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