Genome-Wide Analysis of MDHAR Gene Family in Four Cotton Species Provides Insights into Fiber Development via Regulating AsA Redox Homeostasis

Overexpression of sweetpotato glutamylcysteine synthetase (IbGCS) in Arabidopsis confers tolerance to drought and salt stresses

Various environmental stresses induce the production of reactive oxygen species (ROS), which have deleterious effects on plant cells. Glutathione (GSH) is an antioxidant used to counteract reactive oxygen species. Glutathione is produced by glutamylcysteine synthetase (GCS) and glutathione synthetase (GS). However, evidence for the GCS gene in sweetpotato remains scarce. In this study, the full-length cDNA sequence of IbGCS isolated from sweetpotato cultivar Xu18 was 1566 bp in length, which encodes 521 amino acids. The qRT-PCR analysis revealed a significantly higher expression of the IbGCS in sweetpotato flowers, and the gene was induced by salinity, abscisic acid (ABA), drought, extreme temperature and heavy metal stresses. The seed germination rate, root elongation and fresh weight were promoted in T3 Arabidopsis IbGCS-overexpressing lines (OEs) in contrast to wild type (WT) plants under mannitol and salt stresses. In addition, the soil drought and salt stress experiment results indicated that IbGCS overexpression in Arabidopsis reduced the malondialdehyde (MDA) content, enhanced the levels of GCS activity, GSH and AsA content, and antioxidant enzyme activity. In summary, overexpressing IbGCS in Arabidopsis showed improved salt and drought tolerance.

The ascorbate-glutathione cycle coming of age

Concepts regarding the operation of the ascorbate-glutathione cycle and the associated water/water cycle in the processing of metabolically generated hydrogen peroxide and other forms of reactive oxygen species (ROS) are well established in the literature. However, our knowledge of the functions of these cycles and their component enzymes continues to grow and evolve. Recent insights include participation in the intrinsic environmental and developmental signalling pathways that regulate plant growth, development, and defence. In addition to ROS processing, the enzymes of the two cycles not only support the functions of ascorbate and glutathione, they also have 'moonlighting' functions. They are subject to post-translational modifications and have an extensive interactome, particularly with other signalling proteins. In this assessment of current knowledge, we highlight the central position of the ascorbate-glutathione cycle in the network of cellular redox systems that underpin the energy-sensitive communication within the different cellular compartments and integrate plant signalling pathways.

Genome-wide identification and analysis of ascorbate peroxidase (APX) gene family in hemp (Cannabis sativa L.) under various abiotic stresses

Ascorbate peroxidase (APX) plays a critical role in molecular mechanisms such as plant development and defense against abiotic stresses. As an important economic crop, hemp (Cannabis sativa L.) is vulnerable to adverse environmental conditions, such as drought, cold, salt, and oxidative stress, which lead to a decline in yield and quality. Although APX genes have been characterized in a variety of plants, members of the APX gene family in hemp have not been completely identified. In this study, we (1) identified eight members of the CsAPX gene family in hemp and mapped their locations on the chromosomes using bioinformatics analysis; (2) examined the physicochemical characteristics of the proteins encoded by these CsAPX gene family members; (3) investigated their intraspecific collinearity, gene structure, conserved domains, conserved motifs, and cis-acting elements; (4) constructed a phylogenetic tree and analyzed interspecific collinearity; and (5) ascertained expression differences in leaf tissue subjected to cold, drought, salt, and oxidative stresses using quantitative real-time-PCR (qRT-PCR). Under all four stresses, CsAPX6, CsAPX7, and CsAPX8 consistently exhibited significant upregulation, whereas CsAPX2 displayed notably higher expression levels under drought stress than under the other stresses. Taken together, the results of this study provide basic genomic information on the expression of the APX gene family and pave the way for studying the role of APX genes in abiotic stress.

