CRISPR/Cas9-Based Genome Editing Toolbox for Arabidopsis thaliana

Application of multiple sgRNAs boosts efficiency of CRISPR/Cas9-mediated gene targeting in Arabidopsis

BACKGROUND

Precise gene targeting (GT) is a powerful tool for heritable precision genome engineering, enabling knock-in or replacement of the endogenous sequence via homologous recombination. We recently established a CRISPR/Cas9-mediated approach for heritable GT in Arabidopsis thaliana (Arabidopsis) and rice and reported that the double-strand breaks (DSBs) frequency of Cas9 influences the GT efficiency. However, the relationship between DSBs and GT at the same locus was not examined. Furthermore, it has never been investigated whether an increase in the number of copies of sgRNAs or the use of multiple sgRNAs would improve the efficiency of GT.

RESULTS

Here, we achieved precise GT at endogenous loci Embryo Defective 2410 (EMB2410) and Repressor of Silencing 1 (ROS1) using the sequential transformation strategy and the combination of sgRNAs. We show that increasing of sgRNAs copy number elevates both DSBs and GT efficiency. On the other hand, application of multiple sgRNAs does not always enhance GT efficiency. Our results also suggested that some inefficient sgRNAs would play a role as a helper to facilitate other sgRNAs DSBs activity.

CONCLUSIONS

The results of this study clearly show that DSB efficiency, rather than mutation pattern, is one of the most important key factors determining GT efficiency. This study provides new insights into the relationship between sgRNAs, DSBs, and GTs and the molecular mechanisms of CRISPR/Cas9-mediated GTs in plants.

Exploring aquaporin functions during changes in leaf water potential

Maintenance of optimal leaf tissue humidity is important for plant productivity and food security. Leaf humidity is influenced by soil and atmospheric water availability, by transpiration and by the coordination of water flux across cell membranes throughout the plant. Flux of water and solutes across plant cell membranes is influenced by the function of aquaporin proteins. Plants have numerous aquaporin proteins required for a multitude of physiological roles in various plant tissues and the membrane flux contribution of each aquaporin can be regulated by changes in protein abundance, gating, localisation, post-translational modifications, protein:protein interactions and aquaporin stoichiometry. Resolving which aquaporins are candidates for influencing leaf humidity and determining how their regulation impacts changes in leaf cell solute flux and leaf cavity humidity is challenging. This challenge involves resolving the dynamics of the cell membrane aquaporin abundance, aquaporin sub-cellular localisation and location-specific post-translational regulation of aquaporins in membranes of leaf cells during plant responses to changes in water availability and determining the influence of cell signalling on aquaporin permeability to a range of relevant solutes, as well as determining aquaporin influence on cell signalling. Here we review recent developments, current challenges and suggest open opportunities for assessing the role of aquaporins in leaf substomatal cavity humidity regulation.

Two florigens and a florigen-like protein form a triple regulatory module at the shoot apical meristem to promote reproductive transitions in rice

Many plant species monitor and respond to changes in day length (photoperiod) for aligning reproduction with a favourable season. Day length is measured in leaves and, when appropriate, leads to the production of floral stimuli called florigens that are transmitted to the shoot apical meristem to initiate inflorescence development¹. Rice possesses two florigens encoded by HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1)². Here we show that the arrival of Hd3a and RFT1 at the shoot apical meristem activates FLOWERING LOCUS T-LIKE 1 (FT-L1), encoding a florigen-like protein that shows features partially differentiating it from typical florigens. FT-L1 potentiates the effects of Hd3a and RFT1 during the conversion of the vegetative meristem into an inflorescence meristem and organizes panicle branching by imposing increasing determinacy to distal meristems. A module comprising Hd3a, RFT1 and FT-L1 thus enables the initiation and balanced progression of panicle development towards determinacy.

CRISPR/Cas9 mediated gene editing in non-model nematode Panagrolaimus sp. PS1159

The phylum Nematoda harbors a huge diversity of species in a broad range of ecosystems and habitats. Nematodes share a largely conserved Bauplan but major differences have been found in early developmental processes. The development of the nematode model organism Caenorhabditis elegans has been studied in great detail for decades. These efforts have provided the community with a large number of protocols and methods. Unfortunately, many of these tools are not easily applicable in non-Caenorhabditis nematodes. In recent years it has become clear that many crucial genes in the C. elegans developmental toolkit are absent in other nematode species. It is thus necessary to study the developmental program of other nematode species in detail to understand evolutionary conservation and novelty in the phylum. Panagrolaimus sp. PS1159 is a non-parasitic nematode exhibiting parthenogenetic reproduction and we are establishing the species to comparatively study evolution, biodiversity, and alternative reproduction and survival strategies. Here, we demonstrate the first successful application of the CRISPR/Cas9 system for genome editing in Panagrolaimus sp. PS1159 and the closely related hermaphroditic species Propanagrolaimus sp. JU765 applying the non-homologous end joining and the homology-directed repair (HDR) mechanisms. Using microinjections and modifying published protocols from C. elegans and P. pacificus we induced mutations in the orthologue of unc-22. This resulted in a visible uncoordinated twitching phenotype. We also compared the HDR efficiency following the delivery of different single-stranded oligodeoxynucleotides (ssODNs). Our work will expand the applicability for a wide range of non-model nematodes from across the tree and facilitate functional analysis into the evolution of parthenogenesis, changes in the developmental program of Nematoda, and cryptobiosis.

