Enhanced chilling tolerance at the booting stage in rice by transgenic overexpression of the ascorbate peroxidase gene, OsAPXa

Insights on the enhancement of chilling tolerance in Rice through over-expression and knock-out studies of OsRBCS3

Chilling stress is an important environmental factor that affects rice (Oryza sativa L.) growth and yield, and the booting stage is the most sensitive stage of rice to chilling stress. In this study, we focused on OsRBCS3, a rice gene related to chilling tolerance at the booting stage, which encodes the key enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit in photosynthesis. The aim of this study was to elucidate the role and mechanism of OsRBCS3 in rice chilling tolerance at the booting stage. The expression levels of OsRBCS3 under chilling stress were compared in two japonica rice cultivars with different chilling tolerances: Kongyu131 (KY131) and Longjing11 (LJ11). A positive correlation was found between OsRBCS3 expression and chilling tolerance. Over-expression (OE) and knock-out (KO) lines of OsRBCS3 were constructed using over-expression and CRISPR/Cas9 technology, respectively, and their chilling tolerance was evaluated at the seedling and booting stages. The results showed that OE lines exhibited higher chilling tolerance than wild-type (WT) lines at both seedling and booting stages, while KO lines showed lower chilling tolerance than WT lines. Furthermore, the antioxidant enzyme activities, malondialdehyde (MDA) content and Rubisco activity of four rice lines under chilling stress were measured, and it was found that OE lines had stronger antioxidant and photosynthetic capacities, while KO lines had the opposite effects. This study validated that OsRBCS3 plays an important role in rice chilling tolerance at the booting stage, providing new molecular tools and a theoretical basis for rice chilling tolerance breeding.

Ascorbic acid: a metabolite switch for designing stress-smart crops

Plant growth and productivity are continually being challenged by a diverse array of abiotic stresses, including: water scarcity, extreme temperatures, heavy metal exposure, and soil salinity. A common theme in these stresses is the overproduction of reactive oxygen species (ROS), which disrupts cellular redox homeostasis causing oxidative damage. Ascorbic acid (AsA), commonly known as vitamin C, is an essential nutrient for humans, and also plays a crucial role in the plant kingdom. AsA is synthesized by plants through the d-mannose/l-galactose pathway that functions as a powerful antioxidant and protects plant cells from ROS generated during photosynthesis. AsA controls several key physiological processes, including: photosynthesis, respiration, and carbohydrate metabolism, either by acting as a co-factor for metabolic enzymes or by regulating cellular redox-status. AsA's multi-functionality uniquely positions it to integrate and recalibrate redox-responsive transcriptional/metabolic circuits and essential biological processes, in accordance to developmental and environmental cues. In recognition of this, we present a systematic overview of current evidence highlighting AsA as a central metabolite-switch in plants. Further, a comprehensive overview of genetic manipulation of genes involved in AsA metabolism has been provided along with the bottlenecks and future research directions, that could serve as a framework for designing "stress-smart" crops in future.

MORN motif-containing protein OsMORN1 and OsMORN2 are crucial for rice pollen viability and cold tolerance

The pollen viability directly affects the pollination process and the ultimate grain yield of rice. Here, we identified that the MORN motif-containing proteins, OsMORN1 and OsMORN2, had a crucial role in maintaining pollen fertility. Compared with the wild type (WT), the pollen viability of the osmorn1 and osmorn2 mutants was reduced, and pollen germination was abnormal, resulting in significantly lower spikelet fertility, seed-setting rate, and grain yield per plant. Further investigation revealed that OsMORN1 was localized to the Golgi apparatus and lipid droplets. Lipids associated with pollen viability underwent alterations in osmorn mutants, such as the diacylglyceride (18:3_18:3) was 5.1-fold higher and digalactosyldiacylglycerol (18:2_18:2) was 5.2-fold lower in osmorn1, while the triacylglycerol (TG) (16:0_18:2_18:3) was 8.3-fold higher and TG (16:0_18:1_18:3) was 8.5-fold lower in osmorn2 than those in WT. Furthermore, the OsMORN1/2 was found to be associated with rice cold tolerance, as osmorn1 and osmorn2 mutants were more sensitive to chilling stress than WT. The mutants displayed increased hydrogen peroxide accumulation, reduced antioxidant enzyme activities, elevated malondialdehyde content, and a significantly decreased seedling survival rate. Lipidomics analysis revealed distinct alterations in lipids under low temperature, highlighting significant changes in TG (18:2_18:3_18:3) and TG (18:4_18:2_18:2) in osmorn1, TG (16:0_18:2_18:2) and PI (17:2_18:3) in osmorn2 compared to the WT. Therefore, it suggested that OsMORN1 and OsMORN2 regulate both pollen viability and cold tolerance through maintaining lipid homeostasis.

OsEIN2-OsEIL1/2 pathway negatively regulates chilling tolerance by attenuating OsICE1 function in rice

Low temperature severely affects rice development and yield. Ethylene signal is essential for plant development and stress response. Here, we reported that the OsEIN2-OsEIL1/2 pathway reduced OsICE1-dependent chilling tolerance in rice. The overexpressing plants of OsEIN2, OsEIL1 and OsEIL2 exhibited severe stress symptoms with excessive reactive oxygen species (ROS) accumulation under chilling, while the mutants (osein2 and oseil1) and OsEIL2-RNA interference plants (OsEIL2-Ri) showed the enhanced chilling tolerance. We validated that OsEIL1 and OsEIL2 could form a heterxodimer and synergistically repressed OsICE1 expression by binding to its promoter. The expression of OsICE1 target genes, ROS scavenging- and photosynthesis-related genes were downregulated by OsEIN2 and OsEIL1/2, which were activated by OsICE1, suggesting that OsEIN2-OsEIL1/2 pathway might mediate ROS accumulation and photosynthetic capacity under chilling by attenuating OsICE1 function. Moreover, the association analysis of the seedling chilling tolerance with the haplotype showed that the lower expression of OsEIL1 and OsEIL2 caused by natural variation might confer chilling tolerance on rice seedlings. Finally, we generated OsEIL2-edited rice with an enhanced chilling tolerance. Taken together, our findings reveal a possible mechanism integrating OsEIN2-OsEIL1/2 pathway with OsICE1-dependent cascade in regulating chilling tolerance, providing a practical strategy for breeding chilling-tolerant rice.