Modulation in gene expression and enzyme activity suggested the roles of monodehydroascorbate reductase in development and stress response in bread wheat

Monodehydroascorbate reductase (MDHAR) is a crucial enzymatic antioxidant of the ascorbate-glutathione pathway involved in reactive oxygen species scavenging. Herein, we identified 15 TaMDHAR genes in bread wheat. Phylogenetic analysis revealed their clustering into three groups, which are also related to the subcellular localization in the peroxisome matrix, peroxisome membrane, and chloroplast. Each TaMDHAR protein consisted of two conserved domains; Pyr_redox and Pyr_redox_2 of the pyridine nucleotide disulfide oxidoreductase family. The occurrence of diverse groups of cis-regulatory elements in the promoter region and their interaction with numerous transcription factors suggest assorted functions of TaMDHARs in growth and development and in light, phytohormones, and stress responses. Expression analysis in various tissues further revealed their importance in vegetative and reproductive development. In addition, the differential gene expression and enhanced enzyme activity during drought, heat, and salt treatments exposed their role in abiotic stress response. Interaction of MDHARs with various antioxidant enzymes and biochemicals related to the ascorbate-glutathione cycle exposed their synchronized functioning. Interaction with auxin indicated the probability of cross-talk between antioxidants and auxin signaling. The miR168a, miR169, miR172 and others interaction with various TaMDHARs further directed their association with developmental processes and stress responses. The current study provides extensive information about the importance of TaMDHARs, moreover, the precise role of each gene needs to be established in future studies.

Genome-Wide Identification and Comparative Transcriptome Methods Reveal FaMDHAR50 Regulating Ascorbic Acid Regeneration and Quality Formation of Strawberry Fruits

Ascorbic acid (AsA) is a crucial water-soluble antioxidant in strawberry fruit, but limited research is currently available on the identification and functional validation of key genes involved in AsA metabolism in strawberries. This study analyzed the FaMDHAR gene family identification, which includes 168 genes. Most of the products of these genes are predicted to exist in the chloroplast and cytoplasm. The promoter region is rich in cis-acting elements related to plant growth and development, stress and light response. Meanwhile, the key gene FaMDHAR50 that positively regulates AsA regeneration was identified through comparative transcriptome analysis of 'Benihoppe' strawberry (WT) and its natural mutant (MT) with high AsA content (83 mg/100 g FW). The transient overexpression experiment further showed that overexpression of FaMDHAR50 significantly enhanced the AsA content by 38% in strawberry fruit, with the upregulated expression of structural genes involved in AsA biosynthesis (FaGalUR and FaGalLDH) and recycling and degradation (FaAPX, FaAO and FaDHAR) compared with that of the control. Moreover, increased sugar (sucrose, glucose and fructose) contents and decreased firmness and citric acid contents were observed in the overexpressed fruit, which were accompanied by the upregulation of FaSNS, FaSPS, FaCEL1 and FaACL, as well as the downregulation of FaCS. Additionally, the content of pelargonidin 3-glucoside markedly decreased, while cyanidin chloride increased significantly. In summary, FaMDHAR50 is a key positive regulatory gene involved in AsA regeneration in strawberry fruit, which also plays an important role in the formation of fruit flavor, apperance and texture during strawberry fruit ripening.

Gene expression profiling in Rosa roxburghii fruit and overexpressing RrGGP2 in tobacco and tomato indicates the key control point of AsA biosynthesis

Rosa roxburghii Tratt. is an important commercial horticultural crop endemic to China, which is recognized for its extremely high content of L-ascorbic acid (AsA). To understand the mechanisms underlying AsA overproduction in fruit of R. roxburghii, content levels, accumulation rate, and the expression of genes putatively in the biosynthesis of AsA during fruit development have been characterized. The content of AsA increased with fruit weight during development, and AsA accumulation rate was found to be highest between 60 and 90 days after anthesis (DAA), with approximately 60% of the total amount being accumulated during this period. In vitro incubating analysis of 70DAA fruit flesh tissues confirmed that AsA was synthesized mainly via the L-galactose pathway although L-Gulono-1, 4-lactone was also an effective precursor elevating AsA biosynthesis. Furthermore, in transcript level, AsA content was significantly associated with GDP-L-galactose phosphorylase (RrGGP2) gene expression. Virus-induced RrGGP2 silencing reduced the AsA content in R. roxburghii fruit by 28.9%. Overexpressing RrGGP2 increased AsA content by 8-12-fold in tobacco leaves and 2.33-3.11-fold in tomato fruit, respectively, and it showed enhanced resistance to oxidative stress caused by paraquat in transformed tobacco. These results further justified the importance of RrGGP2 as a major control step to AsA biosynthesis in R. roxburghii fruit.

