Base Editing Landscape Extends to Perform Transversion Mutation.

Base editors have drawn considerable academic and industrial attention in recent years because of their ability to alter single DNA bases with precision. However, the existing cytosine and adenine base editors can only install transition mutations. Three recent studies (Kurt et al.,Zhao et al., and Chen et al.) expand the base editing toolbox by developing cytosine transversion base editors.

Understanding sheath blight resistance in rice: the road behind and the road ahead.

Rice sheath blight disease, caused by the basidiomycetous necrotroph Rhizoctonia solani, became one of the major threats to the rice cultivation worldwide, especially after the adoption of high-yielding varieties. The pathogen is challenging to manage because of its extensively broad host range and high genetic variability and also due to the inability to find any satisfactory level of natural resistance from the available rice germplasm. It is high time to find remedies to combat the pathogen for reducing rice yield losses and subsequently to minimize the threat to global food security. The development of genetic resistance is one of the alternative means to avoid the use of hazardous chemical fungicides. This review mainly focuses on the effort of better understanding the host-pathogen relationship, finding the gene loci/markers imparting resistance response and modifying the host genome through transgenic development. The latest development and trend in the R. solani-rice pathosystem research with gap analysis are provided.

Green tissue-specific co-expression of chitinase and oxalate oxidase 4 genes in rice for enhanced resistance against sheath blight.

MAIN CONCLUSION: Green tissue-specific simultaneous overexpression of two defense-related genes ( OsCHI11 & OsOXO4 ) in rice leads to significant resistance against sheath blight pathogen ( R. solani ) without distressing any agronomically important traits. Overexpressing two defense-related genes (OsOXO4 and OsCHI11) cloned from rice is effective at enhancing resistance against sheath blight caused by Rhizoctonia solani. These genes were expressed under the control of two different green tissue-specific promoters, viz. maize phosphoenolpyruvate carboxylase gene promoter, PEPC, and rice cis-acting 544-bp DNA element, immediately upstream of the D54O translational start site, P D54O-544 . Putative T0 transgenic rice plants were screened by PCR and integration of genes was confirmed by Southern hybridization of progeny (T1) rice plants. Successful expression of OsOXO4 and OsCHI11 in all tested plants was confirmed. Expression of PR genes increased significantly following pathogen infection in overexpressing transgenic plants. Following infection, transgenic plants exhibited elevated hydrogen peroxide levels, significant changes in activity of ROS scavenging enzymes and reduced membrane damage when compared to their wild-type counterpart. In a Rhizoctonia solani toxin assay, a detached leaf inoculation test and an in vivo plant bioassay, transgenic plants showed a significant reduction in disease symptoms in comparison to non-transgenic control plants. This is the first report of overexpression of two different PR genes driven by two green tissue-specific promoters providing enhanced sheath blight resistance in transgenic rice.

Development of pod borer-resistant transgenic chickpea using a pod-specific and a constitutive promoter-driven fused cry1Ab/Ac gene.

KEY MESSAGE: We studied pod-specific msg promoter from soybean and developed different transgenic lines of chickpea expressing fused cry1Ab/Ac constitutively and pod specifically for resistance against the destructive pest Helicoverpa armigera. Crystal (Cry) proteins derived from the soil bacterium Bacillus thuringiensis (Bt) play an important role in controlling infestation of Helicoverpa armigera, which has been considered a serious problem in chickpea productivity. This study was undertaken to overcome the problem by introducing fused cry1Ab/Ac insecticidal gene under the control of pod-specific soybean msg promoter as well as rice actin1 promoter into chickpea var. DCP 92-3 by Agrobacterium-mediated transformation. Transgenic chickpea lines were characterized by real-time PCR, ELISA and insect bioassay. Expression of fused cry gene under constitutive and pod-specific promoter results in increase of 77- and 110-fold, respectively, compared to non-transgenic control plants. Levels of Cry toxins produced under the control of actin1 and soybean msg promoter were also estimated by ELISA in the leaves and pods, respectively. The higher expression of fused cry gene caused a lethal effect in larvae. The results of insect bioassay study revealed significant reduction in the survival rate of H. armigera reared on transgenic chickpea twigs as well as on pods. Pod-specific promoter-driven fused cry gene provides better and significant management strategy of pest control of chickpea without phenotypic cost.

