Transcriptomic dynamics provide an insight into the mechanism for silicon-mediated alleviation of salt stress in cucumber plants

Silicon nanoparticles (SiNPs) stimulated secondary metabolism and mitigated toxicity of salinity stress in basil (Ocimum basilicum) by modulating gene expression: a sustainable approach for crop protection

The underlying mechanisms through which silicon oxide nanoparticles (SiNPs) can confer salinity resistance to plants are poorly understood. This study explored the efficacy of supplementing nutrient solution with SiNPs (20-30 nm; 10 mg kg-1 soil) to stimulate metabolism and alleviate the risks associated with salinity (0.73 g kg-1 soil) in basil seedlings. For this purpose, variations in photosynthetic indices, proline osmoprotectant, antioxidant markers, phenylpropanoid metabolism, and transcriptional behaviors of genes were investigated. SiNPs increased shoot fresh weight (38%) and mitigated the risk associated with the salinity stress by 14%. SiNPs alleviated the inhibitory effects of salinity on the total chlorophyll concentration by 15%. The highest increase (twofold) in proline content was recorded in the SiNP-treated seedlings grown under salinity. The nano-supplement enhanced the activity of enzymatic antioxidants, including peroxidase (2.5-fold) and catalase (4.7-fold). SiNPs induced the expression of gamma-cadinene synthase (CDS) and caffeic acid O-methyltransferase (COMT) genes by 6.5- and 18.3-fold, respectively. SiNPs upregulated the eugenol synthase (EGS1) and fenchol synthase (FES) genes by six- and nine-fold, respectively. Salinity transcriptionally downregulated the geraniol synthase (GES) gene, while this gene displayed an upward trend in response to SiNPs by eight-fold. The nano-supplement transcriptionally stimulated the R-linalool synthase (LIS) gene by 3.3-fold. The terpinolene synthase (TES) gene displayed a similar trend to that of the GES gene. The highest expression (25-fold) of the phenylalanine ammonia-lyase (PAL) gene was recorded in seedlings supplemented with SiNPs. The physiological and molecular assessments demonstrated that employing SiNPs is a sustainable strategy for improving plant primary/secondary metabolism and crop protection.

The Physiological and Molecular Mechanisms of Silicon Action in Salt Stress Amelioration

Salinity is one of the most common abiotic stress factors affecting different biochemical and physiological processes in plants, inhibiting plant growth, and greatly reducing productivity. During the last decade, silicon (Si) supplementation was intensively studied and now is proposed as one of the most convincing methods to improve plant tolerance to salt stress. In this review, we discuss recent papers investigating the role of Si in modulating molecular, biochemical, and physiological processes that are negatively affected by high salinity. Although multiple reports have demonstrated the beneficial effects of Si application in mitigating salt stress, the exact molecular mechanism underlying these effects is not yet well understood. In this review, we focus on the localisation of Si transporters and the mechanism of Si uptake, accumulation, and deposition to understand the role of Si in various relevant physiological processes. Further, we discuss the role of Si supplementation in antioxidant response, maintenance of photosynthesis efficiency, and production of osmoprotectants. Additionally, we highlight crosstalk of Si with other ions, lignin, and phytohormones. Finally, we suggest some directions for future work, which could improve our understanding of the role of Si in plants under salt stress.

Genome-wide characterization and function analysis of ginger (Zingiber officinale Roscoe) ZoGRFs in responding to adverse stresses

Growth-regulating factors (GRFs) play crucial roles in plant growth, development, hormone signaling, and stress response. Despite their significance, the roles of GRFs in ginger remain largely unknown. Herein, 31 ginger ZoGRFs were identified and designated as ZoGRF1-ZoGRF31 according to their phylogenetic relationships. All ZoGRFs were characterized as unstable, hydrophilic proteins, with 29 predicted to be located in the nucleus. Functional cis-elements related to growth and development were enriched in ZoGRF's promoter regions. RNA-seq and RT-qPCR analysis revealed that ZoGRF12, ZoGRF24, and ZoGRF28 were highly induced in various growth and development stages, displaying differential regulation under waterlogging, chilling, drought, and salt stresses, indicating diverse expression patterns of ZoGRFs. Transient expression analysis in Nicotiana benthamiana indicated that overexpressing ZoGRF28 regulated the transcription levels of salicylic acid, jasmonic acid, and pattern-triggered immunity-related genes, increased chlorophyll content and contributed to reduced disease lesions and an increased net photosynthetic rate. This research lays the foundation for further understanding the biological roles of ZoGRFs.

Transcriptome profiling reveals multiple regulatory pathways of Tamarix chinensis in response to salt stress

KEY MESSAGE

Multiple regulatory pathways of T. chinensis to salt stress were identified through transcriptome data analysis. Tamarix chinensis (Tamarix chinensis Lour.) is a typical halophyte capable of completing its life cycle in soils with medium to high salinity. However, the mechanisms underlying its resistance to high salt stress are still largely unclear. In this study, transcriptome profiling analyses in different organs of T. chinensis plants in response to salt stress were carried out. A total number of 2280, 689, and 489 differentially expressed genes (DEGs) were, respectively, identified in roots, stems, and leaves, with more DEGs detected in roots than in stems and leaves. Gene Ontology (GO) term analysis revealed that they were significantly enriched in "biological processes" and "molecular functions". Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that "Beta-alanine metabolism" was the most differentially enriched pathway in roots, stems, and leaves. In pair-to-pair comparison of the most differentially enriched pathways, a total of 14 pathways, including 5 pathways in roots and leaves, 6 pathways in roots and stems, and 3 pathways in leaves and stems, were identified. Furthermore, genes encoding transcription factor, such as bHLH, bZIP, HD-Zip, MYB, NAC, WRKY, and genes associated with oxidative stress, starch and sucrose metabolism, and ion homeostasis, were differentially expressed with distinct organ specificity in roots, stems, and leaves. Our findings in this research provide a novel approach for exploring the salt tolerance mechanism of halophytes and identifying new gene targets for the genetic breeding of new plant cultivars with improved resistance to salt stress.

Genome-Wide Identification and Expression Analysis of the Cucumber FKBP Gene Family in Response to Abiotic and Biotic Stresses

The FKBP (FK506-binding protein) gene family is an important member of the PPlase protease family and plays a vital role during the processes of plant growth and development. However, no studies of the FKBP gene family have been reported in cucumber. In this study, 19 FKBP genes were identified in cucumber, which were located on chromosomes 1, 3, 4, 6, and 7. Phylogenetic analysis divided the cucumber FKBP genes into three subgroups. The FKBP genes in the same subgroup exhibited similar structures and conserved motifs. The cis-acting elements analysis revealed that the promoters of cucumber FKBP genes contained hormone-, stress-, and development-related cis-acting elements. Synteny analysis of the FKBP genes among cucumber, Arabidopsis, and rice showed that 12 kinds of syntenic relationships were detected between cucumber and Arabidopsis FKBP genes, and 3 kinds of syntenic relationships were observed between cucumber and rice FKBP genes. The tissue-specific expression analysis showed that some FKBP genes were expressed in all tissues, while others were only highly expressed in part of the 10 types of tissues. The expression profile analysis of cucumber FKBP genes under 13 types of stresses showed that the CsaV3_1G007080 gene was differentially expressed under abiotic stresses (high temperature, NaCl, silicon, and photoperiod) and biotic stresses (downy mildew, green mottle mosaic virus, Fusarium wilt, phytophthora capsica, angular leaf spot, and root-knot nematode), which indicated that the CsaV3_1G007080 gene plays an important role in the growth and development of cucumber. The interaction protein analysis showed that most of the proteins in the FKBP gene family interacted with each other. The results of this study will lay the foundation for further research on the molecular biological functions of the cucumber FKBP gene family.

