Accumulation of glycine betaine in transplastomic potato plants expressing choline oxidase confers improved drought tolerance

Choline Dehydrogenase Contributes to Salt Tolerance in Dunaliella through Betaine Synthesis

In Dunaliella tertiolecta, a microalga renowned for its extraordinary tolerance to high salinity levels up to 4.5 M NaCl, the mechanisms underlying its stress response have largely remained a mystery. In a groundbreaking discovery, this study identifies a choline dehydrogenase enzyme, termed DtCHDH, capable of converting choline to betaine aldehyde. Remarkably, this is the first identification of such an enzyme not just in D. tertiolecta but across the entire Chlorophyta. A 3D model of DtCHDH was constructed, and molecular docking with choline was performed, revealing a potential binding site for the substrate. The enzyme was heterologously expressed in E. coli Rosetta (DE3) and subsequently purified, achieving enzyme activity of 672.2 U/mg. To elucidate the role of DtCHDH in the salt tolerance of D. tertiolecta, RNAi was employed to knock down DtCHDH gene expression. The results indicated that the Ri-12 strain exhibited compromised growth under both high and low salt conditions, along with consistent levels of DtCHDH gene expression and betaine content. Additionally, fatty acid analysis indicated that DtCHDH might also be a FAPs enzyme, catalyzing reactions with decarboxylase activity. This study not only illuminates the role of choline metabolism in D. tertiolecta's adaptation to high salinity but also identifies a novel target for enhancing the NaCl tolerance of microalgae in biotechnological applications.

Genome-wide identification of 2-oxoglutarate and Fe (II)-dependent dioxygenase family genes and their expression profiling under drought and salt stress in potato

The 2-Oxoglutatrate-dependent dioxygenases (2OGDs) comprise the 2-Oxoglutatrate and Fe(II)-dependent dioxygenases (2ODD) enzyme families that facilitate the biosynthesis of various compounds like gibberellin, ethylene, etc. The 2OGDs are also involved in various catabolism pathways, such as auxin and salicylic acid catabolism. Despite their important roles, 2ODDs have not been studied in potato, which is the third most important crop globally. In this study, a comprehensive genome wide analysis was done to identify all 2ODDs in potatoes, and the putative genes were analysed for the presence of the signature 2OG-FeII_Oxy (PF03171) domain and the conserved DIOX_N (PF14226) domain. A total of 205 St2ODDs were identified and classified into eight groups based on their function. The physiochemical properties, gene structures, and motifs were analysed, and gene duplication events were also searched for St2ODDs. The active amino acid residues responsible for binding with 2-oxoglutarate and Fe (II) were conserved throughout the St2ODDs. The three-dimensional (3D) structures of the representative members of flavanol synthase (FNS), 1-aminocyclopropane-1-carboxylic acid oxidases (ACOs), and gibberellin oxidases (GAOXs) were made and docked with their respective substrates, and the potential interactions were visualised. The expression patterns of the St2ODDs under abiotic stressors such as heat, salt, and drought were also analysed. We found altered expression levels of St2ODDs under abiotic stress conditions, which was further confirmed for drought and salt stress using qRT-PCR. The expression levels of St2ODD115, St2ODD34, and St2ODD99 were found to be upregulated in drought stress with 2.2, 1.8, and 2.6 fold changes, respectively. After rewatering, the expression levels were normal. In salt stress, the expression levels of St2ODD151, St2ODD76, St2ODD91, and St2ODD34 were found to be upregulated after 24 hours (h), 48 hours (h), 72 hours (h), and 96 hours (h). Altogether, the elevated expression levels suggest the importance of St2ODDs under abiotic stresses, i.e., drought and salt. Overall, our study provided a knowledge base for the 2ODD gene family in potato, which can be used further to study the important roles of 2ODDs in potato plants.

Chloroplast Genome Engineering: A Plausible Approach to Combat Chili Thrips and Other Agronomic Insect Pests of Crops

The world population's growing demand for food is expected to increase dramatically by 2050. The agronomic productivity for food is severely affected due to biotic and abiotic constraints. At a global level, insect pests alone account for ~20% loss in crop yield every year. Deployment of noxious chemical pesticides to control insect pests always has a threatening effect on human health and environmental sustainability. Consequently, this necessitates for the establishment of innovative, environmentally friendly, cost-effective, and alternative means to mitigate insect pest management strategies. According to a recent study, using chloroplasts engineered with double-strand RNA (dsRNA) is novel successful combinatorial strategy deployed to effectively control the most vexing pest, the western flower thrips (WFT: Frankliniella occidentalis). Such biotechnological avenues allowed us to recapitulate the recent progress of research methods, such as RNAi, CRISPR/Cas, mini chromosomes, and RNA-binding proteins with plastid engineering for a plausible approach to effectively mitigate agronomic insect pests. We further discussed the significance of the maternal inheritance of the chloroplast, which is the major advantage of chloroplast genome engineering.