Potential gene editing targets for developing haploid inducer stocks in rice and wheat with high haploid induction frequency.

Doubled haploid (DH) breeding is a powerful technique to ensure global food security via accelerated crop improvement. DH can be produced in planta by employing haploid inducer stock (HIS). Widely used HIS in maize is known to be governed by ZmPLA, ZmDMP, ZmPLD3, and ZmPOD65 genes. To develop such HIS in rice and wheat, we have identified putative orthologs of these genes using in silico approaches. The OsPLD1; TaPLD1, and OsPOD6; TaPOD8 were identified as putative orthologs of ZmPLD3 and ZmPOD65 in rice and wheat, respectively. Despite being closely related to ZmPLD3, OsPLD1 and TaPLD1 have shown higher anther-specific expression. Similarly, OsPOD6 and TaPOD8 were found closely related to the ZmPOD65 based on both phylogenetic and expression analysis. However, unlike ZmPLD3 and ZmPOD65, two ZmDMP orthologs have been found for each crop. OsDMP1 and OsDMP2 in rice and TaDMP3 and TaDMP13 in wheat have shown similarity to ZmDMP in terms of both sequence and expression pattern. Furthermore, analogs to maize DMP proteins, these genes possess four transmembrane helices making them best suited to be regarded as ZmDMP orthologs. Modifying these predicted orthologous genes by CRISPR/Cas9-based genome editing can produce a highly efficient HIS in both rice and wheat. Besides revealing the genetic mechanism of haploid induction, the development of HIS would advance the genetic improvement of these crops. Supplementary Information: The online version contains supplementary material available at 10.1007/s13205-023-03857-9.

Genome editing for improving nutritional quality, post-harvest shelf life and stress tolerance of fruits, vegetables, and ornamentals.

Agricultural production relies on horticultural crops, including vegetables, fruits, and ornamental plants, which sustain human life. With an alarming increase in human population and the consequential need for more food, it has become necessary for increased production to maintain food security. Conventional breeding has subsidized the development of improved verities but to enhance crop production, new breeding techniques need to be acquired. CRISPR-Cas9 system is a unique and powerful genome manipulation tool that can change the DNA in a precise way. Based on the bacterial adaptive immune system, this technique uses an endonuclease that creates double-stranded breaks (DSBs) at the target loci under the guidance of a single guide RNA. These DSBs can be repaired by a cellular repair mechanism that installs small insertion and deletion (indels) at the cut sites. When equated to alternate editing tools like ZFN, TALENs, and meganucleases, CRISPR- The cas-based editing tool has quickly gained fast-forward for its simplicity, ease to use, and low off-target effect. In numerous horticultural and industrial crops, the CRISPR technology has been successfully used to enhance stress tolerance, self-life, nutritional improvements, flavor, and metabolites. The CRISPR-based tool is the most appropriate one with the prospective goal of generating non-transgenic yields and avoiding the regulatory hurdles to release the modified crops into the market. Although several challenges for editing horticultural, industrial, and ornamental crops remain, this new novel nuclease, with its crop-specific application, makes it a dynamic tool for crop improvement.

Overexpression of Setaria italica phosphoenolpyruvate carboxylase gene in rice positively impacts photosynthesis and agronomic traits.