Novel insights into the mechanism(s) of silicon-induced drought stress tolerance in lentil plants revealed by RNA sequencing analysis

BACKGROUND

Lentil is an essential cool-season food legume that offers several benefits in human nutrition and cropping systems. Drought stress is the major environmental constraint affecting lentil plants' growth and productivity by altering various morphological, physiological, and biochemical traits. Our previous research provided physiological and biochemical evidence showing the role of silicon (Si) in alleviating drought stress in lentil plants, while the molecular mechanisms are still unidentified. Understanding the molecular mechanisms of Si-mediated drought stress tolerance can provide fundamental information to enhance our knowledge of essential gene functions and pathways modulated by Si during drought stress in plants. Thus, the present study compared the transcriptomic characteristics of two lentil genotypes (drought tolerant-ILL6002; drought sensitive-ILL7537) under drought stress and investigated the gene expression in response to Si supplementation using high-throughput RNA sequencing.

RESULTS

This study identified 7164 and 5576 differentially expressed genes (DEGs) from drought-stressed lentil genotypes (ILL 6002 and ILL 7537, respectively), with Si treatment. RNA sequencing results showed that Si supplementation could alter the expression of genes related to photosynthesis, osmoprotection, antioxidant systems and signal transduction in both genotypes under drought stress. Furthermore, these DEGs from both genotypes were found to be associated with the metabolism of carbohydrates, lipids and proteins. The identified DEGs were also linked to cell wall biosynthesis and vasculature development. Results suggested that Si modulated the dynamics of biosynthesis of alkaloids and flavonoids and their metabolism in drought-stressed lentil genotypes. Drought-recovery-related DEGs identified from both genotypes validated the role of Si as a drought stress alleviator. This study identified different possible defense-related responses mediated by Si in response to drought stress in lentil plants including cellular redox homeostasis by reactive oxygen species (ROS), cell wall reinforcement by the deposition of cellulose, lignin, xyloglucan, chitin and xylan, secondary metabolites production, osmotic adjustment and stomatal closure.

CONCLUSION

Overall, the results suggested that a coordinated interplay between various metabolic pathways is required for Si to induce drought tolerance. This study identified potential genes and different defence mechanisms involved in Si-induced drought stress tolerance in lentil plants. Si supplementation altered various metabolic functions like photosynthesis, antioxidant defence system, osmotic balance, hormonal biosynthesis, signalling, amino acid biosynthesis and metabolism of carbohydrates and lipids under drought stress. These novel findings validated the role of Si in drought stress mitigation and have also provided an opportunity to enhance our understanding at the genomic level of Si's role in alleviating drought stress in plants.

Deciphering the molecular basis of abiotic stress response in cucumber (Cucumis sativus L.) using RNA-Seq meta-analysis, systems biology, and machine learning approaches

Abiotic stress in cucumber (Cucumis sativus L.) may trigger distinct transcriptome responses, resulting in significant yield loss. More insight into the molecular underpinnings of the stress response can be gained by combining RNA-Seq meta-analysis with systems biology and machine learning. This can help pinpoint possible targets for engineering abiotic tolerance by revealing functional modules and key genes essential for the stress response. Therefore, to investigate the regulatory mechanism and key genes, a combination of these approaches was utilized in cucumber subjected to various abiotic stresses. Three significant abiotic stress-related modules were identified by gene co-expression network analysis (WGCNA). Three hub genes (RPL18, δ-COP, and EXLA2), ten transcription factors (TFs), one transcription regulator, and 12 protein kinases (PKs) were introduced as key genes. The results suggest that the identified PKs probably govern the coordination of cellular responses to abiotic stress in cucumber. Moreover, the C2H2 TF family may play a significant role in cucumber response to abiotic stress. Several C2H2 TF target stress-related genes were identified through co-expression and promoter analyses. Evaluation of the key identified genes using Random Forest, with an area under the curve of ROC (AUC) of 0.974 and an accuracy rate of 88.5%, demonstrates their prominent contributions in the cucumber response to abiotic stresses. These findings provide novel insights into the regulatory mechanism underlying abiotic stress response in cucumber and pave the way for cucumber genetic engineering toward improving tolerance ability under abiotic stress.

Integrated physio-biochemical and transcriptomic analysis revealed mechanism underlying of Si-mediated alleviation to cadmium toxicity in wheat

Cadmium (Cd) contamination has resulted in serious reduction of crop yields. Silicon (Si), as a beneficial element, regulates plant growth to heavy metal toxicity mainly through reducing metal uptake and protecting plants from oxidative injury. However, the molecular mechanism underlying Si-mediated Cd toxicity in wheat has not been well understood. This study aimed to reveal the beneficial role of Si (1 mM) in alleviating Cd-induced toxicity in wheat (Triticum aestivum) seedlings. The results showed that exogenous supply of Si decreased Cd concentration by 67.45% (root) and 70.34% (shoot), and maintained ionic homeostasis through the function of important transporters, such as Lsi, ZIP, Nramp5 and HIPP. Si ameliorated Cd-induced photosynthetic performance inhibition through up-regulating photosynthesis-related genes and light harvesting-related genes. Si minimized Cd-induced oxidative stress by decreasing MDA contents by 46.62% (leaf) and 75.09% (root), and helped re-establish redox homeostasis by regulating antioxidant enzymes activities, AsA-GSH cycle and expression of relevant genes through signal transduction pathway. The results revealed molecular mechanism of Si-mediated wheat tolerance to Cd toxicity. Si fertilizer is suggested to be applied in Cd contaminated soil for food safety production as a beneficial and eco-friendly element.

Genome-Wide Identification and Expression Analysis of NPF Genes in Cucumber (Cucumis sativus L.)

The NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER family (NPF) proteins perform an essential role in regulating plant nitrate absorption and distribution and in improving plant nitrogen use efficiency. In this study, cucumber (Cucumis sativus L.) NPF genes were comprehensively analyzed at the whole genome level, and 54 NPF genes were found to be unevenly distributed on seven chromosomes in the cucumber genome. The phylogenetic analysis showed that these genes could be divided into eight subfamilies. We renamed all CsNPF genes according to the international nomenclature, based on their homology with AtNPF genes. By surveying the expression profiles of CsNPF genes in various tissues, we found that CsNPF6.4 was specifically expressed in roots, indicating that CsNPF6.4 may play a role in N absorption; CsNPF6.3 was highly expressed in petioles, which may be related to NO₃^- storage in petioles; and CsNPF2.8 was highly expressed in fruits, which may promote NO₃^- transport to the embryos. We further examined their expression patterns under different abiotic stress and nitrogen conditions, and found that CsNPF7.2 and CsNPF7.3 responded to salt, cold, and low nitrogen stress. Taken together, our study lays a foundation for further exploration of the molecular and physiological functions of cucumber nitrate transporters.