Foliar Spraying of Glycine Betaine Alleviated Growth Inhibition, Photoinhibition, and Oxidative Stress in Pepper (Capsicum annuum L.) Seedlings under Low Temperatures Combined with Low Light

Low temperature combined with low light (LL stress) is a typical environmental stress that limits peppers' productivity, yield, and quality in northwestern China. Glycine betaine (GB), an osmoregulatory substance, has increasingly valuable effects on plant stress resistance. In this study, pepper seedlings were treated with different concentrations of GB under LL stress, and 20 mM of GB was the best treatment. To further explore the mechanism of GB in response to LL stress, four treatments, including CK (normal temperature and light, 28/18 °C, 300 μmol m^-2 s^-1), CB (normal temperature and light + 20 mM GB), LL (10/5 °C, 100 μmol m^-2 s^-1), and LB (10/5 °C, 100 μmol m^-2 s^-1 + 20 mM GB), were investigated in terms of pepper growth, biomass accumulation, photosynthetic capacity, expression levels of encoded proteins Capsb, cell membrane permeability, antioxidant enzyme gene expression and activity, and subcellular localization. The results showed that the pre-spraying of GB under LL stress significantly alleviated the growth inhibition of pepper seedlings; increased plant height by 4.64%; increased root activity by 63.53%; and decreased photoinhibition by increasing the chlorophyll content; upregulating the expression levels of encoded proteins Capsb A, Capsb B, Capsb C, Capsb D, Capsb S, Capsb P1, and Capsb P2 by 30.29%, 36.69%, 18.81%, 30.05%, 9.01%, 6.21%, and 16.45%, respectively; enhancing the fluorescence intensity (OJIP curves), the photochemical efficiency (Fv/Fm, Fv'/Fm'), qP, and NPQ; improving the light energy distribution of PSΠ (Y(II), Y(NPQ), and Y(NO)); and increasing the photochemical reaction fraction and reduced heat dissipation, thereby increasing plant height by 4.64% and shoot bioaccumulation by 13.55%. The pre-spraying of GB under LL stress also upregulated the gene expression of CaSOD, CaPOD, and CaCAT; increased the activity of the ROS-scavenging ability in the pepper leaves; and coordinately increased the SOD activity in the mitochondria, the POD activity in the mitochondria, chloroplasts, and cytosol, and the CAT activity in the cytosol, which improved the LL resistance of the pepper plants by reducing excess H₂O₂, O₂^-, MDA, and soluble protein levels in the leaf cells, leading to reduced biological membrane damage. Overall, pre-spraying with GB effectively alleviated the negative effects of LL stress in pepper seedlings.

CBL-Interacting Protein Kinases 18 (CIPK18) Gene Positively Regulates Drought Resistance in Potato

Sensor-responder complexes comprising calcineurin B-like (CBL) proteins and CBL-interacting protein kinases (CIPKs) are plant-specific Ca²⁺ receptors, and the CBL-CIPK module is widely involved in plant growth and development and a large number of abiotic stress response signaling pathways. In this study, the potato cv. "Atlantic" was subjected to a water deficiency treatment and the expression of StCIPK18 gene was detected by qRT-PCR. The subcellular localization of StCIPK18 protein was observed by a confocal laser scanning microscope. The StCIPK18 interacting protein was identified and verified by yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC). StCIPK18 overexpression and StCIPK18 knockout plants were constructed. The phenotypic changes under drought stress were indicated by water loss rate, relative water content, MDA and proline contents, and CAT, SOD and POD activities. The results showed that StCIPK18 expression was upregulated under drought stress. StCIPK18 is localized in the cell membrane and cytoplasm. Y2H shows the interaction between StCIPK18 and StCBL1, StCBL4, StCBL6 and StCBL8. BiFC further verifies the reliability of the interaction between StCIPK18 and StCBL4. Under drought stress, StCIPK18 overexpression decreased the water loss rate and MDA, and increased RWC, proline contents and CAT, SOD and POD activities; however, StCIPK18 knockout showed opposite results, compared with the wild type, in response to drought stress. The results can provide information for the molecular mechanism of the StCIPK18 regulating potato response to drought stress.

StLTO1, a lumen thiol oxidoreductase in Solanum tuberosum L., enhances the cold resistance of potato plants

Cold stress reduces plant photosynthesis and increases the accumulation of reactive oxygen species (ROS) in plants, thereby dramatically affecting plant growth, crop productivity and quality. Here, we report that lumen thiol oxidoreductase 1 (StLTO1), a vitamin K epoxide reductase (VKOR)-like protein in the thylakoid membrane of Solanum tuberosum L., enhances the cold tolerance of potato plants. Under normal conditions, overexpression of StLTO1 in plants promoted plant growth. In addition, potato plants overexpressing StLTO1 displayed enhanced photosynthetic capacity and increased capacity for scavenging ROS compared to StLTO1 knockdown and wild-type potato plants under cold conditions. Overexpression of StLTO1 in potato plants also improved cold-regulated (COR) gene expression after cold stress. Our results suggest that StLTO1 acts as a positive regulator of cold resistance in potato plants.

Chloroplast Engineering: Fundamental Insights and Its Application in Amelioration of Environmental Stress

Chloroplasts are specialized organelle that are responsible for converting light energy to chemical energy, thereby driving the carbon dioxide fixation. Apart from photosynthesis, chloroplast is the site for essential cellular processes that determine the plant adaptation to changing environment. Owing to the presence of their own expression system, it provides an optimum platform for engineering valued traits as well as site for synthesis of bio-compounds. Advancements in technology have further enhanced the scope of using chloroplast as a multifaceted tool for the biotechnologist to develop stress-tolerant plants and ameliorate environmental stress. Focusing on chloroplast biotechnology, this review discusses the advances in chloroplast engineering and its application in enhancing plant adaptation and resistance to environmental stress and the development of new bioproducts and processes. This is accomplished through analysis of its biogenesis and physiological processes, highlighting the chloroplast engineering and recent developments in chloroplast biotechnology. In the first part of the review, the evolution and principles of structural organization and physiology of chloroplast are discussed. In the second part, the chief methods and mechanisms involved in chloroplast transformation are analyzed. The last part represents an updated analysis of the application of chloroplast engineering in crop improvement and bioproduction of industrial and health compounds.