Insights into the molecular mechanisms of CRISPR/Cas9-mediated gene targeting at multiple loci in Arabidopsis

Homologous recombination-mediated gene targeting (GT) enables precise sequence knockin or sequence replacement, and thus is a powerful tool for heritable precision genome engineering. We recently established a clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR/Cas9)-mediated approach for heritable GT in Arabidopsis (Arabidopsis thaliana), but its broad utility was not tested, and the underlying molecular mechanism was unclear. Here, we achieved precise GT at 14 out of 27 tested endogenous target loci using the sequential transformation approach and obtained vector-free GT plants by backcrossing. Thus, the sequential transformation GT method provides a broadly applicable technology for precise genome manipulation. We show that our approach generates heritable GT in the egg cell or early embryo of T1 Arabidopsis plants. Analysis of imprecise GT events suggested that single-stranded transfer DNA (T-DNA)/VirD2 complexes produced during the Agrobacterium (Agrobacterium tumefaciens) transformation process may serve as the donor templates for homologous recombination-mediated repair in the GT process. This study provides new insights into the molecular mechanisms of CRISPR/Cas9-mediated GT in Arabidopsis.

Dual-sgRNA CRISPR/Cas9 knockout of PD-L1 in human U87 glioblastoma tumor cells inhibits proliferation, invasion, and tumor-associated macrophage polarization

Programmed death ligand 1 (PD-L1) plays a key role in glioblastoma multiforme (GBM) immunosuppression, vitality, proliferation, and migration, and is therefore a promising target for treating GBM. CRISPR/Cas9-mediated genomic editing can delete both cell surface and intracellular PD-L1. This systemic deliverable genomic PD-L1 deletion system can be used as an effective anti-GBM therapy by inhibiting tumor growth and migration, and overcoming immunosuppression. To target PD-L1 for CRISPR/Cas9 gene editing, we first identified two single guide RNA (sgRNA) sequences located on PD-L1 exon 3. The first sgRNA recognizes the forward strand of human PD-L1 near the beginning of exon 3 that allows editing by Cas9 at approximately base pair 82 (g82). The second sgRNA recognizes the forward strand of exon 3 that directs cutting at base pair 165 (g165). A homology-directed repair template (HDR) combined with the dual-sgRNAs was used to improve PD-L1 knockout specificity and efficiency. sgRNAs g82 and g165 were cloned into the multiplex CRISPR/Cas9 assembly system and co-transfected with the HDR template in human U87 GBM cells (g82/165 + HDR). T7E1 analysis suggests that the dual-sgRNA CRISPR/Cas9 strategy with a repair template was capable of editing the genomic level of PD-L1. This was further confirmed by examining PD-L1 protein levels by western blot and immunofluorescence assays. Western blot analysis showed that the dual-sgRNAs with the repair template caused a 64% reduction of PD-L1 protein levels in U87 cells, while immunostaining showed a significant reduction of intracellular PD-L1. PD-L1 deletion inhibited proliferation, growth, invasion and migration of U87 cells, indicating intracellular PD-L1 is necessary for tumor progression. Importantly, U87 cells treated with g82/165 + HDR polarized tumor-associated macrophages (TAM) toward an M1 phenotype, as indicated by an increase in TNF-α and a decrease in IL-4 secretions. This was further confirmed with flow cytometry that showed an increase in the M1 markers Ly6C + and CD80 +, and a decrease in the M2 marker CD206 + both in vitro and in vivo. Utilizing dual-sgRNAs and an HDR template with the CRISPR/Cas9 gene-editing system is a promising avenue for the treatment of GBM.

Progression and application of CRISPR-Cas genomic editors

Base editing technology is an efficient tool for genome editing, particularly in the correction of base mutations. Diverse base editing systems were developed according to the dCas9 or nCas9 linked with different deaminase or reverse transcriptase in the editors, including ABEs, CBEs, PEs and dual-functional of base editor (such as CGBE1, A&C-BEmax, ACBE, etc.). Currently, Base editing technology has been widely applied to various fields such as microorganisms, plants, animals and medicine for basic research and therapeutics. Here, we reviewed the advancement of base editing technology. We also discussed the application of base editors in different areas in the future.