Genome-wide characterization, phylogenetic and expression analysis of Histone gene family in cucumber (Cucumis sativus L.)

Histones are essential components of chromatin and play an important role in regulating gene transcription and participating in DNA replication. Here, we performed a comprehensive analysis of this gene family. In this study, we identified 37 CsHistones that were classified into five groups (H1, H2A, H2B, H3 and H4). The closely linked subfamilies exhibited more similarity in terms of motifs and intron/exon numbers. Segmental duplication (SD) is the main driving force of cucumber CsHistones expansion. Analysis of cis-regulatory elements in the promoter region of CsHistones showed that CsHistones can respond to a variety of stresses. RNA-Seq analysis indicated that the expression of most CsHistones was associated with different stresses, including downy mildew, powdery mildew, wilt, heat, cold, salt stress, and waterlogging. Expression analysis showed that several genes of H3 group were highly expressed in different reproductive organs. Notably, CsCENH3 (CsHistone30) has the characteristics of a variant histone, and we demonstrated that CsCENH3 was localized on the nucleus and its proteins were expressed in centromere region. These findings provide valuable information for the identification and potential functions of Histone genes and ideas for the cultivation of CENH3-mediated haploid induction lines in cucumber.

Silicon nanoparticles in higher plants: Uptake, action, stress tolerance, and crosstalk with phytohormones, antioxidants, and other signalling molecules

Silicon is absorbed as uncharged mono-silicic acid by plant roots through passive absorption of Lsi1, an influx transporter belonging to the aquaporin protein family. Lsi2 then actively effluxes silicon from root cells towards the xylem from where it is exported by Lsi6 for silicon distribution and accumulation to other parts. Recently, it was proposed that silicon nanoparticles (SiNPs) might share a similar route for their uptake and transport. SiNPs then initiate a cascade of morphophysiological adjustments that improve the plant physiology through regulating the expression of many photosynthetic genes and proteins along with photosystem I (PSI) and PSII assemblies. Subsequent improvement in photosynthetic performance and stomatal behaviour correspond to higher growth, development, and productivity. On many occasions, SiNPs have demonstrated a protective role during stressful environments by improving plant-water status, source-sink potential, reactive oxygen species (ROS) metabolism, and enzymatic profile. The present review comprehensively discusses the crop improvement potential of SiNPs stretching their role during optimal and abiotic stress conditions including salinity, drought, temperature, heavy metals, and ultraviolet (UV) radiation. Moreover, in the later section of this review, we offered the understanding that most of these upgrades can be explained by SiNPs intricate correspondence with phytohormones, antioxidants, and signalling molecules. SiNPs can modulate the endogenous phytohormones level such as abscisic acid (ABA), auxins (IAAs), cytokinins (CKs), ethylene (ET), gibberellins (GAs), and jasmonic acid (JA). Altered phytohormones level affects plant growth, development, and productivity at various organ and tissue levels. Similarly, SiNPs regulate the activities of catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD), and ascorbate-glutathione (AsA-GSH) cycle leading to an upgraded defence system. At the cellular and subcellular levels, SiNPs crosstalk with various signalling molecules such as Ca²⁺, K⁺, Na⁺, nitric oxide (NO), ROS, soluble sugars, and transcription factors (TFs) was also explained.

Transcriptomic and metabolomic reveals silicon enhances adaptation of rice under dry cultivation by improving flavonoid biosynthesis, osmoregulation, and photosynthesis

Dry cultivation is a new rice crop mode used to alleviate water shortage and develop water-saving agriculture. There is obvious genetic difference compared with drought-tolerant rice. Silicon (Si) plays an important role in plant adaptation to adverse environmental conditions and can significantly improve the drought tolerance and yield of rice. However, the regulatory mechanism via which Si provides plant tolerance or adaptation under dry cultivation is not well understood. The present study investigated the changes in plant growth, photosynthetic gas exchange, and oxidative stress of the rice cultivar "Suijing 18" under dry cultivation. Si improved photosynthetic performance and antioxidant enzyme activity and subsequently reduced lipid peroxidation of rice seedlings, promoted LAI and promoted leaf growth under dry cultivation. Further, transcriptomics combined with quasi-targeted metabolomics detected 1416 and 520 differentially expressed genes (DEGs), 38 and 41 differentially accumulated metabolites (DAMs) in the rice leaves and roots, respectively. Among them, 13 DEGs were involved in flavonoid biosynthesis, promoting the accumulation of flavonoids, anthocyanins, and flavonols in the roots and leaves of rice under dry cultivation. Meanwhile, 14 DEGs were involved in photosynthesis, promoting photosystem I and photosystem II responses, increasing the abundance of metabolites in leaves. On the other hand, 24 DAMs were identified involved in osmoregulatory processes, significantly increasing amino acids and carbohydrates and their derivatives in roots. These results provide new insight into the role of Si in alleviating to adverse environmental, Si enhanced the accumulation of flavonoids and osmoregulatory metabolites, thereby alleviating drought effect on the roots. On the other hand, improving dehydration resistance of leaves, guaranteeing normal photosynthesis and downward transport of organic matter. In conclusion, Si promoted the coordinated action between the above-ground and below-ground plant parts, improved the root/shoot ratio (R/S) of rice and increased the sugar content and enhancing rice adaptability under dry cultivation conditions. The establishment of the system for increasing the yield of rice under dry cultivation provides theoretical and technical support thereby promoting the rapid development of rice in Northeast China, and ensuring national food security.

Ion absorption, distribution and salt tolerance threshold of three willow species under salt stress

To investigate the response mechanism and salt tolerance threshold of three willow seedlings (Salix matsudana, Salix gordejevii, Salix linearistipularis), the absorption, transport and distribution of salt ions (Na⁺, K⁺, Ca²⁺) were studied under hydroponic conditions with different salt concentrations (CK, 171, 342, 513, and 684 mm) and treatment times (1, 3, 5, 8, 11, and 15 days). Salix linearistipularis has the weakest ability to maintain its apparent shape, while Salix matsudana has the strongest ability. The three plants have a certain Na⁺ interception ability, and the interception abilities of Salix matsudana and Salix gordejevii are higher than that of Salix linearistipularis. The leaf S _AK,Na of Salix linearistipularis were higher than those of Salix matsudana and Salix gordejevii. The leaf selection ability was the highest, and the selection ability of the root system was the lowest in Salix linearistipularis. The long-term low salt concentration and the short-term high salt concentration can increase the root and leaf salinity. Salix matsudana grows more stably in a long-term high-salt stress environment, and Salix gordejevii grows stably in a short-term high-salt stress environment. However, Salix linearistipularis is more suitable for planting as an indicative plant because of its sensitivity to salt stress. The root Na⁺ content of Salix matsudana and Salix gordejevii was 34.21 mg/g, which was the maximum root retention capacity. Once the accumulation of Na⁺ content in roots exceeds this value, the rejection capacity of roots is broken through, and the selective ion absorption capacity will rapidly become weak, which easily leads to the death of plants.