C4 plants have the inherent capacity to concentrate atmospheric CO2 in the vicinity of RuBisCo, thereby increasing carboxylation, and inhibiting photorespiration. Carbonic anhydrase (CA), the first enzyme of C4 photosynthesis, converts atmospheric CO2 to HCO3-, which is utilized by PEPC to produce C4 acids. Bioengineering of C4 traits into C3 crops is an attractive strategy to increase photosynthesis and water use efficiency. In the present study, we isolated the PEPC gene from the C4 plant Setaria italica and transferred it to C3 rice. Overexpression of SiPEPC resulted in a 2-6-fold increment in PEPC enzyme activity in transgenic lines with respect to non-transformed control. Photosynthetic efficiency was enhanced in transformed plants, which was associated with increased ФPSII, ETR, lower NPQ, and higher chlorophyll accumulation. Water use efficiency was increased by 16-22% in PEPC transgenic rice lines. Increased PEPC activity enhanced quantum yield and carboxylation efficiency of PEPC transgenic lines. Transgenic plants exhibited higher light saturation photosynthesis rate and lower CO2 compensation point, as compared to non-transformed control. An increase in net photosynthesis increased the yield by (23-28.9%) and biomass by (24.1-29%) in transgenic PEPC lines. Altogether, our findings indicate that overexpression of C4-specific SiPEPC enzyme is able to enhance photosynthesis and related parameters in transgenic rice.

A detailed landscape of CRISPR-Cas-mediated plant disease and pest management.

Genome editing technology has rapidly evolved to knock-out genes, create targeted genetic variation, install precise insertion/deletion and single nucleotide changes, and perform large-scale alteration. The flexible and multipurpose editing technologies have started playing a substantial role in the field of plant disease management. CRISPR-Cas has reduced many limitations of earlier technologies and emerged as a versatile toolbox for genome manipulation. This review summarizes the phenomenal progress of the use of the CRISPR toolkit in the field of plant pathology. CRISPR-Cas toolbox aids in the basic studies on host-pathogen interaction, in identifying virulence genes in pathogens, deciphering resistance and susceptibility factors in host plants, and engineering host genome for developing resistance. We extensively reviewed the successful genome editing applications for host plant resistance against a wide range of biotic factors, including viruses, fungi, oomycetes, bacteria, nematodes, insect pests, and parasitic plants. Recent use of CRISPR-Cas gene drive to suppress the population of pathogens and pests has also been discussed. Furthermore, we highlight exciting new uses of the CRISPR-Cas system as diagnostic tools, which rapidly detect pathogenic microorganism. This comprehensive yet concise review discusses innumerable strategies to reduce the burden of crop protection.

Morphophysiological alterations in transgenic rice lines expressing PPDK and ME genes from the C4 model Setaria italica.

C4 plants are superior to C3 plants in terms of productivity and limited photorespiration. PPDK (pyruvate orthophosphate dikinase) and NADP-ME (NADP-dependent malic enzyme) are two important photosynthetic C4-specific enzymes present in the mesophyll cells of C4 plants. To evaluate the effect of C4 enzymes in rice, we developed transgenic rice lines by separately introducing Setaria italica PPDK [SiPPDK] and S. italica ME [SiME] gene constructs under the control of the green tissue-specific maize PPDK promoter. Rice plant lines for both constructs were screened using the polymerase chain reaction (PCR), Southern hybridization, and expression analysis. The best transgenic plant lines for each case were selected for physiological and biochemical characterization. The results from qRT-PCR and enzyme activity analysis revealed higher expression and activity of both PPDK and NADP-ME genes compared with the nontransformed and empty-vector-transformed plants. The average photosynthetic efficiency of transgenic plant lines carrying the PPDK and NADP-ME genes increased by 18% and 12%, respectively, and was positively correlated with the increased accumulation of photosynthetic pigment. The decrease in Fv/Fm, increased electron transport rate (ETR), and increased photochemical quenching (qP) compared with nontransformed control plants suggest that transgenic rice plants transferred more absorbed light energy to photochemical reactions than wild-type plants. SiME-transgenic plants displayed reduced leaf malate content and superior performance under water deficit conditions. Interestingly, the transgenic plants showed yield enhancement by exhibiting increased plant height, panicle length, panicle weight and thousand grain weight. Overall, the exogenous foxtail millet C4 gene PPDK enhanced photosynthesis and yield to a greater extent than NADP-ME.

