Negatively Charged\nCarbon Dots Employed Symplastic\nand Apoplastic Pathways to Enable Better Plant Delivery than Positively\nCharged Carbon Dots

Efficient delivery of nanoparticles (NPs) to plants is\nimportant\nfor agricultural application. However, to date, we still lack knowledge\nabout how NPs’ charge matters for its translocation pathway,\ni.e., symplastic and apoplastic pathways, in plants. In this study,\nwe synthesized and used negatively charged citrate sourced carbon\ndots (C-CDs, −37.97 ± 1.89 mV), Cy5 coated C-CDs (Cy5-C-CDs,\n−41.90 ± 2.55 mV), positively charged PEI coated carbon\ndots (P-CDs, +43.03 ± 1.71 mV), and Cy5 coated P-CDs (Cy5-P-CDs,\n+48.80 ± 1.21 mV) to investigate the role of surface charges\nand coatings on the employed translocation pathways (symplastic and\napoplastic pathways) of charged NPs in plants. Our results showed\nthat, different from the higher fluorescence intensity of P-CDs and\nCy5-P-CDs in extracellular than intracellular space, the fluorescence\nintensity of C-CDs and Cy5-C-CDs was similar between intracellular\nand extracellular space in cucumber and cotton roots. It suggests\nthat the negatively charged CDs were translocated via both symplastic\nand apoplastic pathways, but the positively charged CDs were mainly\ntranslocated via the apoplastic pathway. Furthermore, our results\nshowed that root applied negatively charged C-CDs demonstrated higher\nleaf fluorescence than did positively charged P-CDs in both cucumber\n(8.09 ± 0.99 vs 3.75 ± 0.23) and cotton (7.27 ± 1.06\nvs 3.23 ± 0.22), indicating that negatively charged CDs have\na higher translocation efficiency from root to leaf than do positively\ncharged CDs. It should be noted that CDs do not affect root cell activities,\nROS level, and photosynthetic performance in cucumber and cotton,\nshowing its good biocompatibility. Overall, this study not only figured\nout that root applied negatively charged CDs employed both symplastic\nand apoplastic pathways to do the transportation in roots compared\nwith mainly the employment of apoplastic pathway for positively charge\nCDs, but also found that negatively charge CDs could be more efficiently\ntranslocated from root to leaf than positively charged CDs, indicating\nthat imparting negative charge to NPs, at least CDs, matters for its\nefficient delivery in crops.