Differential Impact of Nitric Oxide and Abscisic Acid on the Cellular and Physiological Functioning of sub1A QTL Bearing Rice Genotype under Salt Stress

Hydroponic culture containing 200 mM NaCl was used to induce oxidative stress in seedlings of cultivars initially primed with 1 mM SNP and 10 µM ABA. Exogenous application of sodium nitroprusside (SNP - a nitric oxide donor) and abscisic acid (ABA) was well sensitized more in cv. Swarna Sub1 than cv. Swarna and also reflected in different cellular responses. The major effects of salinity, irrespective of the cultivar, were lowering the water relation, including relative water content and osmotic potential, and decreasing the compatible solutes like alanine, gamma-aminobutyric acid, and glycine betaine. The accumulated polyamines were reduced more in cv. Swarna with a concomitant decrease in photosynthetic reserves. NADP-malic enzyme activity, sucrose accumulation, ascorbate peroxidase, and glutathione S-transferase activities gradually declined under NaCl stress and the catabolizing enzymes like invertase (both wall and cytosolic forms) also declined. On the contrary, plants suffered from oxidative stress through superoxide, hydrogen peroxide, and their biosynthetic enzymes like NADP(H) oxidase. Moderation of Na⁺/K⁺ by both SNP and ABA were correlated with other salt sensitivities in the plants. The maximum effects of SNP and ABA were found in the recovery of antioxidation pathways, osmotic tolerance, and carbohydrate metabolism. Findings predict the efficacy of SNP and ABA either independently or cumulatively in overcoming NaCl toxicity in rice.

Improving boron use efficiency via different application techniques for optimum production of good quality potato (Solanum tuberosum L.) in alkaline soil

Boron (B) deficiency is a widespread problem in alkaline soils which affects yield and quality of potato but is often ignored by the growers. That's why, we compared the impact of different methods of boron application (foliar spray, fertigation and soil dressing) along with control on boron use efficiency (BUE), quality and yield of potato in alkaline soils. Boron (0.5 kg ha-1) applied as a foliar spray had significantly increased plant height, tuber per plant, tuber volume and enhanced the quality in terms of vitamin C, starch and B content of potato compared to other methods. Moreover, foliar applied B significantly improved B uptake and it use efficiency over other application methods. B concentration in tubers were strongly correlated with vitamin C and starch contents. The application methods were ranked as foliar spray>fertigation>soil dressing in term of their effectiveness towards potato yield and quality improvement. Thus, for optimum production of good quality potato, B should be applied as foliar spray at the rate of 0.5 kg B ha-1 in existing agro-climatic conditions.

Heterologous Production of Glycine Betaine Using Synechocystis sp. PCC 6803-Based Chassis Lacking Native Compatible Solutes

Among compatible solutes, glycine betaine has various applications in the fields of nutrition, pharmaceuticals, and cosmetics. Currently, this compound can be extracted from sugar beet plants or obtained by chemical synthesis, resulting in low yields or high carbon footprint, respectively. Hence, in this work we aimed at exploring the production of glycine betaine using the unicellular cyanobacterium Synechocystis sp. PCC 6803 as a photoautotrophic chassis. Synechocystis mutants lacking the native compatible solutes sucrose or/and glucosylglycerol-∆sps, ∆ggpS, and ∆sps∆ggpS-were generated and characterized. Under salt stress conditions, the growth was impaired and accumulation of glycogen decreased by ∼50% whereas the production of compatible solutes and extracellular polymeric substances (capsular and released ones) increased with salinity. These mutants were used as chassis for the implementation of a synthetic device based on the metabolic pathway described for the halophilic cyanobacterium Aphanothece halophytica for the production of the compatible solute glycine betaine. Transcription of ORFs comprising the device was shown to be stable and insulated from Synechocystis' native regulatory network. Production of glycine betaine was achieved in all chassis tested, and was shown to increase with salinity. The introduction of the glycine betaine synthetic device into the ∆ggpS background improved its growth and enabled survival under 5% NaCl, which was not observed in the absence of the device. The maximum glycine betaine production [64.29 µmol/gDW (1.89 µmol/mg protein)] was reached in the ∆ggpS chassis grown under 3% NaCl. Taking into consideration this production under seawater-like salinity, and the identification of main key players involved in the carbon fluxes, this work paves the way for a feasible production of this, or other compatible solutes, using optimized Synechocystis chassis in a pilot-scale.

Ingeniería genética contra estrés abiótico en cultivos neotropicales: osmolitos, factores de transcripción y CRISPR/Cas9

El neotrópico es sitio de origen de gran variedad de plantas que actualmente son cultivadas con éxito en diferentes regiones del mundo. Sin embargo, condiciones climáticas adversas, que se pueden ver acrecentadas por efectos del cambio climático antropogénico, pueden afectar su rendimiento y productividad debido a las situaciones de estrés abiótico que se pueden generar. Como alternativa para contrarrestar estos efectos, se ha experimentado con modificaciones genéticas, particularmente en genes relacionados con la producción de osmolitos y factores de transcripción que han llevado a que estas plantas, a nivel experimental, tengan mayor tolerancia a estrés oxidativo, altas y bajas temperaturas y fotoinhibición, sequía y salinidad, mediante la acumulación de osmoprotectores, la regulación en la expresión de genes y cambios en el fenotipo. En este trabajo se presentan y describen las estrategias metodológicas planteadas con estos fines y se complementan con ejemplos de trabajos realizados en cultivos de origen neotropical de importancia económica, como maíz, algodón, papa y tomate. Además, y debido a la novedad y potencial que ofrece la edición génica por medio del sistema CRISPR/Cas9, también se mencionan trabajos realizados en plantas con origen neotropical, enfocados en comprender e implementar mecanismos de tolerancia a sequía. Las metodologías aquí descritas podrían constituirse en opciones prácticas para mejorar la seguridad alimentaria con miras a contrarrestar las consecuencias negativas del cambio climático antropogénico.