Antimicrobial Peptides - Small but Mighty Weapons for Plants to Fight Phytopathogens.

Antimicrobial Peptides (AMPs) have diverse structures, varied modes of actions, and can inhibit the growth of a wide range of pathogens at low concentrations. Plants are constantly under attack by a wide range of phytopathogens causing massive yield losses worldwide. To combat these pathogens, nature has armed plants with a battery of defense responses including Antimicrobial Peptides (AMPs). These peptides form a vital component of the two-tier plant defense system. They are constitutively expressed as part of the pre-existing first line of defense against pathogen entry. When a pathogen overcomes this barrier, it faces the inducible defense system, which responds to specific molecular or effector patterns by launching an arsenal of defense responses including the production of AMPs. This review emphasizes the structural and functional aspects of different plant-derived AMPs, their homology with AMPs from other organisms, and how their biotechnological potential could generate durable resistance in a wide range of crops against different classes of phytopathogens in an environmentally friendly way without phenotypic cost.

Development of a rapid and highly efficient Agrobacterium-mediated transformation system for pigeon pea [Cajanus cajan (L.) Millsp].

An efficient genetic transformation system is a prerequisite for studying gene functions, molecular breeding program, and introducing new traits. Agrobacterium tumefaciens-mediated genetic transformation is a widely preferred and accepted method for many plants, including pigeon pea. However, the efficiency of transformation of pigeon pea using the existing protocols is low and time-consuming. In the present study, we developed a rapid and highly efficient transformation system of pigeon pea, using embryonic axis-attached cotyledons as explants. We systematically investigated the influence of varying optical densities of Agrobacterium suspension, duration of incubation, and co-cultivation on the transformation efficiency. In our system, a transformation efficiency of approximately 83% was achieved using Agrobacterium cells at an optical density (OD600) of 0.25, infection time of 15 min, and co-culturing with explants for 72 h in the light with 100µM acetosyringone. The entire procedure, starting from seed to establishment of transformed plants in soil, was achieved in 35-40 days. This is a rapid and highly efficient protocol for Agrobacterium-mediated transformation of pigeon pea, which could potentially be a useful reference, not only for the genetic improvement of pigeon pea but also for other recalcitrant leguminous plants.

Proteo-metabolomic investigation of transgenic rice unravels metabolic alterations and accumulation of novel proteins potentially involved in defence against Rhizoctonia solani.

The generation of sheath blight (ShB)-resistant transgenic rice plants through the expression of Arabidopsis NPR1 gene is a significant development for research in the field of biotic stress. However, to our knowledge, regulation of the proteomic and metabolic networks in the ShB-resistant transgenic rice plants has not been studied. In the present investigation, the relative proteome and metabolome profiles of the non-transformed wild-type and the AtNPR1-transgenic rice lines prior to and subsequent to the R. solani infection were investigated. Total proteins from wild type and transgenic plants were investigated using two-dimensional gel electrophoresis (2-DE) followed by mass spectrometry (MS). The metabolomics study indicated an increased abundance of various metabolites, which draws parallels with the proteomic analysis. Furthermore, the proteome data was cross-examined using network analysis which identified modules that were rich in known as well as novel immunity-related prognostic proteins, particularly the mitogen-activated protein kinase 6, probable protein phosphatase 2C1, probable trehalose-phosphate phosphatase 2 and heat shock protein. A novel protein, 14-3-3GF14f was observed to be upregulated in the leaves of the transgenic rice plants after ShB infection, and the possible mechanistic role of this protein in ShB resistance may be investigated further.

Dual gene expression cassette is superior than single gene cassette for enhancing sheath blight tolerance in transgenic rice.