Sucrose synthase gene SUS3 could enhance cold tolerance in tomato

Tomatoes are susceptible to damage from cold temperatures in all stages of growth. Therefore, it is important to identify genetic resources and genes that can enhance tomato's ability to tolerate cold. In this study, a population of 223 tomato accessions was used to identify the sensitivity or tolerance of plants to cold stress. Transcriptome analysis of these accessions revealed that SUS3, a member of the sucrose synthase gene family, was induced by cold stress. We further investigated the role of SUS3 in cold stress by overexpression (OE) and RNA interference (RNAi). Compared with the wild type, SUS3-OE lines accumulated less MDA and electrolyte leakage and more proline and soluble sugar, maintained higher activities of SOD and CAT, reduced superoxide radicals, and suffered less membrane damage under cold. Thus, our findings indicate that SUS3 plays a crucial role in the response to cold stress. This study indicates that SUS3 may serve as a direct target for genetic engineering and improvement projects, which aim to augment the cold tolerance of tomato crops.

Transcriptome Analysis and QTL Mapping Identify Candidate Genes and Regulatory Mechanisms Related to Low-Temperature Germination Ability in Maize

Low-temperature germination ability (LTGA) is an important characteristic for spring sowing maize. However, few maize genes related to LTGA were confirmed, and the regulatory mechanism is less clear. Here, maize-inbred lines Ye478 and Q1 with different LTGA were used to perform transcriptome analysis at multiple low-temperature germination stages, and a co-expression network was constructed by weighted gene co-expression network analysis (WGCNA). Data analysis showed that 7964 up- and 5010 down-regulated differentially expressed genes (DEGs) of Ye478 were identified at low-temperature germination stages, while 6060 up- and 2653 down-regulated DEGs of Q1 were identified. Gene ontology (GO) enrichment analysis revealed that ribosome synthesis and hydrogen peroxide metabolism were enhanced and mRNA metabolism was weakened under low-temperature stress for Ye478, while hydrogen peroxide metabolism was enhanced and mRNA metabolism was weakened for Q1. DEGs pairwise comparisons between the two genotypes found that Ye478 performed more ribosome synthesis at low temperatures compared with Q1. WGCNA analysis based on 24 transcriptomes identified 16 co-expressed modules. Of these, the MEbrown module was highly correlated with Ye478 at low-temperature stages and catalase and superoxide dismutase activity, and the MEred, MEgreen, and MEblack modules were highly correlated with Ye478 across low-temperature stages, which revealed a significant association between LTGA and these modules. GO enrichment analysis showed the MEbrown and MEred modules mainly functioned in ribosome synthesis and cell cycle, respectively. In addition, we conducted quantitative trait loci (QTL) analysis based on a doubled haploid (DH) population constructed by Ye478 and Q1 and identified a major QTL explanting 20.6% of phenotype variance on chromosome 1. In this QTL interval, we found three, four, and three hub genes in the MEbrown, MEred, and MEgreen modules, of which two hub genes (Zm00001d031951, Zm00001d031953) related to glutathione metabolism and one hub gene (Zm00001d031617) related to oxidoreductase activity could be the candidate genes for LTGA. These biological functions and candidate genes will be helpful in understanding the regulatory mechanism of LTGA and the directional improvement of maize varieties for LTGA.

Comparative physiological, antioxidant and proteomic investigation reveal robust response to cold stress in Digitalis purpurea L

BACKGROUND OF THE STUDY

Digitalis purpurea (L) is an important medicinal plant growing at Alpine region of Himalayas and withstands low temperatures and harsh climatic conditions existing at high altitude. It serves as an ideal plant system to decipher the tolerance to cold stress (CS) in plants from high altitudes.

METHODS AND RESULTS

To understand the complexity of plant response to CS, we performed a comparative physiological and biochemical study complemented with proteomics in one-month-old D. purpurea grown at 25 °C (control) and 4 °C (CS). We observed an enhanced accumulation of different osmo-protectants (glycine betaine, soluble sugar and proline) and higher transcription (mRNA levels) of various antioxidant enzymes with an increased antioxidant enzyme activity in D. purpurea when exposed to CS. Furthermore, higher concentrations of non-enzymatic antioxidants (flavonoids, phenolics) was also associated with the response to CS. Differential proteomic analysis revealed the role of various proteins primarily involved in redox reactions, protein stabilization, quinone and sterol metabolism involved in CS response in D. purpurea..

CONCLUSION

Our results provide a framework for better understanding the physiological and molecular mechanism of CS response in D. purpurea at high altitudes.