Multicopper oxidases GbAO and GbSKS are involved in the Verticillium dahliae resistance in Gossypium barbadense

Ascorbate oxidase (AO) and skewed5 (SKU5)-similar (SKS) proteins belong to the multicopper oxidase (MCO) family and play important roles in plants in response to environmental stress via modulation of oxidoreduction homeostasis. Currently, reports on the response of Gossypium barbadense MCO to Verticillium wilt (VW) caused by Verticillium dahliae are still limited. Herein, RNA sequencing of two G. barbadense cultivars of VW-resistant XH21 and VW-susceptible XH7 under V. dahliae treatment, combined with physiological and genetic analysis, was performed to analyze the function and mechanism of multicopper oxidases GbAO and GbSKS involved in V. dahliae resistance. The identified differentially expressed genes are mainly involved in the regulation of oxidoreduction reaction, and extracellular components and signaling. Interestingly, ascorbate oxidase family members were discovered as the most significantly upregulated genes after V. dahliae treatment, including GbAO3A/D, GbSKS3A/D, and GbSKS16A/D. H₂O₂ and Asc contents, especially reductive Asc in both XH21 and XH7, were shown to be increased. Silenced expression of respective GbAO3A/D, GbSKS3A/D, and GbSKS16A/D in virus-induced gene silencing (VIGS) cotton plants significantly decreased the resistance to V. dahliae, coupled with the reduced contents of pectin and lignin. Our results indicate that AO might be involved in cotton VW resistance via the regulation of cell wall components.

Morphological characteristics, anatomical structure, and dynamic change of ascorbic acid under different storage conditions of celery

Ascorbic acid (AsA) is a crucial antioxidant in vegetables. Celery (Apium graveolens L.) is a vegetable of Apiaceae and is rich in AsA. Till now, the effects of different storage conditions on celery morphological characteristics, anatomical features, and antioxidant accumulation are unclear. Here, the celery cvs. 'Sijixiaoxiangqin' and 'Liuhehuangxinqin' were selected as experimental materials, and the two celery plants grown for 65 days were harvested from soils and stored in light at room temperature (25 °C), darkness at low temperature (4 °C), and darkness at room temperature (25 °C) for 0, 6, 24, 30, 48, and 54 h, respectively. The results showed that celery in darkness had better water retention capacity than celery in light. Morphological changes in celery mesophyll, leaf veins, and petioles were the least in darkness at low temperature (4 °C). The weight loss rate and wilting degree in darkness at low temperature (4 °C) were the lowest, and the AsA content remained at a high level. The expression patterns of GDP-D-mannose pyrophosphorylase (AgGMP) and L-galactose dehydrogenase (AgGalDH) were similar to the change of AsA content. The results indicated that low temperature and dark was the optimized storage condition for 'Sijixiaoxiangqin' and 'Liuhehuangxinqin' celery. AgGMP and AgGalDH genes may play an important role in the accumulation of AsA in celery. This paper will provide potential references for prolonging the shelf life of celery and other horticultural crops.

Physio-biochemical and transcriptomic analysis reveals that the mechanism of Bacillus cereus G2 alleviated oxidative stress of salt-stressed Glycyrrhiza uralensis Fisch. seedlings