Can Grafting Manage Fusarium Wilt Disease of Cucumber and Increase Productivity under Heat Stress?

Cucumber production is considered a crucial problem under biotic and abiotic stress, particularly in arid and semi-arid zones. The current study investigated the impact of grafted cucumber plants on five cucurbit rootstocks under infection with Fusarium oxysporum f. sp. cucumerinum alone and in combination with heat stress in two different locations (i.e., Kafr El-Sheikh and Sidi Salem) during the year of 2021. The rootstock of VSS-61 F₁ displayed the highest level of resistance with values 20.8 and 16.6% for wilt incidence and 79.2 and 83.4% for the wilt reduction, respectively for both locations. This rootstock showed the lowest disease severity of fusarium wilt (15.3 and 12%), and high grafting efficiency (85 and 88%), respectively in both locations. Grafting also improved plant vigor and cucumber production under heat stress (40-43 °C). The rootstocks VSS-61 F₁, Ferro and Super Shintoza significantly increased the total yield of cucumber plants compared to non-grafted cucumber and the rootstock Bottle gourd in both locations. Further studies are needed on grafted plants under multiple stresses in terms of plant biological levels, including physiological, biochemical and genetic attributes.

Multidimensional Role of Silicon to Activate Resilient Plant Growth and to Mitigate Abiotic Stress

Sustainable agricultural production is critically antagonistic by fluctuating unfavorable environmental conditions. The introduction of mineral elements emerged as the most exciting and magical aspect, apart from the novel intervention of traditional and applied strategies to defend the abiotic stress conditions. The silicon (Si) has ameliorating impacts by regulating diverse functionalities on enhancing the growth and development of crop plants. Si is categorized as a non-essential element since crop plants accumulate less during normal environmental conditions. Studies on the application of Si in plants highlight the beneficial role of Si during extreme stressful conditions through modulation of several metabolites during abiotic stress conditions. Phytohormones are primary plant metabolites positively regulated by Si during abiotic stress conditions. Phytohormones play a pivotal role in crop plants' broad-spectrum biochemical and physiological aspects during normal and extreme environmental conditions. Frontline phytohormones include auxin, cytokinin, ethylene, gibberellin, salicylic acid, abscisic acid, brassinosteroids, and jasmonic acid. These phytohormones are internally correlated with Si in regulating abiotic stress tolerance mechanisms. This review explores insights into the role of Si in enhancing the phytohormone metabolism and its role in maintaining the physiological and biochemical well-being of crop plants during diverse abiotic stresses. Moreover, in-depth information about Si's pivotal role in inducing abiotic stress tolerance in crop plants through metabolic and molecular modulations is elaborated. Furthermore, the potential of various high throughput technologies has also been discussed in improving Si-induced multiple stress tolerance. In addition, a special emphasis is engrossed in the role of Si in achieving sustainable agricultural growth and global food security.

Transcriptome and Physiological Analysis of Rootstock Types and Silicon Affecting Cold Tolerance of Cucumber Seedlings

Cucumbers grafted on rootstocks with different de-blooming capacity show varying levels of cold tolerance. The content of fruit bloom correlates with its silicon-metabolizing capacity, and rootstock grafting can alter not only the cold tolerance but also the silicon-metabolizing capacity of the scion. The molecular mechanisms responsible for resistance due to rootstocks and silicon and the pathway that affects cold tolerance, however, remain poorly understood. Therefore, we performed physiological and transcriptome analysis to clarify how rootstock types and silicon affect cold tolerance in cucumber seedlings. Then, we randomly selected eight differentially expressed genes (DEGs) for quantitative real time PCR (qRT-PCR) analysis to proof the reliability of the transcriptome data. The results showed that silicon can enhance the cold tolerance of cucumbers by boosting the phenylpropanoid metabolism, and rootstock grafting can boost the active oxygen scavenging ability and synthesis level of hormones in cucumbers and maintain the stability of the membrane structure to enhance cold tolerance. The difference in cold tolerance between the two rootstocks is because the cold-tolerant one has stronger metabolic and sharp signal transduction ability and can maintain the stability of photosynthesis, thereby contributing to the stability of the cellular system and enhancing tolerance to cold.

Comparative Transcriptome Analysis Unravels Defense Pathways of Fraxinus velutina Torr Against Salt Stress

Fraxinus velutina Torr with high salt tolerance has been widely grown in saline lands in the Yellow River Delta, China. However, the salt-tolerant mechanisms of F. velutina remain largely elusive. Here, we identified two contrasting cutting clones of F. velutina, R7 (salt-tolerant), and S4 (salt-sensitive) by measuring chlorophyll fluorescence characteristics (Fv/Fm ratio) in the excised leaves and physiological indexes in roots or leaves under salt treatment. To further explore the salt resistance mechanisms, we compared the transcriptomes of R7 and S4 from leaf and root tissues exposed to salt stress. The results showed that when the excised leaves of S4 and R7 were, respectively, exposed to 250 mM NaCl for 48 h, Fv/Fm ratio decreased significantly in S4 compared with R7, confirming that R7 is more tolerant to salt stress. Comparative transcriptome analysis showed that salt stress induced the significant upregulation of stress-responsive genes in R7, making important contributions to the high salt tolerance. Specifically, in the R7 leaves, salt stress markedly upregulated key genes involved in plant hormone signaling and mitogen-activated protein kinase signaling pathways; in the R7 roots, salt stress induced the upregulation of main genes involved in proline biosynthesis and starch and sucrose metabolism. In addition, 12 genes encoding antioxidant enzyme peroxidase were all significantly upregulated in both leaves and roots. Collectively, our findings revealed the crucial defense pathways underlying high salt tolerance of R7 through significant upregulation of some key genes involving metabolism and hub signaling pathways, thus providing novel insights into salt-tolerant F. velutina breeding.

Study on the Relationship of Ions (Na, K, Ca) Absorption and Distribution to Photosynthetic Response of Salix matsudana Koidz Under Salt Stress