Achieving abiotic stress tolerance in plants through antioxidative defense mechanisms

Climate change has increased the overall impact of abiotic stress conditions such as drought, salinity, and extreme temperatures on plants. Abiotic stress adversely affects the growth, development, crop yield, and productivity of plants. When plants are subjected to various environmental stress conditions, the balance between the production of reactive oxygen species and its detoxification through antioxidant mechanisms is disturbed. The extent of disturbance depends on the severity, intensity, and duration of abiotic stress. The equilibrium between the production and elimination of reactive oxygen species is maintained due to both enzymatic and non-enzymatic antioxidative defense mechanisms. Non-enzymatic antioxidants include both lipid-soluble (α-tocopherol and β-carotene) and water-soluble (glutathione, ascorbate, etc.) antioxidants. Ascorbate peroxidase (APX), superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GR) are major enzymatic antioxidants that are essential for ROS homeostasis. In this review, we intend to discuss various antioxidative defense approaches used to improve abiotic stress tolerance in plants and the mechanism of action of the genes or enzymes involved.

Ornithine δ-aminotransferase OsOAT is critical for male fertility and cold tolerance during rice plant development

Cold stress is a major factor limiting the production and geographical distribution of rice (Oryza sativa) varieties. However, the molecular mechanisms underlying cold tolerance remain to be elucidated. Here, we report that ornithine δ-aminotransferase (OsOAT) contributes to cold tolerance during the vegetative and reproductive development of rice. osoat mutant was identified as a temperature-sensitive male sterile mutant with deformed floral organs and seedlings sensitive to cold stress. Comparative transcriptome analysis showed that OsOAT mutation and cold treatment of the wild-type plant led to similar changes in the global gene expression profiles in anthers. OsOAT genes in indica rice Huanghuazhan (HHZ) and japonica rice Wuyungeng (WYG) are different in gene structure and response to cold. OsOAT is cold-inducible in WYG but cold-irresponsive in HHZ. Further studies showed that indica varieties carry both WYG-type and HHZ-type OsOAT, whereas japonica varieties mostly carry WYG-type OsOAT. Cultivars carrying HHZ-type OsOAT are mainly distributed in low-latitude regions, whereas varieties carrying WYG-type OsOAT are distributed in both low- and high-latitude regions. Moreover, indica varieties carrying WYG-type OsOAT generally have higher seed-setting rates than those carrying HHZ-type OsOAT under cold stress at reproductive stage, highlighting the favorable selection for WYG-type OsOAT during domestication and breeding to cope with low temperatures.

Deoxynivalenol accumulation and detoxification in cereals and its potential role in wheat-Fusarium graminearum interactions

Deoxynivalenol (DON) is a prominent mycotoxin showing significant accumulation in cereal plants during infection by the phytopathogen Fusarium graminearum. It is a virulence factor that is important in the spread of F. graminearum within cereal heads, and it causes serious yield losses and significant contamination of cereal grains. In recent decades, genetic and genomic studies have facilitated the characterization of the molecular pathways of DON biosynthesis in F. graminearum and the environmental factors that influence DON accumulation. In addition, diverse scab resistance traits related to the repression of DON accumulation in plants have been identified, and experimental studies of wheat-pathogen interactions have contributed to understanding detoxification mechanisms in host plants. The present review illustrates and summarizes the molecular networks of DON mycotoxin production in F. graminearum and the methods of DON detoxification in plants based on the current literature, which provides molecular targets for crop improvement programs. This review also comprehensively discusses recent advances and challenges related to genetic engineering-mediated cultivar improvements to strengthen scab resistance. Furthermore, ongoing advancements in genetic engineering will enable the application of these molecular targets to develop more scab-resistant wheat cultivars with DON detoxification traits.

Recent Molecular Aspects and Integrated Omics Strategies for Understanding the Abiotic Stress Tolerance of Rice

Rice is an important staple food crop for over half of the world's population. However, abiotic stresses seriously threaten rice yield improvement and sustainable production. Breeding and planting rice varieties with high environmental stress tolerance are the most cost-effective, safe, healthy, and environmentally friendly strategies. In-depth research on the molecular mechanism of rice plants in response to different stresses can provide an important theoretical basis for breeding rice varieties with higher stress resistance. This review presents the molecular mechanisms and the effects of various abiotic stresses on rice growth and development and explains the signal perception mode and transduction pathways. Meanwhile, the regulatory mechanisms of critical transcription factors in regulating gene expression and important downstream factors in coordinating stress tolerance are outlined. Finally, the utilization of omics approaches to retrieve hub genes and an outlook on future research are prospected, focusing on the regulatory mechanisms of multi-signaling network modules and sustainable rice production.

OsLUX Confers Rice Cold Tolerance as a Positive Regulatory Factor

During the early seedling stage, rice (Oryza sativa L.) must overcome low-temperature stress. While a few cold-tolerance genes have been characterized, further excavation of cold-resistance genes is still needed. In this study, we identified a cold-induced transcription factor—LUX ARRHYTHMO (LUX)—in rice. OsLUX was found to be specifically expressed in leaf blades and upregulated by both cold stress and circadian rhythm. The full-length OsLUX showed autoactivation activity, and the OsLUX protein localized throughout the entire onion cell. Overexpressing OsLUX resulted in increased cold tolerance and reduced ion leakage under cold-stress conditions during the seedling stage. In contrast, the knockout of OsLUX decreased seedling cold tolerance and showed higher ion leakage compared to the wild type. Furthermore, overexpressing OsLUX upregulated the expression levels of oxidative stress-responsive genes, which improved reactive oxygen species (ROS) scavenging ability and enhanced tolerance to chilling stress. Promoter analysis showed that the OsLUX promoter contains two dehydration-responsive element binding (DREB) motifs at positions −510/−505 (GTCGGa) and −162/−170 (cCACCGccc), which indicated that OsDREB1s and OsDREB2s probably regulate OsLUX expression by binding to the motif to respond to cold stress. Thus, OsLUX may act as a downstream gene of the DREB pathway. These results demonstrate that OsLUX serves as a positive regulatory factor of cold stress and that overexpressing OsLUX could be used in rice breeding programs to enhance abiotic stress tolerance.