Glycinebetaine mitigates drought stress-induced oxidative damage in pears

Glycinebetaine (GB) is an osmoprotectant found in plants under environmental stresses that incorporates drought and is associated with drought tolerance in several plants, such as the woody pear. However, how GB improves drought tolerance in pears remains unclear. In the current study, we explored the mechanism by which GB enhances drought tolerance of whole pear plants (Pyrus bretschneideri Redh. cv. Suli) supplied with exogenous GB. The results showed that on the sixth day after withholding water, levels of O₂·⁻, H₂O₂, malonaldehyde (MDA) and electrolyte leakage in the leaves were substantially increased by 143%, 38%, 134% and 155%, respectively. Exogenous GB treatment was substantially reduced O₂·⁻, H₂O₂, MDA and electrolyte leakage (38%, 24%, 38% and 36%, respectively) in drought-stressed leaves. Furthermore, exogenous GB induced considerably higher antioxidant enzyme activity in dry-stressed leaves than drought-stressed treatment alone on the sixth day after withholding water, such as superoxide dismutase (SOD) (201%) and peroxidase (POD) (127%). In addition, these GB-induced phenomena led to increased endogenous GB levels in the leaves of the GB 100 + drought and GB 500 + drought treatment groups by 30% and 78%, respectively, compared to drought treatment alone. The findings obtained were confirmed by the results of the disconnected leaf tests, in which GB contributed to a substantial increase in SOD activity and parallel dose- and time-based decreases in MDA levels. These results demonstrate that GB-conferred drought resistance in pears may be due in part to minimizing symptoms of oxidative harm incurred in response to drought by the activities of antioxidants and by reducing the build-up of ROS and lipid peroxidation.

The mechanisms of silicon on maintaining water balance under water deficit stress

Water deficit stress severely threatens crop yield and numerous reports have shown silicon could enhance plants resistance to water deficit. One of the most important mechanisms is that silicon maintains the water balance. In this review, we summarized advanced research to elucidate the effect of silicon on plant water transport processes, including leaf water loss, vessel water transport, and root water uptake. In leaves, the deposition of silica phytolith on cuticle and stomata decreases transpirational water loss under water deficit stress. However, accumulating evidence suggest that silicon maintaining leaf water content is not through reducing water loss, but through osmotic adjustments, enhancing water transport and uptake. Enhancement of stem water transport efficiency by silicon is due to silica phytolith depositing in the cell wall of vessel tubes and pits, which support it avoiding to collapse and embolism, respectively. The improvement of root water uptake capacity by silicon acts as a key role in maintaining water balance. The underlying mechanisms include (i) enlargement of the root water uptake area, (ii) improvement of the water driving force, (iii) the prevention of water loss from root to soil, and (iv) the up-regulation of aquaporin activity. This review provides three simple models to understand the mechanism of silicon on water balance and highlights the future research area.

Genome-Wide Analysis of NF-Y Genes in Potato and Functional Identification of StNF-YC9 in Drought Tolerance

The nuclear factor Y (NF-Y) family is comprised of transcription factors that have been implicated in multiple plant biological processes. However, little is known about this family in potato. In the present study, a total of 41 StNF-Y genes were identified in the potato genome. In addition, the phylogenetic, gene structure, motif, and chromosomal location of this family were analyzed. The tissue expression profiles based on RNA-seq data showed that 27 StNF-Y genes had tissue-specific expression, while the remaining 14 had low expression in all tissues. Publicly available transcriptomics data from various abiotic stresses revealed several stress-responsive StNF-Y genes, which were further verified via quantitative real-time polymerase chain reaction experiments. Furthermore, the StNF-YC9 gene was highly induced by dehydration and drought treatments. StNF-YC9 protein was mainly localized in the nucleus and cytoplasmic membrane. Overexpressing StNF-YC9 potato lines (OxStNF-YC9) had significantly increased in root length and exhibited stronger stomatal closure in potato treated by polyethylene-glycol and abscisic acid. In addition, OxStNF-YC9 lines had higher photosynthetic rates and decreased water loss under short-term drought stress compared to wild-type plants. During long-term drought stress, OxStNF-YC9 lines had higher proline levels, lower malondialdehyde content, and increased activity of several antioxidant enzymes, including superoxide dismutase, catalase, and peroxidase. This study increased our understanding of the StNF-Y gene and suggested that StNF-YC9 played an important role in drought tolerance by increased the photosynthesis rate, antioxidant enzyme activity, and proline accumulation coupled to lowered malondialdehyde accumulation in potato.

Metabolite profiles of energy cane and sugarcane reveal different strategies during the axillary bud outgrowth

Commercial cultivation of sugarcane is usually carried out by planting culm segments (sett) carrying buds in their internodes. However, this is an inefficient practice due to high sprouting irregularity. In this work, we inspect the first stages of the physiological preparation of the culm for sprouting, trying to identify compounds that actively participate in this process. We compared, during the first 48 h, the metabolic profile of sugarcane against energy cane, a cultivar known to have higher sprouting speed and consistency. In fact, during this short period it was possible to observe that energy cane already had a higher physiological activity than sugarcane, with significant changes in the catabolism of amino acids, increased levels of reducing sugars, lipids and metabolic activity in the phenylpropanoid pathway. On the other hand, sugarcane samples had just begun their activity during this same period, with an increase in the level of glutamate as the most significant change, which may be linked to the strategy of these cultivars to develop their roots before leaves, opposite of what is seen for energy cane. These results contribute to the development of strategies for increasing the efficiency of sprouting in sugarcane.