To identify the key indicators for salt tolerance evaluation of Salix matsudana Koidz, we explored the relationship of ion absorption and distribution with chlorophyll, fluorescence parameters (leaf performance index, maximum photochemical efficiency), and photosynthetic gas parameters (net photosynthetic rate, transpiration, stomatal conductance, intercellular carbon dioxide concentration) under salt stress. We established 4 treatment groups and one control group based on salinity levels of NaCl hydroponic solutions (171, 342, 513, and 684 mm). The Na⁺/K⁺, Na⁺/Ca²⁺, chlorophyll fluorescence, and photosynthetic parameters of leaves were measured on the 1st, 3rd, 5th, 8th, 11th, and 15th days to analyze the correlations of chlorophyll, chlorophyll fluorescence and photosynthetic parameters to the ion distribution ratio. The results showed that (1) The ratio of the dry weight of roots to leaves gradually increased with increasing salt concentration, whereas the water content of leaves and roots first increased and then decreased with increasing time. (2) The content of Na⁺, Na^+/K⁺, and Na⁺/Ca²⁺ in roots and leaves increased with increasing salt stress concentration and treatment time, and the difference gradually narrowed. (3) Ca²⁺ was lost more than K⁺ under salt stress, and Na⁺/Ca²⁺ was more sensitive to the salt stress environment than Na⁺/K⁺. (4) Because the root system had a retention effect, both Na⁺/K⁺ and Na⁺/Ca²⁺ in roots under different NaCl concentrations and different treatment times were higher than those in leaves, and Na⁺/Ca²⁺ was much higher than Na⁺/K⁺ in roots. (5) Na^+/K⁺ had a higher correlation with fluorescence parameters than Na⁺/Ca²⁺. Among them, Na⁺/K⁺ had a significantly negative correlation with the maximum photochemical efficiency, and the correlation coefficient R ² was 0.8576. (6) Photosynthetic gas parameters had a higher correlation with Na⁺/Ca²⁺ than with Na⁺/K⁺. Among them, significantly negative correlations were noted between Na⁺/Ca²⁺ and Gs as well as between Na⁺/Ca²⁺ and E under salt stress. The correlation between Na⁺/Ca²⁺ and Gs was the highest with a correlation coefficient of 0.9368. (7) Na⁺/K⁺ and Na⁺/Ca²⁺ had no significant correlation with chlorophylls. Na⁺/Ca²⁺ was selected as a key index to evaluate the salt tolerance of S. matsudana Koidz, and the results provided a reference for analyzing the relationship between ion transport and distribution for photosynthesis.

Comparative transcriptomic and metabolic profiling provides insight into the mechanism by which the autophagy inhibitor 3-MA enhances salt stress sensitivity in wheat seedlings

BACKGROUND

Salt stress hinders plant growth and production around the world. Autophagy induced by salt stress helps plants improve their adaptability to salt stress. However, the underlying mechanism behind this adaptability remains unclear. To obtain deeper insight into this phenomenon, combined metabolomics and transcriptomics analyses were used to explore the coexpression of differentially expressed-metabolite (DEM) and gene (DEG) between control and salt-stressed wheat roots and leaves in the presence or absence of the added autophagy inhibitor 3-methyladenine (3-MA).

RESULTS

The results indicated that 3-MA addition inhibited autophagy, increased ROS accumulation, damaged photosynthesis apparatus and impaired the tolerance of wheat seedlings to NaCl stress. A total of 14,759 DEGs and 554 DEMs in roots and leaves of wheat seedlings were induced by salt stress. DEGs were predominantly enriched in cellular amino acid catabolic process, response to external biotic stimulus, regulation of the response to salt stress, reactive oxygen species (ROS) biosynthetic process, regulation of response to osmotic stress, ect. The DEMs were mostly associated with amino acid metabolism, carbohydrate metabolism, phenylalanine metabolism, carbapenem biosynthesis, and pantothenate and CoA biosynthesis. Further analysis identified some critical genes (gene involved in the oxidative stress response, gene encoding transcription factor (TF) and gene involved in the synthesis of metabolite such as alanine, asparagine, aspartate, glutamate, glutamine, 4-aminobutyric acid, abscisic acid, jasmonic acid, ect.) that potentially participated in a complex regulatory network in the wheat response to NaCl stress. The expression of the upregulated DEGs and DEMs were higher, and the expression of the down-regulated DEGs and DEMs was lower in 3-MA-treated plants under NaCl treatment.

CONCLUSION

3-MA enhanced the salt stress sensitivity of wheat seedlings by inhibiting the activity of the roots and leaves, inhibiting autophagy in the roots and leaves, increasing the content of both H₂O₂ and O₂•-, damaged photosynthesis apparatus and changing the transcriptome and metabolome of salt-stressed wheat seedlings.

Identification and functional deciphering suggested the regulatory roles of long intergenic ncRNAs (lincRNAs) in increasing grafting pepper resistance to Phytophthora capsici

BACKGROUND

As a popular and valuable technique, grafting is widely used to protect against soil-borne diseases and nematodes in vegetable production. Growing evidences have revealed that long intergenic ncRNAs (lincRNAs) are strictly regulated and play essential roles in plants development and stress responses. Nevertheless, genome-wide identification and function deciphering of pepper lincRNAs, especially for their roles in improving grafting pepper resistance to Phytophthora capsici is largely unknown.

RESULTS

In this study, RNA-seq data of grafting and control pepper plants with or without P. capsici inoculation were used to identify lincRNAs. In total, 2,388 reliable lincRNAs were identified. They were relatively longer and contained few exons than protein-coding genes. Similar to coding genes, lincRNAs had higher densities in euchromatin regions; and longer chromosome transcribed more lincRNAs. Expression pattern profiling suggested that lincRNAs commonly had lower expression than mRNAs. Totally, 607 differentially expressed lincRNAs (DE-lincRANs) were identified, of which 172 were found between P. capsici resistance grafting pepper sample GR and susceptible sample LDS. The neighboring genes of DE-lincRNAs and miRNAs competitively sponged by DE-lincRNAs were identified. Subsequently, the expression level of DE-lincRNAs was further confirmed by qRT-PCR and regulation patterns between DE-lincRNAs and neighboring mRNAs were also validated. Function annotation revealed that DE-lincRNAs increased the resistance of grafting prepper to P. capsici by modulating the expression of disease-defense related genes through cis-regulating and/or lincRNA-miRNA-mRNA interaction networks.

CONCLUSIONS

This study identified pepper lincRNAs and suggested their potential roles in increasing the resistance level of grafting pepper to P. capsici.

Comparative transcriptomic and metabolomic analyses reveal the protective effects of silicon against low phosphorus stress in tomato plants

Phosphorus (P) is an essential nutrient controlling plant growth and development through the regulation of basic metabolic processes. Soil P deficiency is one of the major limiting factors for sustainable crop production worldwide. Previous studies have demonstrated that silicon (Si), as a beneficial element, promotes plant nutrition, growth, development, and responses to low P (LP) stress; however, the molecular mechanisms underlying Si-mediated LP tolerance remain largely unclear. Here, we found that LP + Si treatment increased the net photosynthetic rate and shoot fresh weight by 34.3%, and 121.3%, respectively compared with LP alone. RNA-sequencing and metabolomic analyses were subsequently performed with tomato plants grown under control and P depleted conditions with or without Si amendment. RNA-sequencing showed that Si supply alters not only the expression of genes involved in the metabolism of carbon (C), nitrogen (N), and P but also phosphorylation processes and metabolism of glutathione and reactive active oxygen in tomato roots. Si also affected the expression of genes encoding major transcription factors such as WRKY and MYB under LP stress. Moreover, a set of genes encoding the enzymes or regulators of organic acid (OA) metabolism or secretion were differentially expressed in Si-treated P deficient roots compared with those in LP stress alone. Furthermore, the metabolomic analysis showed that the levels of several OAs were significantly elevated in Si-treated P deficient roots. Taken together, these results indicate that exogenous Si increases the secretion of OAs by modulating C/N metabolism in LP-treated tomato roots and thereby improving plant growth under LP stress.

