Cas12a-mediated gene targeting by sequential transformation strategy in Arabidopsis thaliana.

Gene targeting (GT) allows precise manipulation of genome sequences, such as knock-ins and sequence substitutions, but GT in seed plants remains a challenging task. Engineered sequence-specific nucleases (SSNs) are known to facilitate GT via homology-directed repair (HDR) in organisms. Here, we demonstrate that Cas12a and a temperature-tolerant Cas12a variant (ttCas12a) can efficiently establish precise and heritable GT at two loci in Arabidopsis thaliana (Arabidopsis) through a sequential transformation strategy. As a result, ttCas12a showed higher GT efficiency than unmodified Cas12a. In addition, the efficiency of transcriptional and translational enhancers for GT via sequential transformation strategy was also investigated. These enhancers and their combinations were expected to show an increase in GT efficiency in the sequential transformation strategy, similar to previous reports of all-in-one strategies, but only a maximum twofold increase was observed. These results indicate that the frequency of double strand breaks (DSBs) at the target site is one of the most important factors determining the efficiency of genetic GT in plants. On the other hand, a higher frequency of DSBs does not always lead to higher efficiency of GT, suggesting that some additional factors are required for GT via HDR. Therefore, the increase in DSB can no longer be expected to improve GT efficiency, and a new strategy needs to be established in the future. This research opens up a wide range of applications for precise and heritable GT technology in plants.

Chromosome-scale assembly and gene editing of Solanum americanum genome reveals the basis for thermotolerance and fruit anthocyanin composition.

Solanum americanum serves as a promising source of resistance genes against potato late blight and is considered as a leafy vegetable for complementary food and nutrition. The limited availability of high-quality genome assemblies and gene annotations has hindered the exploration and exploitation of stress-resistance genes in S. americanum. Here, we present a chromosome-level genome assembly of a thermotolerant S. americanum ecotype and identify a crucial heat-inducible transcription factor gene, SaHSF17, essential for heat tolerance. The CRISPR/Cas9 system-mediated knockout of SaHSF17 results in remarkably reduced thermotolerance in S. americanum, exhibiting a significant suppression of multiple HSP gene expressions under heat treatment. Furthermore, our transcriptome analysis and anthocyanin component investigation of fruits indicated that delphinidins are the major anthocyanins accumulated in the mature dark-purple fruits. The accumulation of delphinidins and other pigment components during fruit ripening in S. americanum coincides with the transcriptional regulation of key genes, particularly the F3'5'H and F3'H genes, in the anthocyanin biosynthesis pathway. By integrating existing knowledge, the development of this high-quality reference genome for S. americanum will facilitate the identification and utilization of novel abiotic and biotic stress-resistance genes for improvement of Solanaceae and other crops.

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.

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.

Precise and heritable gene targeting in rice using a sequential transformation strategy.

Gene targeting (GT) is a powerful tool for modifying endogenous genomic sequences of interest, such as sequence replacement and gene knockin. Although the efficiency of GT is extremely low in higher plants, engineered sequence-specific nucleases (SSNs)-mediated double-strand breaks (DSBs) can improve GT frequency. We recently reported a CRISPR-Cas9-mediated approach for heritable GT in Arabidopsis, called the "sequential transformation" strategy. For efficient establishment of GT via the sequential transformation method, strong Cas9 activity and robust DSBs are required in the plant cells being infected with Agrobacterium carrying sgRNA and donor DNA. Accordingly, we generated two independent parental lines with maize Ubiquitin 1 promoter-driven Cas9 and established sequential transformation-mediated GT in the Japonica rice cultivar Oryza sativa Nipponbare. We achieved precise GFP knockin into the endogenous OsFTL1 and OsROS1a loci. We believe that our GT technology could be widely utilized in rice research and breeding applications.

Natural variation in SlSOS2 promoter hinders salt resistance during tomato domestication.

Increasing soil salinization seriously impairs plant growth and development, resulting in crop loss. The Salt-Overly-Sensitive (SOS) pathway is indispensable to the mitigation of Na + toxicity in plants under high salinity. However, whether natural variations of SOS2 contribute to salt tolerance has not been reported. Here a natural variation in the SlSOS2 promoter region was identified to be associated with root Na+/K+ ratio and the loss of salt resistance during tomato domestication. This natural variation contains an ABI4-binding cis-element and plays an important role in the repression of SlSOS2 expression. Genetic evidence revealed that SlSOS2 mutations increase root Na+/K+ ratio under salt stress conditions and thus attenuate salt resistance in tomato. Together, our findings uncovered a critical but previously unknown natural variation of SOS2 in salt resistance, which provides valuable natural resources for genetic breeding for salt resistance in cultivated tomatoes and other crops.

The Sm core protein SmEb regulates salt stress responses through maintaining proper splicing of RCD1 pre-mRNA in Arabidopsis.

Salt stress adversely impacts crop production. Several spliceosome components have been implicated in regulating salt stress responses in plants, however, the underlying molecular basis is still unclear. Here we report that the spliceosomal core protein SmEb is essential to salt tolerance in Arabidopsis. Transcriptome analysis showed that SmEb modulates alternative splicing of hundreds of pre-mRNAs in plant response to salt stress. Further study revealed that SmEb is crucial in maintaining proper ratio of two RCD1 splicing variants (RCD1.1/RCD1.2) important for salt stress response. In addition, RCD1.1 but not RCD1.2 is able to interact with the stress regulators and attenuates salt-sensitivity by decreasing salt-induced cell death in smeb-1 mutant. Together, our findings uncovered the essential role of SmEb in the regulation of alternative pre-mRNA splicing in salt stress response.