Morpho-Physiological Testing of NaCl Sensitivity of Tobacco Plants Overexpressing Choline Oxidase Gene

In this study the transgenic lines (TLs) of tobacco (Nicotianatabacum L.), which overexpress the heterologous gene encoding the bacterial enzyme choline oxidase were evaluated. The goal of our work is to study the effect of choline oxidase gene expression on the sensitivity of plant tissues to the action of NaCl. The regenerative capacity, rhizogenesis, the amount of photosynthetic pigments and osmotically active compounds (proline and glycine betaine) were assessed by in vitro cell culture methods using biochemical and morphological parameters. Transgenic lines with confirmed expression were characterized by high regeneration capacity from callus in the presence of 200 mmol NaCl, partial retention of viability at 400 mmol NaCl. These data correlated with the implicit response of regenerants and whole plants to the harmful effects of salinity. They turned out to be less sensitive to the presence of 200 mmol NaCl in the cultivation medium, in contrast to the WT plants.

Delineation of mechanistic approaches employed by plant growth promoting microorganisms for improving drought stress tolerance in plants

Drought stress is expected to increase in intensity, frequency, and duration in many parts of the world, with potential negative impacts on plant growth and productivity. The plants have evolved complex physiological and biochemical mechanisms to respond and adjust to water-deficient environments. The physiological and biochemical mechanisms associated with water-stress tolerance and water-use efficiency have been extensively studied. Besides these adaptive and mitigating strategies, the plant growth-promoting rhizobacteria (PGPR) play a significant role in alleviating plant drought stress. These beneficial microorganisms colonize the endo-rhizosphere/rhizosphere of plants and enhance drought tolerance. The common mechanism by which these microorganisms improve drought tolerance included the production of volatile compounds, phytohormones, siderophores, exopolysaccharides, 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase), accumulation of antioxidant, stress-induced metabolites such as osmotic solutes proline, alternation in leaf and root morphology and regulation of the stress-responsive genes. The PGPR is an easy and efficient alternative approach to genetic manipulation and crop enhancement practices because plant breeding and genetic modification are time-consuming and expensive processes for obtaining stress-tolerant varieties. In this review, we will elaborate on PGPR's mechanistic approaches in enhancing the plant stress tolerance to cope with the drought stress.

Betaine Aldehyde Dehydrogenase (BADH) vs. Flavodoxin (Fld): Two Important Genes for Enhancing Plants Stress Tolerance and Productivity

Abiotic stresses, mainly salinity and drought, are the most important environmental threats that constrain worldwide food security by hampering plant growth and productivity. Plants cope with the adverse effects of these stresses by implementing a series of morpho-physio-biochemical adaptation mechanisms. Accumulating effective osmo-protectants, such as proline and glycine betaine (GB), is one of the important plant stress tolerance strategies. These osmolytes can trigger plant stress tolerance mechanisms, which include stress signal transduction, activating resistance genes, increasing levels of enzymatic and non-enzymatic antioxidants, protecting cell osmotic pressure, enhancing cell membrane integrity, as well as protecting their photosynthetic apparatus, especially the photosystem II (PSII) complex. Genetic engineering, as one of the most important plant biotechnology methods, helps to expedite the development of stress-tolerant plants by introducing the key tolerance genes involved in the biosynthetic pathways of osmolytes into plants. Betaine aldehyde dehydrogenase (BADH) is one of the important genes involved in the biosynthetic pathway of GB, and its introduction has led to an increased tolerance to a variety of abiotic stresses in different plant species. Replacing down-regulated ferredoxin at the acceptor side of photosystem I (PSI) with its isofunctional counterpart electron carrier (flavodoxin) is another applicable strategy to strengthen the photosynthetic apparatus of plants under stressful conditions. Heterologous expression of microbially-sourced flavodoxin (Fld) in higher plants compensates for the deficiency of ferredoxin expression and enhances their stress tolerance. BADH and Fld are multifunctional transgenes that increase the stress tolerance of different plant species and maintain their production under stressful situations by protecting and enhancing their photosynthetic apparatus. In addition to increasing stress tolerance, both BADH and Fld genes can improve the productivity, symbiotic performance, and longevity of plants. Because of the multigenic and complex nature of abiotic stresses, the concomitant delivery of BADH and Fld transgenes can lead to more satisfying results in desired plants, as these two genes enhance plant stress tolerance through different mechanisms, and their cumulative effect can be much more beneficial than their individual ones. The importance of BADH and Fld genes in enhancing plant productivity under stress conditions has been discussed in detail in the present review.

Advancing organelle genome transformation and editing for crop improvement

Plant cells contain three organelles that harbor DNA: the nucleus, plastids, and mitochondria. Plastid transformation has emerged as an attractive platform for the generation of transgenic plants, also referred to as transplastomic plants. Plastid genomes have been genetically engineered to improve crop yield, nutritional quality, and resistance to abiotic and biotic stresses, as well as for recombinant protein production. Despite many promising proof-of-concept applications, transplastomic plants have not been commercialized to date. Sequence-specific nuclease technologies are widely used to precisely modify nuclear genomes, but these tools have not been applied to edit organelle genomes because the efficient homologous recombination system in plastids facilitates plastid genome editing. Unlike plastid transformation, successful genetic transformation of higher plant mitochondrial genome transformation was tested in several research group, but not successful to date. However, stepwise progress has been made in modifying mitochondrial genes and their transcripts, thus enabling the study of their functions. Here, we provide an overview of advances in organelle transformation and genome editing for crop improvement, and we discuss the bottlenecks and future development of these technologies.