Role of silicon in elevating resistance against sheath blight and blast diseases in rice (Oryza sativa L.)

Rice blast caused by Magnaporthe oryzae and sheath blight caused by Rhizoctonia solani, are the two major diseases of rice that cause enormous losses in rice production worldwide. Identification and utilization of broad-spectrum resistance resources have been considered sustainable and effective strategies. However, the majority of the resistance genes and QTLs identified have often been found to be race-specific, and their resistance is frequently broken down due to continuous exposure to the pathogen. Therefore, integrated approaches to improve plant resistance against such devastating pathogen have great importance. Silicon (Si), a beneficial element for plant growth, has shown to provide a prophylactic effect against many pathogens. The application of Si helps the plants to combat the disease-causing pathogens, either through its deposition in different parts of the plant or through modulation/induction of specific defense genes by yet an unknown mechanism. Some reports have shown that Si imparts resistance to rice blast and sheath blight. The present review summarizes the mechanism of Si transport and deposition and its effect on rice growth and development. A special emphasis has been given to explore the existing evidence showing Si mediated blast and sheath blight resistance and the mechanism involved in resistance. This review will help to understand the prophylactic effects of Si against sheath blight and blast disease at the mechanical, physiological, and genetic levels. The information provided here will help develop a strategy to explore Si derived benefits for sustainable rice production.

Downregulation of Polyamine and Diamine Oxidases in Silicon-Treated Cucumber

Silicon (Si) is a ubiquitous element in soil with well-known beneficial effects under certain conditions, in several plant species, if supplied in available form for uptake. It may alleviate damage in various stress situations and may also promote growth when no obvious stressors are applied. Effects of Si are often linked to mitigation of oxidative stress, in particular to the induction of antioxidant defense mechanisms. In the work presented, the impact of silicon provision on pro-oxidant systems was investigated in cucumber. Plants of the F1 cultivar hybrid 'Joker' were grown under in vitro conditions in the absence of any applied external stressor. Silicon provision decreased H2O2 content and lowered lipid peroxidation in the leaves of the treated plants. This was paralleled by declining polyamine oxidase (PAO) and diamine oxidase (DAO) activities. Several PAO as well as lipoxygenase (LOX) genes were coordinately downregulated in Si-treated plants. Unlike in similar systems studied earlier, the Si effect was not associated with an increased transcript level of gene coding for antioxidant enzymes. These results suggest an inhibitory effect of Si provision on pro-oxidant amine oxidases, which may decrease the level of reactive oxygen species by retarding their production. This extends the molecular mechanisms linked to silicon effects onto redox balance in plants.

Fascinating role of silicon to combat salinity stress in plants: An updated overview

Salt stress limits plant growth and productivity by severely impacting the fundamental physiological processes. Silicon (Si) supplementation is considered one of the promising methods to improve plant resilience under salt stress. Here, the role of Si in modulating physiological and biochemical processes that get adversely affected by high salinity, is discussed. Although numerous reports show the beneficial effects of Si under stress, the precise molecular mechanism underlying this is not well understood. Questions like whether all plants are equally benefitted with Si supplementation despite having varying Si uptake capability and salinity tolerance are still elusive. This review illustrates the Si uptake and accumulation mechanism to understand the direct or indirect participation of Si in different physiological processes. Evaluation of plant responses at transcriptomics and proteomics levels are promising in understanding the role of Si. Integration of physiological understanding with omics scale information highlighted Si supplementation affecting the phytohormonal and antioxidant responses under salinity as a key factor defining improved resilience. Similarly, the crosstalk of Si with lignin and phenolic content under salt stress also seems to be an important phenomenon helping plants to reduce the stress. The present review also addressed various crucial mechanisms by which Si application alleviates salt stress, such as a decrease in oxidative damage, decreased lipid peroxidation, improved photosynthetic ability, and ion homeostasis. Besides, the application and challenges of using Si-nanoparticles have also been addressed. Comprehensive information and discussion provided here will be helpful to better understand the role of Si under salt stress.

Nano-selenium, silicon and H2O2 boost growth and productivity of cucumber under combined salinity and heat stress

The production of cucumber under combined salinity and heat stress is a crucial challenge facing many countries particularly in arid environments. This challenge could be controlled through exogenous foliar application of some bio-stimulants or anti-stressors. This study was carried out to investigate the management and improving cucumber production under combined salinity and heat stress. Nano-selenium (nano-Se, 25 mg L^-1), silicon (Si, 200 mg L^-1) and hydrogen peroxide (H₂O₂, 20 mmol L^-1) were foliar applied on cucumber plants as anti-stress compounds. The results revealed that studied anti-stressors improved growth and productivity of cucumber grown in saline soil regardless the kind of anti-stressor under heat stress. The foliar application of nano-Se (25 mg L^-1) clearly improved cucumber growth parameters (plant height and leaf area) compared to other anti-stressor and control. Foliar Si application showed the greatest impact on enzymatic antioxidant capacities among the other anti-stressor treatments. This applied rate of Si also showed the greatest increase in marketable fruit yield and yield quality (fruit firmness and total soluble solids) compared to untreated plants. These increases could be due to increasing nutrient uptake particularly N, P, K, and Mg, as well as Se (by 40.2% and 43%) in leaves and Si (by 11.2% and 22.1% in fruits) in both seasons, respectively. The potential role of Si in mitigating soil salinity under heat stress could be referred to high Si content found in leaf which regulates water losses via transpiration as well as high nutrient uptake of other nutrients (N, P, K, Mg and Se). The distinguished high K⁺ content found in cucumber leaves might help stressed plants to tolerate studied stresses by regulating the osmotic balance and controlling stomatal opening, which support cultivated plants to adapt to soil salinity under heat stress. Further studies are needed to be carried out concerning the different response of cultivated plants to combined stresses.

Combined Transcriptomic and Metabolomic Analysis Reveals the Role of Phenylpropanoid Biosynthesis Pathway in the Salt Tolerance Process of Sophora alopecuroides

Salt stress is the main abiotic stress that limits crop yield and agricultural development. Therefore, it is imperative to study the effects of salt stress on plants and the mechanisms through which plants respond to salt stress. In this study, we used transcriptomics and metabolomics to explore the effects of salt stress on Sophora alopecuroides. We found that salt stress incurred significant gene expression and metabolite changes at 0, 4, 24, 48, and 72 h. The integrated transcriptomic and metabolomic analysis revealed that the differentially expressed genes (DEGs) and differential metabolites (DMs) obtained in the phenylpropanoid biosynthesis pathway were significantly correlated under salt stress. Of these, 28 DEGs and seven DMs were involved in lignin synthesis and 23 DEGs and seven DMs were involved in flavonoid synthesis. Under salt stress, the expression of genes and metabolites related to lignin and flavonoid synthesis changed significantly. Lignin and flavonoids may participate in the removal of reactive oxygen species (ROS) in the root tissue of S. alopecuroides and reduced the damage caused under salt stress. Our research provides new ideas and genetic resources to study the mechanism of plant responses to salt stress and further improve the salt tolerance of plants.