Oligomeric Proanthocyanidins Confer Cold Tolerance in Rice through Maintaining Energy Homeostasis

Oligomeric proanthocyanidins (OPCs) are abundant polyphenols found in foods and botanicals that benefit human health, but our understanding of the functions of OPCs in rice plants is limited, particularly under cold stress. Two rice genotypes, named Zhongzao39 (ZZ39) and its recombinant inbred line RIL82, were subjected to cold stress. More damage was caused to RIL82 by cold stress than to ZZ39 plants. Transcriptome analysis suggested that OPCs were involved in regulating cold tolerance in the two genotypes. A greater increase in OPCs content was detected in ZZ39 than in RIL82 plants under cold stress compared to their respective controls. Exogenous OPCs alleviated cold damage of rice plants by increasing antioxidant capacity. ATPase activity was higher and poly (ADP-ribose) polymerase (PARP) activity was lower under cold stress in ZZ39 than in RIL82 plants. Importantly, improvements in cold tolerance were observed in plants treated with the OPCs and 3-aminobenzamide (PARP inhibitor, 3ab) combination compared to the seedling plants treated with H₂O, OPCs, or 3ab alone. Therefore, OPCs increased ATPase activity and inhibited PARP activity to provide sufficient energy for rice seedling plants to develop antioxidant capacity against cold stress.

Coronatine Enhances Chilling Tolerance of Tomato Plants by Inducing Chilling-Related Epigenetic Adaptations and Transcriptional Reprogramming

Low temperature is an important environmental factor limiting the widespread planting of tropical and subtropical crops. The application of plant regulator coronatine, which is an analog of Jasmonic acid (JA), is an effective approach to enhancing crop's resistance to chilling stress and other abiotic stresses. However, the function and mechanism of coronatine in promoting chilling resistance of tomato is unknown. In this study, coronatine treatment was demonstrated to significantly increase tomato chilling tolerance. Coronatine increases H3K4me3 modifications to make greater chromatin accessibility in multiple chilling-activated genes. Corresponding to that, the expression of CBFs, other chilling-responsive transcription factor (TF) genes, and JA-responsive genes is significantly induced by coronatine to trigger an extensive transcriptional reprogramming, thus resulting in a comprehensive chilling adaptation. These results indicate that coronatine enhances the chilling tolerance of tomato plants by inducing epigenetic adaptations and transcriptional reprogramming.

Cold-adaptive evolution at the reproductive stage in Geng/japonica subspecies reveals the role of OsMAPK3 and OsLEA9

Cold stress at the reproductive stage severely affects the production and geographic distribution of rice. The Geng/japonica subpopulation gradually developed stronger cold adaptation than the Xian/indica subpopulation during the long-term domestication of cultivated rice. However, the evolutionary path and natural alleles underlying the cold adaptability of intra-Geng subspecies remain largely unknown. Here, we identified MITOGEN-ACTIVATED PROTEIN KINASE 3 (OsMAPK3) and LATE EMBRYOGENESIS ABUNDANT PROTEIN 9 (OsLEA9) as two important regulators for the cold adaptation of Geng subspecies from a combination of transcriptome analysis and genome-wide association study. Transgenic validation showed that OsMAPK3 and OsLEA9 confer cold tolerance at the reproductive stage. Selection and evolution analysis suggested that the Geng version of OsMAPK3 (OsMAPK3^Geng ) directly evolved from Chinese Oryza rufipogon III and was largely retained in high-latitude and high-altitude regions with low temperatures during domestication. Later, the functional nucleotide polymorphism (FNP-776) in the Kunmingxiaobaigu and Lijiangxiaoheigu version of the OsLEA9 (OsLEA9^KL ) promoter originated from novel variation of intra-Geng was selected and predominantly retained in temperate Geng to improve the adaptation of Geng together with OsMAPK3^Geng to colder climatic conditions in high-latitude areas. Breeding potential analysis suggested that pyramiding of OsMAPK3^Geng and OsLEA9^KL enhanced the cold tolerance of Geng and promotes the expansion of cultivated rice to colder regions. This study not only highlights the evolutionary path taken by the cold-adaptive differentiation of intra-Geng, but also provides new genetic resources for rice molecular breeding in low-temperature areas.

Effects of Chilling Stress on Morphological, Physiological, and Biochemical Attributes of Silage Corn Genotypes during Seedling Establishment

Chilling stress is one of the major abiotic stresses which hinder seedling emergence and growth. Herein, we investigated the effects of chilling/low temperature stress on the morphological, physiological, and biochemical attributes of two silage corn genotypes during the seedling establishment phase. The experiment was conducted in a growth chamber, and silage corn seedlings of Yukon-R and A4177G-RIB were grown at optimum temperature up to V3 stage and then subjected to five temperature regimes (25 °C as control, 20 °C, 15 °C, 10 °C, and 5 °C) for 5 days. After the temperature treatment, the morphological, physiological, and biochemical parameters were recorded. Results indicated that temperatures of 15 °C and lower significantly affected seedling growth, photosynthesis system, reactive oxygen species (ROS) accumulation, and antioxidant enzyme activities. Changes in seedlings’ growth parameters were in the order of 25 °C > 20 °C > 15 °C > 10 °C > 5 °C, irrespective of genotypes. The chlorophyll content, photosynthetic rate, and maximal photochemical efficiency of PS-II (Fv/Fm) were drastically decreased under chilling conditions. Moreover, chilling stress induced accumulation of hydrogen peroxide (H2O2)and malonaldehyde (MDA) contents. Increased proline content and enzymatic antioxidants, including superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxide (APX), were found to alleviate oxidative damage under chilling stress. However, the genotype of Yukon-R exhibited better adaption to chilling stress than A4177G3-RIB. Yukon-R showed significantly higher proline content and enzymatic antioxidant activities than A4177G3-RIB under severe chilling conditions (temperature ≤ 10 °C). Similarly, Yukon-R expressed low temperature-induced ROS accumulation. Furthermore, the interaction effects were found between temperature treatment and genotype on the ROS accumulation, proline content and antioxidant enzyme activities. In summary, the present study indicated that Yukon-R has shown better adaptation and resilience against chilling temperature stress, and therefore could be considered a potential candidate genotype to be grown in the boreal climate.