Genome-wide identification and characterization of WOX genes in Cucumis sativus

WUSCHEL-related homeobox (WOX) proteins are plant-specific transcription factors that are profoundly involved in regulation of plant development and stress responses. In this study, we totally identified 11 WOX transcription factor family members in cucumber (Cucumis sativus, CsWOX) genome and classified them into three clades with nine subclades based on phylogenetic analysis results. Alignment of amino acid sequences revealed that all WOX members in cucumber contained the typical homeodomain, which consists of 60-66 amino acids and is folded into a helix-turn-helix structure. Gene duplication event analysis indicated that CsWOX1a and CsWOX1b were a segment duplication pair, which might affect the number of WOX members in cucumber genome. The expression profiles of CsWOX genes in different tissues demonstrated that the members sorted into the ancient clade (CsWOX13a and CsWOX13b) were constitutively expressed at higher levels in comparison to the others. Cis-element analysis in promoter regions suggested that the expression of CsWOX genes was associated with phytohormone pathways and stress responses, which was further supported by RNA-seq data. Taken together, our results provide new insights into the evolution of cucumber WOX genes and improve our understanding about the biological functions of the CsWOX gene family.

Glycinebetaine-Induced Alteration in Gaseous Exchange Capacity and Osmoprotective Phenomena in Safflower (Carthamus tinctorius L.) under Water Deficit Conditions

Several inorganic and organic compounds including glycine betaine (GB) are presently being used as an exogenous application to enhance tolerance in plants to different environmental stresses. The current study assessed to what extent exogenously applied GB could improve the gaseous exchange capacity and primary and secondary metabolites in two accessions (16178 and 16180) of safflower (Carthamus tinctorius L.) plants under drought stress. Three-week-old plants of both safflower accessions were subjected to well-watered (control) or water-deficit conditions (60% field capacity (FC)). Three levels of GB (control, 50 mM and 100 mM) were sprayed to the foliage of the control and stressed plants after one month of drought application. After two weeks of foliar application of GB, gas exchange characteristics and other biochemical parameters were determined. The results showed that water deficiency markedly suppressed plant biomass, chlorophyll contents, photosynthesis rate (A), water use efficiency (A/E), stomatal conductance (gs) and relative water contents (RWC) of both accessions of safflower, while it enhanced the levels of osmolytes (GB and proline), hydrogen peroxide (H2O2) and total phenolics. Foliar application of GB was effective in enhancing the plant biomass, chlorophyll contents, gs, sub-stomatal CO2 concentration (Ci), Ci/Ca ratio, osmolytes, H2O2, ascorbic acid (AsA), total phenolics and RWC in safflower plants under water shortage. Thus, exogenous application of GB could be used as an effective strategy to improve plant growth, photosynthetic attributes and secondary metabolites in safflower plants under water deficit conditions.

Comparative effects of glycinebetaine on the thermotolerance in codA- and BADH-transgenic tomato plants under high temperature stress

We propose that codA tomato plants exhibited higher degrees of enhanced thermotolerance than BADH tomato plants, and H₂O₂ as a signaling molecule also plays an important role in heat resistance. Betaine aldehyde dehydrogenase (BADH) and choline oxidase (COD) are key enzymes in glycinebetaine (GB) synthesis. In this study, two kinds of transgenic tomato plants, which were transformed with BADH gene and codA gene, respectively, were used to explore their thermotolerance. Our results showed that the levels of GB in leaves of the fourteen independent transgenic lines ranged from 1.9 μmol g^-1 fresh weight to 3.4 μmol g^-1 fresh weight, while GB was almost undetectable in leaves of WT plants. CO₂ assimilation and photosystem II (PSII) photochemical activity in transgenic plants were more thermotolerant than WT plants, especially the codA-transgenic plants showed the most. Significant accumulation of hydrogen peroxide (H₂O₂), superoxide anion radical (O₂·^-), and malondialdehyde (MDA) were more in WT plants than transgenic plants, while this accumulation in codA-transgenic plant was the least. Furthermore, the expression of the heat response genes and the accumulation of heat shock protein 70 (HSP70) were found to be more in transgenic plants than that in WT plants during heat stress, as well as showing the most expression and accumulation of HSP70 in the codA-transgenic plants. Taken together, our results suggest that the enhanced thermotolerance in transgenic plants is due to the positive role of GB in response to heat stress. And interestingly, in addition to the major role of GB in codA-transgenic plants, H₂O₂ as a signaling molecule may also play an important role in heat resistance, leading to higher thermotolerance compared to BADH-transgenic plants.

The roles of chloroplast membrane lipids in abiotic stress responses

Plant chloroplasts have complex membrane systems. Among these, thylakoids serve as the sites for photosynthesis and photosynthesis-related adaptation. In addition to the photosynthetic membrane complexes and associated molecules, lipids in the thylakoid membranes, are predominantly composed of MGDG (monogalactosyldiacylglycerol), DGDG (digalactosyldiacylglycerol), SQDG (sulfoquinovosyldiacylglycerol) and PG (phosphatidylglycerol), play essential roles in shaping the thylakoid architecture, electron transfer, and photoregulation. In this review, we discuss the effect of abiotic stress on chloroplast structure, the changes in membrane lipid composition, and the degree of unsaturation of fatty acids. Advanced understanding of the mechanisms regulating chloroplast membrane lipids and unsaturated fatty acids in response to abiotic stresses is indispensable for improving plant resistance and may inform the strategies of crop breeding.