Salt stress severely affects the growth and productivity of Glycyrrhiza uralensis. Our previous research found that the endophyte Bacillus cereus G2 alleviated the osmotic and oxidative stress in G. uralensis exposed to salinity. However, the mechanism is still unclear. Here, a pot experiment was conducted to analyse the change in parameters related to osmotic adjustment and antioxidant metabolism by G2 in salt-stressed G. uralensis at the physio-biochemistry and transcriptome levels. The results showed that G2 significantly increased proline content by 48 %, glycine betaine content by 75 % due to activated expression of BADH1, and soluble sugar content by 77 % due to upregulated expression of α-glucosidase and SS, which might help to decrease the cell osmotic potential, enable the cell to absorb water, and stabilize the cell's protein and membrane structure, thereby alleviating osmotic stress. Regarding antioxidant metabolism, G2 significantly decreased malondialdehyde (MDA) content by 27 %, which might be ascribed to the increase in superoxide dismutase (SOD) activity that facilitated the decrease in the superoxide radical (O₂^‾) production rate; it also increased the activities of catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GPX), which helped stabilize the normal level of hydrogen peroxide (H₂O₂). G2 also increased glutathione (GSH) content by 65 % due to increased glutathione reductase (GR) activity and GSH/GSSG ratio, but G2 decreased oxidized glutathione (GSSG) content by 13 % due to decreased activity of dehydroascorbate reductase (DHAR), which could provide sufficient substrates for the ascorbate-glutathione (AsA-GSH) cycle to eliminate excess H₂O₂ that was not cleared in a timely manner by the antioxidant enzyme system. Taken together, G2 alleviated osmotic stress by increasing proline, soluble sugar, and glycine betaine contents and alleviated oxidative stress by the synergistic effect of antioxidant enzymes and the AsA-GSH cycle. Therefore, the results may be useful for explaining the mechanism by which endophyte inoculation regulates the salt tolerance of crops.

Genome-Wide Identification and Analysis of the Ascorbate Peroxidase (APX) Gene Family of Winter Rapeseed (Brassica rapa L.) Under Abiotic Stress

Winter Brassica rapa (B. rapa) is an important oilseed crop in northern China, but the mechanism of its cold resistance remains unclear. Ascorbate peroxidase (APX) plays important roles in the response of this plant to abiotic stress and in scavenging free radicals. In this study, the roles of APX proteins in the cold response and superoxide metabolism pathways in rapeseed species were investigated, and a comprehensive analysis of phylogeny, chromosome distribution, motif identification, sequence structure, gene duplication, and RNA-seq expression profiles in the APX gene family was conducted. Most BrAPX genes were specifically expressed under cold stress and behaved significantly differently in cold-tolerant and weakly cold-resistant varieties. Quantitative real-time-PCR (qRT-PCR) was also used to verify the differences in expression between these two varieties under cold, freezing, drought and heat stress. The expression of five BrAPX genes was significantly upregulated in growth cones at 3 h of cold stress, while their expression was significantly lower at 24 h than at 3 h. The expression of Bra015403 and Bra003918 was significantly higher in “Longyou-7” growth cones than in other treatments. Five BrAPXs (Bra035235, Bra003918, Bra033040, Bra017120, and Bra031934) were closely associated with abiotic stress responses in B. rapa. These candidate genes may play important roles in the response of B. rapa to low temperature stress and provide new information for the elucidation of the cold resistance mechanism in B. rapa.

Regulatory Network of Cotton Genes in Response to Salt, Drought and Wilt Diseases (Verticillium and Fusarium): Progress and Perspective

In environmental conditions, crop plants are extremely affected by multiple abiotic stresses including salinity, drought, heat, and cold, as well as several biotic stresses such as pests and pathogens. However, salinity, drought, and wilt diseases (e.g., Fusarium and Verticillium) are considered the most destructive environmental stresses to cotton plants. These cause severe growth interruption and yield loss of cotton. Since cotton crops are central contributors to total worldwide fiber production, and also important for oilseed crops, it is essential to improve stress tolerant cultivars to secure future sustainable crop production under adverse environments. Plants have evolved complex mechanisms to respond and acclimate to adverse stress conditions at both physiological and molecular levels. Recent progresses in molecular genetics have delivered new insights into the regulatory network system of plant genes, which generally includes defense of cell membranes and proteins, signaling cascades and transcriptional control, and ion uptake and transport and their relevant biochemical pathways and signal factors. In this review, we mainly summarize recent progress concerning several resistance-related genes of cotton plants in response to abiotic (salt and drought) and biotic (Fusarium and Verticillium wilt) stresses and classify them according to their molecular functions to better understand the genetic network. Moreover, this review proposes that studies of stress related genes will advance the security of cotton yield and production under a changing climate and that these genes should be incorporated in the development of cotton tolerant to salt, drought, and fungal wilt diseases (Verticillium and Fusarium).