Sheath blight, caused by the necrotrophic fungal pathogen Rhizoctonia solani, is a serious and destructive disease of the rice. In order to improve sheath blight resistance, we developed three different kinds of transgenic rice lines. The first transgenic line overexpresses the rice chitinase gene (OsCHI11); the second contains the Arabidopsis NPR1 (AtNPR1) gene and, the third has pyramided constructs with both the genes (OsCHI11 and AtNPR1). This is a comparative study between the single-gene transgenic lines and the double gene transgenic in terms of their ability to activate the plant defense system. Rice plants of each individual construct were screened via PCR, Southern hybridization, activity assays, and expression analysis. The best transgenic lines of each construct were chosen for comparative study. The fold change in qRT-PCR and activity assays revealed that the pyramided transgenic rice plants show a significant upregulation of defense-related genes, PR genes, and antioxidant marker genes as compared to the single transgene. Simultaneous co-expression of both the genes was found to be more efficient in tolerating oxidative stress. In R. solani (RS) toxin assay, mycelial agar disc bioassay, and in vivo plant bioassay, pyramided transgenic plant lines were more competent at restricting the pathogen development and enhancing sheath blight tolerance as compared to single gene transformants.

Tissue-specific expression of Arabidopsis NPR1 gene in rice for sheath blight resistance without compromising phenotypic cost.

Rice sheath blight disease, caused by the fungus Rhizoctonia solani, is considered the second most important disease of rice after blast. NPR1 (non expressor of PR1) is the central regulator of systemic acquired resistance (SAR) conferring broad spectrum resistance to various pathogens. Previous reports have indicated that constitutive expression of the Arabidopsis thaliana NPR1 (AtNPR1) gene results in disease resistance in rice but has a negative impact on growth and agronomic traits. Here, we report that green tissue-specific expression of AtNPR1 in rice confers resistance to the sheath blight pathogen, with no concomitant abnormalities in plant growth and yield parameters. Elevated levels of NPR1 activated the defence pathway in the transgenic plants by inducing expression of endogenous genes such as PR1b, RC24, and PR10A. Enhanced sheath blight resistance of the transgenic plants was evaluated using three different bioassay systems. A partially isolated toxin from R. solani was used in the bioassays to measure the resistance level. Studies of the phenotype and yield showed that the transgenic plants did not exhibit any kind of phenotypic imbalances. Our results demonstrate that green tissue-specific expression of AtNPR1 is an effective strategy for controlling the sheath blight pathogen. The present work in rice can be extended to other crop plants severely damaged by the pathogen.

Rice oxalate oxidase gene driven by green tissue-specific promoter increases tolerance to sheath blight pathogen (Rhizoctonia solani) in transgenic rice.

Rice sheath blight, caused by the necrotrophic fungus Rhizoctonia solani, is one of the most devastating and intractable diseases of rice, leading to a significant reduction in rice productivity worldwide. In this article, in order to examine sheath blight resistance, we report the generation of transgenic rice lines overexpressing the rice oxalate oxidase 4 (Osoxo4) gene in a green tissue-specific manner which breaks down oxalic acid (OA), the pathogenesis factor secreted by R. solani. Transgenic plants showed higher enzyme activity of oxalate oxidase (OxO) than nontransgenic control plants, which was visualized by histochemical assays and sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-PAGE). Transgenic rice leaves were more tolerant than control rice leaves to exogenous OA. Transgenic plants showed a higher level of expression of other defence-related genes in response to pathogen infection. More importantly, transgenic plants exhibited significantly enhanced durable resistance to R. solani. The overexpression of Osoxo4 in rice did not show any detrimental phenotypic or agronomic effect. Our findings indicate that rice OxO can be utilized effectively in plant genetic manipulation for sheath blight resistance, and possibly for resistance to other diseases caused by necrotrophic fungi, especially those that secrete OA. This is the first report of the expression of defence genes in rice in a green tissue-specific manner for sheath blight resistance.