Endogenous accumulation of glycine betaine confers improved low temperature resistance on transplastomic potato plants

Glycine betaine (GB) plays a crucial role in plant response to abiotic stress, and its accumulation in chloroplasts is more effective than in the cytosol in improving the resistance of transgenic plants. Here, we report that the codA gene from Arthrobacter globiformis, which encodes a choline oxidase catalysing the conversion of choline to GB, was successfully introduced into the plastid genome of potato (Solanum tuberosum L.). Transgenic plants with plastid expression of codA showed increased tolerance to low temperature stress compared with the wild type (WT). Further studies revealed that under low temperature stress condition, transgenic plants presented a significantly higher photosynthetic performance by regulating the electron transport and energy distribution in PSII, and higher antioxidant enzyme activities and lower O2- and H2O2 accumulation than did the WT plants. A higher expression of the COR genes was also observed in transgenic plants. Our results suggest that chloroplast biosynthesis of GB could be an effective strategy for the engineering of plants with increased resistance to low temperature stress.

Pentatricopeptide repeat protein PHOTOSYSTEM I BIOGENESIS FACTOR2 is required for splicing of ycf3

To gain a better understanding of the molecular mechanisms of photosystem I (PSI) biogenesis, we characterized the Arabidopsis thaliana photosystem I biogenesis factor 2 (pbf2) mutant, which lacks PSI complex. PBF2 encodes a P-class pentatricopeptide repeat (PPR) protein. In the pbf2 mutants, we observed a striking decrease in the transcript level of only one gene, the chloroplast gene ycf3, which is essential for PSI assembly. Further analysis of ycf3 transcripts showed that PBF2 is specifically required for the splicing of ycf3 intron 1. Computational prediction of binding sequences and electrophoretic mobility shift assays reveal that PBF2 specifically binds to a sequence in ycf3 intron 1. Moreover, we found that PBF2 interacted with two general factors for group II intron splicing CHLOROPLAST RNA SPLICING2-ASSOCIATED FACTOR1 (CAF1) and CAF2, and facilitated the association of these two factors with ycf3 intron 1. Our results suggest that PBF2 is specifically required for the splicing of ycf3 intron 1 through cooperating with CAF1 and CAF2. Our results also suggest that additional proteins are required to contribute to the specificity of CAF-dependent group II intron splicing.

Choline oxidases

Choline oxidase catalyzes the four-electron, two-step, flavin-mediated oxidation of choline to glycine betaine. The enzyme is important both for medical and biotechnological reasons, because glycine betaine is one among a limited number of compatible solutes used by cells to counteract osmotic pressure. From a fundamental standpoint, choline oxidase has emerged as one of the paradigm enzymes for the oxidation of alcohols catalyzed by flavoproteins. Mechanistic, structural, and computational studies have elucidated the mechanism of action of the enzyme from Arthrobacter globiformis at the molecular level. Both choline and oxygen access to the active site cavity are gated and tightly controlled. Amino acid residues involved in substrate binding, and their contribution, have been identified. The mechanism of choline oxidation, with a hydride transfer reaction, an asynchronous transition state, the formation and stabilization of an alkoxide transient species, and a quantum mechanical mode of reaction, has been elucidated. The importance of nonpolar side chains for oxygen localization and of the positive charge harbored on the substrate for activation of oxygen for reaction with the reduced flavin have been recognized. Interesting phenomena, like the formation of a metastable photoinduced flavin-protein adduct, the reversible formation of a bicovalent flavoprotein, and the trapping of the enzyme in inactive conformations, have been described. This review summarizes the current status of our understanding on the structure-function-dynamics of choline oxidase.

Length-dependent accumulation of double-stranded RNAs in plastids affects RNA interference efficiency in the Colorado potato beetle

Transplastomic potato plants expressing double-stranded RNA (dsRNA) targeted against essential genes of the Colorado potato beetle (CPB) can be lethal to larvae by triggering an RNA interference (RNAi) response. High accumulation levels of dsRNAs in plastids are crucial to confer an efficient RNAi response in the insects. However, whether length and sequence of the dsRNA determine the efficacy of RNAi and/or influence the level of dsRNA accumulation in plastids is not known. We compared the RNAi efficacy of different lengths of dsRNA targeted against the CPB β-Actin gene (ACT) by feeding in vitro-synthesized dsRNAs to larvae. We showed that, while the 60 bp dsRNA induced only a relatively low RNAi response in CPB, dsRNAs of 200 bp and longer caused high mortality and similar larval growth retardation. When the dsRNAs were expressed from the plastid (chloroplast) genome of potato plants, we found that their accumulation were negatively correlated with length. The level of dsRNA accumulation was positively associated with the observed mortality, suppression of larval growth, and suppression of target gene expression. Importantly, transplastomic potato plants expressing the 200 bp dsRNA were better protected from CPB than plants expressing the 297 bp dsRNA, the best-performing line in our previous study. Our results suggest that the length of dsRNAs is an important factor that influences their accumulation in plastids and thus determines the strength of the insecticidal RNAi effect. Our findings will aid the design of optimized dsRNA expression constructs for plant protection by plastid-mediated RNAi.