Deciphering the Molecular Mechanisms of Chilling Tolerance in Lsi1-Overexpressing Rice

Improving tolerance to low-temperature stress during the rice seedling stage is of great significance in agricultural science. In this study, using the low silicon gene 1 (Lsi1)-overexpressing (Dular-OE) and wild-type rice (Dular-WT), we showed that Lsi1 overexpression enhances chilling tolerance in Dular-OE. The overexpression of the Lsi1 increases silicon absorption, but it was not the main reason for chilling tolerance in Dular-OE. Instead, our data suggest that the overexpression of a Lsi1-encoding NIP and its interaction with key proteins lead to chilling tolerance in Dular-OE. Additionally, we show that the high-mobility group protein (HMG1) binds to the promoter of Lsi1, positively regulating its expression. Moreover, Nod26-like major intrinsic protein (NIP)’s interaction with α and β subunits of ATP synthase and the 14-3-3f protein was validated by co-immunoprecipitation (Co-IP), bimolecular fluorescent complementary (BiFC), and GST-pulldown assays. Western blotting revealed that the overexpression of NIP positively regulates the ATP-synthase subunits that subsequently upregulate calcineurin B-like interacting protein kinases (CIPK) negatively regulating 14-3-3f. Overall, these NIP-mediated changes trigger corresponding pathways in an orderly manner, enhancing chilling tolerance in Dular-OE.

Low-temperature stress affects reactive oxygen species, osmotic adjustment substances, and antioxidants in rice (Oryza sativa L.) at the reproductive stage

The sensitivity of rice to low-temperature stress (LTS), especially at the reproductive stage, is a primary factor of rice yield fluctuation in cold cultivate region. Here, the changes of reactive oxygen species (ROS), osmotic adjustment substances, and antioxidants in different tissues were analyzed during rice growing under low temperatures (LT) at the reproductive stage. Results showed that LTS increases the levels of proline (Pro), soluble protein (SP), glutathione (GSH), superoxidase (SOD), and ascorbate peroxidase (APX) in LJ25 (LTS-resistant) and LJ11 (LTS-sensitive). The activities of catalase (CAT) and peroxidase (POD) were significantly increased in LJ25 but decreased in LJ11 under LTS, while an opposite trend in ROS and malondialdehyde (MDA) was observed in both varieties. Moreover, most physicochemical properties were higher in flag leaves and panicles compared with those in leaf sheaths. The expression patterns of OsCOIN, OsCATC, OsMAP1, OsPOX1, and OsAPX were the same with phenotypic changes in Pro and the enzymes encoded by them, confirming the accuracy of the physicochemical analysis. Therefore, only CAT and POD increased more in LJ25, suggesting they could be the key factors used for LT-tolerant breeding of rice in cold regions.

OsAPX1 Positively Contributes to Rice Blast Resistance

Ascorbate peroxidases (APXs) maintain cellular reactive oxygen species (ROS) homeostasis through their peroxidase activity. Here, we report that OsAPX1 also promotes ROS production such that a delicate cellular ROS homeostasis is achieved temporally after Magnaporthe oryzae infection. OsAPX1 specifically induces ROS production through increasing respiratory burst oxidase homologs (OsRBOHs) expression and can be inhibited by DPI, a ROS inhibitor. The time-course experiment data show that the simultaneous induction of OsAPX1 and OsRBOHs leads to ROS accumulation at an early stage; whereas a more durable expression of OsAPX1 leads to ROS scavenging at a later stage. By the temporal switching between ROS inducer and eliminator, OsAPX1 triggers an instant ROS burst upon M. oryzae infection and then a timely elimination of ROS toxicity. We find that OsAPX1 is under the control of the miR172a-OsIDS1 regulatory module. OsAPX1 also affects salicylic acid (SA) synthesis and signaling, which contribute to blast resistance. In conclusion, we show that OsAPX1 is a key factor that connects the upstream gene silencing and transcription regulatory routes with the downstream phytohormone and redox pathway, which provides an insight into the sophisticated regulatory network of rice innate immunity.

Rice Putative Pectin Methyltransferase Gene OsPMT10 Is Required for Maintaining the Cell Wall Properties of Pistil Transmitting Tissues via Pectin Modification

Precise directional control of pollen tube growth via mechanical guidance by pistil tissue is critical for the successful fertilization of flowering plants and requires active cell-to-cell communication and maintenance of softness in the transmitting tissue. However, the regulation of transmitting tissue softness as controlled by cell wall properties, especially pectin, has not been reported. Here we report that regulation of pectin methylesterification supports pollen elongation through pistil transmitting tissues in Oryza sativa. The rice pectin methylesterase gene OsPMT10 was strongly expressed in reproductive tissues, especially the pistil. The ospmt10 mutant did not have a significant effect on vegetative growth, but the fertility rate was reduced by approximately half. In the ospmt10 mutant, pollen tube elongation was observed in the transmitting tissue of the style, but approximately half of the pollen tubes did not extend all the way to the ovule. Tissue cross-sections of the upper ovary were prepared, and immunohistochemical staining using LM19 and LM20 showed that methylesterified pectin distribution was decreased in ospmt10 compared with the wild type. The decreased expression of methylesterified pectins in ospmt10 may have resulted in loss of fluidity in the apoplast space of the transmitting tissue, rendering it difficult for the pollen tube to elongate in the transmitting tissue and thereby preventing it from reaching the ovule.

