Research progress of carbon nanotubes in delivery of plant genetic materials

Population expansion, climate change, and growing scarcity of arable land threaten global food security. With the development of biotechnology, transgenic crops with superior features, such as high yield and stress resistance, can ensure food security. Despite decades of progress in biotechnology, genetic modification of most plant species remains challenging. Currently, plant transgenic technologies that rely on Agrobacterium tumefaciens and gene guns are limited by low transfer efficiencies and species dependence. Recently, nanocarriers have received significant attention in the field of plant genetic engineering. Multiple studies have found that nanoparticles can carry nucleic acids (DNA, RNA) and proteins into plant cells. The use of nanomaterials to enhance the efficiency of genetic biomolecule delivery into plant cells is a useful strategy for efficient plant genetic modification. Among the nanomaterials possessing the advantages of stability, biocompatibility, and a high surface-area-to-volume ratio, carbon nanotubes (CNTs) have shown great application potential. Meanwhile, a high aspect ratio causes CNTs to passively traverse the extracted chloroplasts and plant membranes. Various surface modifications render CNTs capable of carrying a range of cargoes and target different organelles, such as the nucleus or chloroplasts. Polyethyleneimine (PEI)-modified CNTs loaded with plasmids enter the nucleus and transiently express exogenous genes without transgene or host range limitations. Using the lipid exchange envelope penetration mechanism, chitosan-complexed CNTs selectively delivered plasmid DNA to chloroplasts. Based on π-π adsorption, CNTs have been designed as an siRNA delivery platform to silence endogenous genes with high efficiency and specificity. Modified CNTs can also protect nucleic acid cargo from degradation by nucleases during delivery. CNTs combine with genome-editing tools, such as clustered regularly interspaced short palindromic repeats (CRISPR), producing a new opportunity to achieve permanent genome editing through independent DNA delivery without transgene integration. However, as a novel nucleic acid delivery technique, CNTs have several drawbacks. CNT-mediated delivery of plasmids into cells exhibits relatively low expression efficiency, which warrants further improvement. There are still technical limitations in carrying large DNA units, such as the CRISPR-associated protein 9 (Cas9) expression box, for gene editing. Furthermore, the cellular uptake mechanism and techniques of CNT modification require further investigation, which will have a significant influence on the targeted delivery to plant cells and organelles. This review provides a comprehensive summary of the structural attributes of CNTs and offers an overview of the primary techniques employed for the surface functionalization of CNTs and advancements in nucleic acid delivery into plants via surface-modified CNTs. In this review, we explored the limiting factors and prospects for the application of CNTs, which will provide new insights for plant genetic transformation technologies and methods.