Three-week-old soil-grown N. benthamiana plants were used in all experiments. PEI-SWCNT prepared were used directly or maximum in the first week of synthesis. PEI-SWCNT were always probe tip sonicated prior to use. PEI-SWCNTs were added to DNA at the ratio of 3:1, 2:1 and 1:1, in infiltration buffer (MgCl 2 and MES, pH 6). The DNA-PEI-SWCNT complexes were infiltrated into the lower epidermis of fully expanded leaves. In all experiments, the ratio of DNA:PEI-SWCNT was 1:3, which was the optimal ratio that generated a stable solution and better expression, except otherwise indicated in the respective figures' legends.