Genome-wide analysis of Dof transcription factors and their response to cold stress in rice (Oryza sativa L.)

Background

Rice (Oryza sativa L.) is a food crop for humans worldwide. However, temperature has an effect during the vegetative and reproductive stages. In high-latitude regions where rice is cultivated, cold stress is a major cause of yield loss and plant death. Research has identified a group of plant-specific transcription factors, DNA binding with one zinc fingers (DOFs), with a diverse range of functions, including stress signaling and stress response during plant growth. The aim of this study was to identify Dof genes in two rice subspecies, indica and japonica, and screen for Dof genes that may be involved in cold tolerance during plant growth.

Results

A total of 30 rice Dofs (OsDofs) were identified using bioinformatics and genome-wide analyses and phylogenetically analyzed. The 30 OsDOFs were classified into six subfamilies, and 24 motifs were identified based on protein sequence alignment. The chromosome locations of OsDofs were determined and nine gene duplication events were identified. A joint phylogenetic analysis was performed on DOF protein sequences obtained from four monocotyledon species to examine the evolutionary relationship of DOF proteins. Expression profiling of OsDofs from two japonica cultivars (Longdao5, which is cold-tolerant, and Longjing11, which is cold-sensitive) revealed that OsDof1 and OsDof19 are cold-inducible genes. We examined the seed setting rates in OsDof1- and OsDof19-overexpression and RNAi lines and found that OsDof1 showed a response to cold stress.

Conclusions

Our investigation identified OsDof1 as a potential target for genetic breeding of rice with enhanced cold tolerance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08104-0.

Convergence and Divergence: Signal Perception and Transduction Mechanisms of Cold Stress in Arabidopsis and Rice

Cold stress, including freezing stress and chilling stress, is one of the major environmental factors that limit the growth and productivity of plants. As a temperate dicot model plant species, Arabidopsis develops a capability to freezing tolerance through cold acclimation. The past decades have witnessed a deep understanding of mechanisms underlying cold stress signal perception, transduction, and freezing tolerance in Arabidopsis. In contrast, a monocot cereal model plant species derived from tropical and subtropical origins, rice, is very sensitive to chilling stress and has evolved a different mechanism for chilling stress signaling and response. In this review, the authors summarized the recent progress in our understanding of cold stress response mechanisms, highlighted the convergent and divergent mechanisms between Arabidopsis and rice plasma membrane cold stress perceptions, calcium signaling, phospholipid signaling, MAPK cascade signaling, ROS signaling, and ICE-CBF regulatory network, as well as light-regulated signal transduction system. Genetic engineering approaches of developing freezing tolerant Arabidopsis and chilling tolerant rice were also reviewed. Finally, the future perspective of cold stress signaling and tolerance in rice was proposed.

OsGATA16, a GATA Transcription Factor, Confers Cold Tolerance by Repressing OsWRKY45-1 at the Seedling Stage in Rice

BACKGROUND

Cold stress is the main abiotic stress in rice, which seriously affects the growth and yield of rice. Identification of cold tolerance genes is of great significance for rice to solve these problems. GATA-family transcription factors involve diverse biological functions, however, their role in cold tolerance in rice remains unclear.

RESULTS

In this study, a GATA-type zinc finger transcription factor OsGATA16, which can improve cold tolerance, was isolated and characterized from rice. OsGATA16 belongs to OsGATA subfamily-II and contains 11 putative phosphorylation sites, a nuclear localization signal (NLS), and other several conserved domains. OsGATA16 was expressed in all plant tissues, with the strongest in panicles. It was induced by cold and ABA treatments, but was repressed by drought, cytokinin and JA, and acted as a transcriptional suppressor in the nucleus. Overexpression of OsGATA16 improves cold tolerance of rice at seedling stage. Under cold stress treatments, the transcription of four cold-related genes OsWRKY45-1, OsSRFP1, OsCYL4, and OsMYB30 was repressed in OsGATA16-overexpressing (OE) rice compared with wild-type (WT). Interestingly, OsGATA16 bound to the promoter of OsWRKY45-1 and repressed its expression. In addition, haplotype analysis showed that OsGATA16 polarized between the two major rice subspecies japonica and indica, and had a non-synonymous SNP8 (336^G) associated with cold tolerance.

CONCLUSION

OsGATA16 is a GATA transcription factor, which improves cold tolerance at seedling stage in rice. It acts as a positive regulator of cold tolerance by repressing some cold-related genes such as OsWRKY45-1, OsSRFP1, OsCYL4 and OsMYB30. Additionally, OsGATA16 has a non-synonymous SNP8 (336^G) associated with cold tolerance on CDS region. This study provides a theoretical basis for elucidating the mechanism of cold tolerance in rice and new germplasm resources for rice breeding.