Integration of silicon and secondary metabolites in plants: a significant association in stress tolerance

As sessile organisms, plants are unable to avoid being subjected to environmental stresses that negatively affect their growth and productivity. Instead, they utilize various mechanisms at the morphological, physiological, and biochemical levels to alleviate the deleterious effects of such stresses. Amongst these, secondary metabolites produced by plants represent an important component of the defense system. Secondary metabolites, namely phenolics, terpenes, and nitrogen-containing compounds, have been extensively demonstrated to protect plants against multiple stresses, both biotic (herbivores and pathogenic microorganisms) and abiotic (e.g. drought, salinity, and heavy metals). The regulation of secondary metabolism by beneficial elements such as silicon (Si) is an important topic. Silicon-mediated alleviation of both biotic and abiotic stresses has been well documented in numerous plant species. Recently, many studies have demonstrated the involvement of Si in strengthening stress tolerance through the modulation of secondary metabolism. In this review, we discuss Si-mediated regulation of the synthesis, metabolism, and modification of secondary metabolites that lead to enhanced stress tolerance, with a focus on physiological, biochemical, and molecular aspects. Whilst mechanisms involved in Si-mediated regulation of pathogen resistance via secondary metabolism have been established in plants, they are largely unknown in the case of abiotic stresses, thus leaving an important gap in our current knowledge.

Silicon confers cucumber resistance to salinity stress through regulation of proline and cytokinins

Salt stress is a continuous threat to global crop production. Here, we studied the alleviation role of exogenous silicon (Si) in NaCl-stressed cucumber, with special emphasis on plant growth, proline (Pro) and hormone metabolisms. The results showed that Si supplementation ameliorated the adverse effects of NaCl on plants growth, biomass, and oxidative stress. Salt stress greatly increased the content of Pro throughout the experiment, while Si regulated Pro content in two distinct ways. Si promoted the salt-induced Pro levels after 3 and 6 days of treatment, but decreased it after 9 and 12 days of treatment. Moreover, P5CS and ProDH activities and P5CS gene play important roles in Si and salt-regulated Pro levels in different stress phase. Under stress condition, Si addition tend to revert the content of ABA, IAA, cytokinin and SA to the control levels in most cases. Further correlation analysis revealed a negative correlation between the root cytokinin and Pro content after 3 days of treatment, suggesting the interaction between cytokinin and Pro metabolism. Exogenous application of Pro and ProDH competitive inhibitor D-Lactate confirmed the possible interplay between Pro and cytokinin metabolism. Further study identified several CKX (Csa4G647490 and Csa1G589070) and IPT (Csa7G392940 and Csa3G150100) genes that may be responsible for the regulation of cytokinin accumulation by Si and/or Pro after short-term of treatment. The results suggested that Pro is a key factor in Si-induced salt tolerance, and Si-increased Pro content may participate in the regulation of cytokinin metabolism under short-term of salt stress.

Genome-wide analysis of the WRKY gene family in the cucumber genome and transcriptome-wide identification of WRKY transcription factors that respond to biotic and abiotic stresses

BACKGROUND

Cucumber (Cucumis sativus L.) is an economically important vegetable crop species. However, it is susceptible to various abiotic and biotic stresses. WRKY transcription factors play important roles in plant growth and development, particularly in the plant response to biotic and abiotic stresses. However, little is known about the expression pattern of WRKY genes under different stresses in cucumber.

RESULTS

In the present study, an analysis of the new assembly of the cucumber genome (v3.0) allowed the identification of 61 cucumber WRKY genes. Phylogenetic and synteny analyses were performed using related species to investigate the evolution of the cucumber WRKY genes. The 61 CsWRKYs were classified into three main groups, within which the gene structure and motif compositions were conserved. Tissue expression profiles of the WRKY genes demonstrated that 24 CsWRKY genes showed constitutive expression (FPKM > 1 in all samples), and some WRKY genes showed organ-specific expression, suggesting that these WRKYs might be important for plant growth and organ development in cucumber. Importantly, analysis of the CsWRKY gene expression patterns revealed that five CsWRKY genes strongly responded to both salt and heat stresses, 12 genes were observed to be expressed in response to infection from downy mildew and powdery mildew, and three CsWRKY genes simultaneously responded to all treatments analysed. Some CsWRKY genes were observed to be induced/repressed at different times after abiotic or biotic stress treatment, demonstrating that cucumber WRKY genes might play different roles during different stress responses and that their expression patterns vary in response to stresses.

CONCLUSIONS

Sixty-one WRKY genes were identified in cucumber, and insight into their classification, evolution, and expression patterns was gained in this study. Responses to different abiotic and biotic stresses in cucumber were also investigated. Our results provide a better understanding of the function of CsWRKY genes in improving abiotic and biotic stress resistance in cucumber.

Genome-wide identification and characterization of SRO gene family in wheat: Molecular evolution and expression profiles during different stresses

SRO (SIMILAR TO RCD ONE), a type of plant-specific small protein family, play important roles in plant growth and development, as well as in response to biotic/abiotic stresses. Although characterization of SROs have been performed in model plants, little is known about their function in wheat, especially under stress conditions. In this study, 30 SRO genes were identified from the wheat genome (TaSRO). They were phylogenetically separated into two groups with distinct structures. The cis-regulatory elements in the promoter region of TaSROs were analyzed and numerous elements functionally associated with stress responding and hormones were interpreted, implying the reason for induction expression patterns of TaSROs during abiotic and biotic stresses in wheat. Whole-genome replication events in the SRO gene family of wheat and seven other species (Arabidopsis thaliana, rice, maize, barley, soybean, upland cotton, and cucumber) were analyzed, resulting in 1, 12, 9, 23, 6, 5, and 3 of gene pairs, respectively. The tissue-specific expression pattern profiling revealed that most TaSROs are highly expressed in one or more tissues and may play an important role in wheat growth and development. In addition, qRT-PCR results further confirmed that these TaSRO genes are involved in wheat stress response. In summary, our study laid a theoretical basis for molecular function deciphering of TaSROs, especially in plant hormones and biotic/abiotic stress responses.

Genome-Wide Mining of Wheat DUF966 Gene Family Provides New Insights Into Salt Stress Responses

Domain of unknown function (DUF) proteins constitute a great deal of families of functionally uncharacterized proteins in eukaryotes. The DUF966 gene family is found in monocotyledons, dicotyledons, mosses, and other species. However, little is known about the functions of DUF966 genes in wheat (Triticum aestivum L.). In this study, we identified and characterized the TaDUF966 gene family members in wheat by in silico analysis. A total of 28 TaDUF966 proteins were identified in wheat. Phylogenetic analysis divided these proteins into two groups (Groups I and II). Proteins in each group showed a highly conserved DUF966 domain and conserved motif distribution, implying their functional conservation. Analysis of gene expression profiling data showed that some TaDUF966 genes were induced by salt stress. We further confirmed the role of TaDUF966-9B in salt stress using virus induced gene silencing (VIGS) assay. Compared with the empty vector control, the TaDUF966-9B knockdown plants exhibited severe leaf curling at 10 days post-inoculation with BSMV under salt stress, suggesting that TaDUF966 genes play a vital role in salt stress tolerance in wheat. Taken together, these results expand our knowledge of the evolution of the DUF966 gene family in wheat and promote the potential application of these genes in wheat genetic improvement.