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.

Gene Targeting Facilitated by Engineered Sequence-Specific Nucleases: Potential Applications for Crop Improvement.

Humans are currently facing the problem of how to ensure that there is enough food to feed all of the world's population. Ensuring that the food supply is sufficient will likely require the modification of crop genomes to improve their agronomic traits. The development of engineered sequence-specific nucleases (SSNs) paved the way for targeted gene editing in organisms, including plants. SSNs generate a double-strand break (DSB) at the target DNA site in a sequence-specific manner. These DSBs are predominantly repaired via error-prone non-homologous end joining and are only rarely repaired via error-free homology-directed repair if an appropriate donor template is provided. Gene targeting (GT), i.e. the integration or replacement of a particular sequence, can be achieved with combinations of SSNs and repair donor templates. Although its efficiency is extremely low, GT has been achieved in some higher plants. Here, we provide an overview of SSN-facilitated GT in higher plants and discuss the potential of GT as a powerful tool for generating crop plants with desirable features.

Origin of Jupiter's cloud-level zonal winds remains a puzzle even after Juno.

How far Jupiter's cloud-level zonal winds penetrate into its interior, a question related to the origin of the winds, has long been a major puzzle about Jupiter. There exist two different views: the shallow scenario in which the cloud-level winds are confined within the thin weather layer at cloud top and the deep scenario in which the cloud-level winds manifest thermal convection in the deep interior. We interpret, using two different models corresponding to the two scenarios, the high-precision measurements of Jupiter's equatorially antisymmetric gravitational field by the Juno spacecraft. We demonstrate, based on the thermal-gravitational wind equation, that both the shallow and deep cloud-level winds models are capable of explaining the measured odd gravitational coefficients within the measured uncertainties, reflecting the nonunique nature of the gravity inverse problem. We conclude that the high-precision Juno gravity measurements cannot provide an answer to the long-standing question about the origin of Jupiter's cloud-level zonal winds.

Using Jupiter's gravitational field to probe the Jovian convective dynamo.

Convective motion in the deep metallic hydrogen region of Jupiter is believed to generate its magnetic field, the strongest in the solar system. The amplitude, structure and depth of the convective motion are unknown. A promising way of probing the Jovian convective dynamo is to measure its effect on the external gravitational field, a task to be soon undertaken by the Juno spacecraft. We calculate the gravitational signature of non-axisymmetric convective motion in the Jovian metallic hydrogen region and show that with sufficiently accurate measurements it can reveal the nature of the deep convection.

Precessing cylinders at the second and third resonance: turbulence controlled by geostrophic flow.

We investigate, via both asymptotic analysis and direct numerical simulation, precessionally driven flow of a homogeneous fluid confined in fluid-filled circular cylinders that rotate rapidly about their symmetry axis and precess about a different axis and that are marked by radius-height aspect ratios Γ=1.045945 and Γ=1.611089. At these radius-height aspect ratios, the Poincaré force resonates directly with the two special inertial modes that have the simplest vertical structure. An asymptotic analytical solution in closed form describing weakly precessing flow is derived in the mantle frame of reference for asymptotically small Ekman numbers, showing quantitative agreement with the result of direct nonlinear numerical simulation. Our numerical simulation makes use of a finite-element method with the three-dimensional tetrahedralization of a cylindrical cavity that allows the construction of dense nodes in the vicinity of the bounding surface of the cavity for resolving the thin viscous boundary layer. It is found that axisymmetric geostrophic flow in the alternating eastward and westward direction can be generated and maintained by nonlinear and viscous effects in the viscous boundary layer. It is also found that, when the precessing rate is moderate and, consequently, the geostrophic flow is weak, nonlinear interaction between the resonant inertial mode and the nonesonant inertial modes driven by the Poincaré force and the boundary-layer influx leads to strongly turbulent flow with irregular temporal-spatial fluctuation. When the cylinders are strongly precessing such that the geostrophic flow becomes predominant, however, the effect of the geostrophic flow controls/stabilizes its nonlinear dynamics, leading to weakly turbulent flow that can be largely described by a dominant quasisteady geostrophic component and a weak nonaxisymmetric component localized in the region where the geostrophic flow is weak.

Swimming motion of rod-shaped magnetotactic bacteria: the effects of shape and growing magnetic moment.

We investigate the swimming motion of rod-shaped magnetotactic bacteria affiliated with the Nitrospirae phylum in a viscous liquid under the influence of an externally imposed, time-dependent magnetic field. By assuming that fluid motion driven by the translation and rotation of a swimming bacterium is of the Stokes type and that inertial effects of the motion are negligible, we derive a new system of the twelve coupled equations that govern both the motion and orientation of a swimming rod-shaped magnetotactic bacterium with a growing magnetic moment in the laboratory frame of reference. It is revealed that the initial pattern of swimming motion can be strongly affected by the rate of the growing magnetic moment. It is also revealed, through comparing mathematical solutions of the twelve coupled equations to the swimming motion observed in our laboratory experiments with rod-shaped magnetotactic bacteria, that the laboratory trajectories of the swimming motion can be approximately reproduced using an appropriate set of the parameters in our theoretical model.