The response mechanism to salt stress in Arabidopsis transgenic lines over-expressing of GmG6PD

Glucose-6-phosphate dehydrogenase (G6PD or G6PDH) plays an important role in response to salt stress in plants. However, much less is known about G6PD proteins in soybean (Glycine max L.). Here, we found that a soybean cytosolic G6PD gene, GmG6PD7, was induced by NaCl. We generated Arabidopsis transgenic lines overexpressing GmG6PD7. The seed germination rate and primary root length of Arabidopsis thaliana over-expressing GmG6PD7 under NaCl treatment were enhanced. Salt stress induced an obvious increase of the total and cytosolic G6PD activity and the marked decrease of ROS levels in the transgenic plants. At the same time, over-expressing GmG6PD7 in Arabidopsis affected the glutathione and NADPH level and activated ROS scavengers, suggesting that GmG6PD7 contributes to increase salinity tolerance by decreasing ROS accumulation. What's more, we found GmG6PD7 overexpression led to the up-regulation of abscisic acid (ABA) degradation gene and the down-regulation of ABA synthesis and ABA-responsive genes, which finally reduced ABA content to improve seed germination rate under salinity stress. It was noteworthy that GmG6PD7 can rescue the seed and root phenotype of Arabidopsis cytosolic G6PD mutant (Atg6pd5 and Atg6pd6) under salt stress, suggesting cytosolic G6PD may have a conserved function in soybean and Arabidopsis.

Zinc Oxide Nanoparticles Alleviate Chilling Stress in Rice (Oryza Sativa L.) by Regulating Antioxidative System and Chilling Response Transcription Factors

As one of the common abiotic stresses, chilling stress has negative effects on rice growth and development. Minimization of these adverse effects through various ways is vital for the productivity of rice. Nanoparticles (NPs) serve as one of the effective alleviation methods against abiotic stresses. In our research, zinc oxide (ZnO) NPs were utilized as foliar sprays on rice leaves to explore the mechanism underlying the effect of NPs against the negative impact of chilling stress on rice seedlings. We revealed that foliar application of ZnO NPs significantly alleviated chilling stress in hydroponically grown rice seedlings, including improved plant height, root length, and dry biomass. Besides, ZnO NPs also restored chlorophyll accumulation and significantly ameliorated chilling-induced oxidative stress with reduced levels of H₂O₂, MDA, proline, and increased activities of major antioxidative enzymes, superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). We further found that foliar application of ZnO NPs induced the chilling-induced gene expression of the antioxidative system (OsCu/ZnSOD1, OsCu/ZnSOD2, OsCu/ZnSOD3, OsPRX11, OsPRX65, OsPRX89, OsCATA, and OsCATB) and chilling response transcription factors (OsbZIP52, OsMYB4, OsMYB30, OsNAC5, OsWRKY76, and OsWRKY94) in leaves of chilling-treated seedlings. Taken together, our results suggest that foliar application of ZnO NPs could alleviate chilling stress in rice via the mediation of the antioxidative system and chilling response transcription factors.

Exogenous glutathione alleviates chilling injury in postharvest bell pepper by modulating the ascorbate-glutathione (AsA-GSH) cycle

We investigated the effect of exogenous glutathione (GSH) on chilling injury (CI) in postharvest bell pepper fruits stored at low temperature and explored the mechanism of this treatment from the perspective of the ascorbate-glutathione (AsA-GSH) cycle. Compared with the control, fruits treated with exogenous GSH before refrigeration displayed only slight CI symptoms and mitigated CI-induced cell damage after 10 d. Moreover, the treated peppers had lower lipid peroxidation product, H₂O₂, and O₂^- content than those did the control. Glutathione treatment enhanced the ascorbate-glutathione cycle by upregulating CaAPX1, CaGR2, CaMDHAR1, and CaDHAR1 and the antioxidant enzymes APX, GR, and MDHAR associated with the ascorbate-glutathione cycle. Glutathione treatment also increased ascorbate and glutathione concentrations. Taken together, our results showed that exogenous GSH treatment could alleviate CI in pepper fruits during cold storage by triggering the AsA-GSH cycle and improving antioxidant capacity.

The Multiple Roles of Ascorbate in the Abiotic Stress Response of Plants: Antioxidant, Cofactor, and Regulator

Ascorbate (ASC) plays a critical role in plant stress response. The antioxidant role of ASC has been well-studied, but there are still several confusing questions about the function of ASC in plant abiotic stress response. ASC can scavenge reactive oxygen species (ROS) and should be helpful for plant stress tolerance. But in some cases, increasing ASC content impairs plant abiotic stress tolerance, whereas, inhibiting ASC synthesis or regeneration enhances plant stress tolerance. This confusing phenomenon indicates that ASC may have multiple roles in plant abiotic stress response not just as an antioxidant, though many studies more or less ignored other roles of ASC in plant. In fact, ACS also can act as the cofactor of some enzymes, which are involved in the synthesis, metabolism, and modification of a variety of substances, which has important effects on plant stress response. In addition, ASC can monitor and effectively regulate cell redox status. Therefore, we believe that ASC has atleast triple roles in plant abiotic stress response: as the antioxidant to scavenge accumulated ROS, as the cofactor to involve in plant metabolism, or as the regulator to coordinate the actions of various signal pathways under abiotic stress. The role of ASC in plant abiotic stress response is important and complex. The detail role of ASC in plant abiotic stress response should be analyzed according to specific physiological process in specific organ. In this review, we discuss the versatile roles of ASC in the response of plants to abiotic stresses.

Differentiation, evolution and utilization of natural alleles for cold adaptability at the reproductive stage in rice

Genetic studies on cold tolerance at the reproductive stage in rice could lead to significant reductions in yield losses. However, knowledge about the genetic basis and adaptive differentiation, as well as the evolution and utilization of the underlying natural alleles, remains limited. Here, 580 rice accessions in two association panels were used to perform genome-wide association study, and 156 loci associated with cold tolerance at the reproductive stage were identified. Os01g0923600 and Os01g0923800 were identified as promising candidate genes in qCTB1t, a major associated locus. Through population genetic analyses, 22 and 29 divergent regions controlling cold adaptive differentiation inter-subspecies (Xian/Indica and Geng/Japonica) and intra-Geng, respectively, were identified. Joint analyses of four cloned cold-tolerance genes showed that they had different origins and utilizations under various climatic conditions. bZIP73 and OsAPX1 differentiating inter-subspecies evolved directly from wild rice, whereas the novel mutations CTB4a and Ctb1 arose in Geng during adaptation to colder climates. The cold-tolerant Geng accessions have undergone stronger selection under colder climate conditions than other accessions during the domestication and breeding processes. Additive effects of dominant allelic variants of four identified genes have been important in adaptation to cold in modern rice varieties. Therefore, this study provides valuable information for further gene discovery and pyramiding breeding to improve cold tolerance at the reproductive stage in rice.