Is Silicon a Panacea for Alleviating Drought and Salt Stress in Crops?

Salinity affects around 20% of all arable land while an even larger area suffers from recurrent drought. Together these stresses suppress global crop production by as much as 50% and their impacts are predicted to be exacerbated by climate change. Infrastructure and management practices can mitigate these detrimental impacts, but are costly. Crop breeding for improved tolerance has had some success but is progressing slowly and is not keeping pace with climate change. In contrast, Silicon (Si) is known to improve plant tolerance to a range of stresses and could provide a sustainable, rapid and cost-effective mitigation method. The exact mechanisms are still under debate but it appears Si can relieve salt stress via accumulation in the root apoplast where it reduces "bypass flow of ions to the shoot. Si-dependent drought relief has been linked to lowered root hydraulic conductance and reduction of water loss through transpiration. However, many alternative mechanisms may play a role such as altered gene expression and increased accumulation of compatible solutes. Oxidative damage that occurs under stress conditions can be reduced by Si through increased antioxidative enzymes while Si-improved photosynthesis has also been reported. Si fertilizer can be produced relatively cheaply and to assess its economic viability to improve crop stress tolerance we present a cost-benefit analysis. It suggests that Si fertilization may be beneficial in many agronomic settings but may be beyond the means of smallholder farmers in developing countries. Si application may also have disadvantages, such as increased soil pH, less efficient conversion of crops into biofuel and reduced digestibility of animal fodder. These issues may hamper uptake of Si fertilization as a routine agronomic practice. Here, we critically evaluate recent literature, quantifying the most significant physiological changes associated with Si in plants under drought and salinity stress. Analyses show that metrics associated with photosynthesis, water balance and oxidative stress all improve when Si is present during plant exposure to salinity and drought. We further conclude that most of these changes can be explained by apoplastic roles of Si while there is as yet little evidence to support biochemical roles of this element.

Genome-wide characterization of the C2H2 zinc-finger genes in Cucumis sativus and functional analyses of four CsZFPs in response to stresses

BACKGROUNDS

C2H2-type zinc finger protein (ZFPs) form a relatively large family of transcriptional regulators in plants, and play many roles in plant growth, development, and stress response. However, the comprehensive analysis of C2H2 ZFPs in cucumber (CsZFPs) and their regulation function in cucumber are still lacking.

RESULTS

In the current study, the whole genome identification and characterization of CsZFPs, including the gene structure, genome localization, phylogenetic relationship, and gene expression were performed. Functional analysis of 4 selected genes by transient transformation were also conducted. A total of 129 full-length CsZFPs were identified, which could be classified into four groups according to the phylogenetic analysis. The 129 CsZFPs unequally distributed on 7 chromosomes. Promoter cis-element analysis showed that the CsZFPs might involve in the regulation of phytohormone and/or abiotic stress response, and 93 CsZFPs were predicted to be targeted by one to 20 miRNAs. Moreover, the subcellular localization analysis indicated that 10 tested CsZFPs located in the nucleus and the transcriptome profiling analysis of CsZFPs demonstrated that these genes are involved in root and floral development, pollination and fruit spine. Furthermore, the transient overexpression of Csa1G085390 and Csa7G071440 into Nicotiana benthamiana plants revealed that they could decrease and induce leave necrosis in response to pathogen attack, respectively, and they could enhance salt and drought stresses through the initial induction of H2O2. In addition, Csa4G642460 and Csa6G303740 could induce cell death after 5 days transformation.

CONCLUSIONS

The identification and function analysis of CsZFPs demonstrated that some key individual CsZFPs might play essential roles in response to biotic and abiotic stresses. These results could lay the foundation for understanding the role of CsZFPs in cucumber development for future genetic engineering studies.

Genome-wide identification, structure characterization, and expression pattern profiling of aquaporin gene family in cucumber

BACKGROUND

Aquaporin (AQP) proteins comprise a group of membrane intrinsic proteins (MIPs) that are responsible for transporting water and other small molecules, which is crucial for plant survival under stress conditions including salt stress. Despite the vital role of AQPs, little is known about them in cucumber (Cucumis sativus L.).

RESULTS

In this study, we identified 39 aquaporin-encoding genes in cucumber that were separated by phylogenetic analysis into five sub-families (PIP, TIP, NIP, SIP, and XIP). Their substrate specificity was then assessed based on key amino acid residues such as the aromatic/Arginine (ar/R) selectivity filter, Froger's positions, and specificity-determining positions. The putative cis-regulatory motifs available in the promoter region of each AQP gene were analyzed and results revealed that their promoter regions contain many abiotic related cis-regulatory elements. Furthermore, analysis of previously released RNA-seq data revealed tissue- and treatment-specific expression patterns of cucumber AQP genes (CsAQPs). Three aquaporins (CsTIP1;1, CsPIP2;4, and CsPIP1;2) were the most transcript abundance genes, with CsTIP1;1 showing the highest expression levels among all aquaporins. Subcellular localization analysis in Nicotiana benthamiana epidermal cells revealed the diverse and broad array of sub-cellular localizations of CsAQPs. We then performed RNA-seq to identify the expression pattern of CsAQPs under salt stress and found a general decreased expression level of root CsAQPs. Moreover, qRT-PCR revealed rapid changes in the expression levels of CsAQPs in response to diverse abiotic stresses including salt, polyethylene glycol (PEG)-6000, heat, and chilling stresses. Additionally, transient expression of AQPs in N. benthamiana increased leaf water loss rate, suggesting their potential roles in the regulation of plant water status under stress conditions.

CONCLUSIONS

Our results indicated that CsAQPs play important roles in response to salt stress. The genome-wide identification and primary function characterization of cucumber aquaporins provides insight to elucidate the complexity of the AQP gene family and their biological functions in cucumber.

Role of Silicon in Mediating Salt Tolerance in Plants: A Review

Salt stress is a major threat for plant growth worldwide. The regulatory mechanisms of silicon in alleviating salt stress have been widely studied using physiological, molecular genetics, and genomic approaches. Recently, progresses have been made in elucidating the alleviative effects of silicon in salt-induced osmotic stress, Na toxicity, and oxidative stress. In this review, we highlight recent development on the impact of silicon application on salt stress responses. Emphasis will be given to the following aspects. (1) Silicon transporters have been experimentally identified in different plant species and their structure feature could be an important molecular basis for silicon permeability. (2) Silicon could mediate salt-induced ion imbalance by (i) regulating Na⁺ uptake, transport, and distribution and (ii) regulating polyamine levels. (3) Si-mediated upregulation of aquaporin gene expression and osmotic adjustment play important roles in alleviating salinity-induced osmotic stress. (4) Silicon application direct/indirectly mitigates oxidative stress via regulating the antioxidant defense and polyamine metabolism. (5) Omics studies reveal that silicon could regulate plants' response to salt stress by modulating the expression of various genes including transcription factors and hormone-related genes. Finally, research areas that require further investigation to provide a deeper understanding of the role of silicon in plants are highlighted.