The sweet sorghum SbWRKY50 is negatively involved in salt response by regulating ion homeostasis

The WRKY transcription factor family is involved in responding to biotic and abiotic stresses. Its members contain a typical WRKY domain and can regulate plant physiological responses by binding to W-boxes in the promoter regions of downstream target genes. We identified the sweet sorghum SbWRKY50 (Sb09g005700) gene, which encodes a typical class II of the WRKY family protein that localizes to the nucleus and has transcriptional activation activity. The expression of SbWRKY50 in sweet sorghum was reduced by salt stress, and its ectopic expression reduced the salt tolerance of Arabidopsis thaliana plants. Compared with the wild type, the germination rate, root length, biomass and potassium ion content of SbWRKY50 over-expression plants decreased significantly under salt-stress conditions, while the hydrogen peroxide, superoxide anion and sodium ion contents increased. Real-time PCR results showed that the expression levels of AtSOS1, AtHKT1 and genes related to osmotic and oxidative stresses in over-expression strains decreased under salt-stress conditions. Luciferase complementation imaging and yeast one-hybrid assays confirmed that SbWRKY50 could directly bind to the upstream promoter of the SOS1 gene in A. thaliana. However, in sweet sorghum, SbWRKY50 could directly bind to the upstream promoters of SOS1 and HKT1. These results suggest that the new WRKY transcription factor SbWRKY50 participates in plant salt response by controlling ion homeostasis. However, the regulatory mechanisms are different in sweet sorghum and Arabidopsis, which may explain their different salt tolerance levels. The data provide information that can be applied to genetically modifying salt tolerance in different crop varieties.

Plastid-expressed Bacillus thuringiensis (Bt) cry3Bb confers high mortality to a leaf eating beetle in poplar

The Bacillus thuringiensis (Bt) cry3Bb gene was successfully introduced into poplar plastid genome, leading to transplastomic poplar with high mortality to Plagiodera versicolora. Poplar (Populus L.) is one of the main resource of woody industry, but being damaged by insect pests. The feasibility and efficiency of plastid transformation technology for controlling two lepidopteran caterpillars have been demonstrated previously. Here, we introduced B. thuringiensis (Bt) cry3Bb into poplar plastid genome by biolistic bombardment for controlling P. versicolora, a widely distributed forest pest. Chimeric cry3Bb gene is controlled by the tobacco plastid rRNA operon promoter combined with the 5'UTR from gene10 of bacteriophage T7 (NtPrrn:T7g10) and the 3'UTR from the E. coli ribosomal RNA operon rrnB (TrrnB). The integration of transgene and homoplasmy of transplastomic poplar plants was confirmed by Southern blot analysis. Northern blot analysis indicated that cry3Bb was transcribed to both read through and shorter length transcripts in plastid. The transplastomic poplar expressing Cry3Bb insecticidal protein showed the highest accumulation level in young leaves, which reach up to 16.8 μg/g fresh weight, and comparatively low levels in mature and old leaves. Feeding the young leaves from Bt-Cry3Bb plastid lines to P. versicolora caused 100% mortality in the first-instar larvae after only 1 day, in the second-instar larvae after 2 days, and in the third-instar larvae for 3 days. Thus, we report a successful extension of plastid engineering poplar against the chrysomelid beetle.

Osmoprotection in plants under abiotic stresses: new insights into a classical phenomenon

Plant osmoprotectants protect against abiotic stresses. Introgression of osmoprotectant genes into crop plants via genetic engineering is an important strategy in developing more productive plants. Plants employ adaptive mechanisms to survive various abiotic stresses. One mechanism, the osmoprotection system, utilizes various groups of low molecular weight compounds, collectively known as osmoprotectants, to mitigate the negative effect of abiotic stresses. Osmoprotectants may include amino acids, polyamines, quaternary ammonium compounds and sugars. These nontoxic compounds stabilize cellular structures and enzymes, act as metabolic signals, and scavenge reactive oxygen species produced under stressful conditions. The advent of recent drastic fluctuations in the global climate necessitates the development of plants better adapted to abiotic stresses. The introgression of genes related to osmoprotectant biosynthesis from one plant to another by genetic engineering is a unique strategy bypassing laborious conventional and classical breeding programs. Herein, we review recent literature related to osmoprotectants and transgenic plants engineered with specific osmoprotectant properties.

Biostimulation of Maize (Zea mays) and Irrigation Management Improved Crop Growth and Water Use under Controlled Environment

Water deficits during the growing season are a major factor limiting crop production. Therefore, reducing water use during crop production by the application of regulated deficit irrigation (RDI) is crucially important in water resources. There are few reports on the biostimulants used for growth and water use efficiency (WUE) in maize (Zea mays Linn.) under RDI. Therefore, the influence of betaine and chitin treatments, alone and in combination, on maize cultivar ‘White Pearl’ was assessed by observing changes in the physiology and morphology of plants exposed to RDI. Plants were grown in plastic pots in greenhouses and maintained under full irrigation (FI) for 1 week until imposing RDI and biostimulants. Plants were then subjected to FI (no water deficiency treatment, field capacity >70%) and RDI (field capacity <50%) conditions until the end of each experiment. Plant agronomic performance, photosynthesis parameters, and WUE values were recorded weekly for 8 weeks and three individual experiments were carried out to assess the efficacy of biostimulants and irrigation treatments. Betaine (0, 50, and 100 mM/plant) was foliage-treated every 2 weeks during Experiment 1, but chitin (0, 2, and 4 g/kg) was applied to the soil at the beginning of Experiment 2. The optimal concentration of each chemical alone or in combination was then applied to the plants as Experiment 3. A factorial experiment design of two factors with different levels under a completely randomized arrangement was used in this investigation. Betaine (50 mM) or chitin (2 g/kg) treatments alone significantly elevated total fresh weight (63.03 or 124.07 g/plant), dry weight (18.00 or 22.34 g/plant), and cob weight (3.15 or 6.04 g/plant) and boosted the water-stress tolerance of the maize under RDI compared to controls. However, a combination treatment of 50 mM betaine and 2 g/kg chitin did not increase plant height, fresh shoot and root weights, dry cob weight, and total dry weight under RDI compared to controls. Soil-plant analysis development (SPAD) values (>30) were effective in detecting plant growth performance and WUE values under RDI. These findings may have greater significance for farming in dry lands and offer information for further physiological studies on maize WUE and water stress tolerance