEI 30 -SWCNT prompted us to investigate in more detail the mechanism of PEI-SWCNT-based DNA delivery and expression in plant cells. In nature, insect vectors are the main source of virus transmission in plants. Insect vectors generally transmit very few copies of the virus particles into plant cells. However, inside the plant cell, the robust replication system of the viruses hijacks the plant cellular machinery to rapidly produce virus particles. We reasoned that the intracellular release of an infectious virus genome from PEI 30 -SWCNT at an efficiency lower than the observed <0.1% efficiency (Fig. ) would be sufficient to initiate virus replication inside the plant cell. Different plant viruses (reporter clones) were selected to distinguish between the two possible mechanisms for the low level in planta expression of the functional DNA unit (i.e., the release of a low number of DNA molecules or low expression of the long functional units from DNA conjugated PEI 30 -SWCNT). Tomato Yellow Leaf Curl Virus (TYLCV) is a DNA virus, and the replication and systemic movement of TYLCV in plants result in leaf thickening, cup-shaped curling, and dwarfing. The binary plasmid (11 kb) harboring the infectious TYLCV clone was conjugated at 1:3 ratio with PEI 30 -SWCNTs and infiltrated into N. benthamiana leaves. The same clone DNA was directly infiltrated as negative control and also transformed into Agrobacterium and infiltrated into N. benthamiana leaves as a positive control.