Identification of candidate genes controlling chilling tolerance of rice in the cold region at the booting stage by BSA-Seq and RNA-Seq

Rice is sensitive to low temperatures, specifically at the booting stage. Chilling tolerance of rice is a quantitative trait loci that is governed by multiple genes, and thus, its precise identification through the conventional methods is an arduous task. In this study, we investigated the candidate genes related to chilling tolerance at the booting stage of rice. The F2 population was derived from Longjing25 (chilling-tolerant) and Longjing11 (chilling-sensitive) cross. Two bulked segregant analysis pools were constructed. A 0.82 Mb region containing 98 annotated genes on chromosomes 6 and 9 was recognized as the candidate region associated with chilling tolerance of rice at the booting stage. Transcriptomic analysis of Longjing25 and Longjing11 revealed 50 differentially expressed genes (DEGs) on the candidate intervals. KEGG pathway enrichment analysis of DEGs was performed. Nine pathways were found to be enriched, which contained 10 DEGs. A total of four genes had different expression patterns or levels between Longjing25 and Longjing11. Four out of the 10 DEGs were considered as potential candidate genes for chilling tolerance. This study will assist in the cloning of the candidate genes responsible for chilling tolerance and molecular breeding of rice for the development of chilling-tolerant rice varieties.

Coordination of light, circadian clock with temperature: The potential mechanisms regulating chilling tolerance in rice

Rice (Oryza sativa L.) is a major staple food crop for over half of the world's population. As a crop species originated from the subtropics, rice production is hampered by chilling stress. The genetic mechanisms of rice responses to chilling stress have attracted much attention, focusing on chilling-related gene mining and functional analyses. Plants have evolved sophisticated regulatory systems to respond to chilling stress in coordination with light signaling pathway and internal circadian clock. However, in rice, information about light-signaling pathways and circadian clock regulation and their roles in chilling tolerance remains elusive. Further investigation into the regulatory network of chilling tolerance in rice is needed, as knowledge of the interaction between temperature, light, and circadian clock dynamics is limited. Here, based on phenotypic analysis of transgenic and mutant rice lines, we delineate the relevant genes with important regulatory roles in chilling tolerance. In addition, we discuss the potential coordination mechanism among temperature, light, and circadian clock in regulating chilling response and tolerance of rice, and provide perspectives for the ongoing chilling signaling network research in rice.

Global analysis of differentially expressed genes between two Japonica rice varieties induced by low temperature during the booting stage by RNA-Seq

As one of the abiotic stresses, low temperature severely threatens rice production during its entire growth period, especially during the booting stage. In the present study, transcriptome analysis was performed comparing Longjing (LJ) 25 (chilling-tolerant) and LJ 11 (chilling-sensitive) rice varieties to identify genes associated with chilling tolerance in rice spikelets. A total of 23 845 expressed genes and 13 205 differentially expressed genes (DEGs) were identified, respectively. Gene ontology (GO) enrichment analyses revealed 'response to cold' (containing 180 DEGs) as the only category enriched in both varieties during the entire cold treatment period. Through MapMan analysis, we identified nine and six DEGs related to the Calvin cycle and antioxidant enzymes, respectively, including OsRBCS3, OsRBCS2, OsRBCS4, OsAPX2 and OsCATC, that under chilling stress were markedly downregulated in LJ11 compared with LJ25. Furthermore, we predicted their protein-protein interaction (PPI) network and identified nine hub genes (the threshold of co-expressed gene number ≥ 11) in Cytoscape, including three RuBisCO-related genes with 14 co-expressed genes. Under chilling stress, antioxidant enzyme activities (peroxidase (POD) and catalase (CAT)) were downregulated in LJ11 compared with LJ25. However, the content of malondialdehyde (MDA) was higher in LJ11 compared with LJ25. Collectively, our findings identify low temperature responsive genes that can be effectively used as candidate genes for molecular breeding programmes to increase the chilling tolerance of rice.

The role of cyanide-resistant respiration in Solanum tuberosum L. against high light stress

Cyanide-resistant respiration in potato mitochondria is an important pathway for energy dissipation. It can be activated by high light; however, it is unclear what roles cyanide-resistant respiration plays in the response to high light stress in potato. We designed a CRISPR vector for the functional gene StAOX of the potato cyanide-resistant respiratory pathway. Agrobacterium tumefaciens GV3101 was transformed into potato. Hydrogen peroxide level, MDA content, antioxidant activity and cyanide-resistant respiratory capacity of potato leaves under high light stress were determined. Photosynthetic efficiency and chlorophyll content were determined. In addition, the operation of the malate-oxaloacetate shuttle route and transcription level of photorespiration-related enzymes were also examined. The results showed that two base substitutions occurred at the sequencing target site on leaves of the transformed potato. Accumulation of ROS and increased membrane lipid peroxidation were detected in the transformed potato leaves and lower photosynthetic efficiency was observed. The transcription level of the malate-oxaloacetate shuttle route and photorespiration-related enzymes also significantly increased. These results indicate that the cyanide-resistant respiration is an important physiological pathway in potato in response to high light stress. It also suggests that plant cyanide-resistant respiration is closely related to photosynthesis. This implies the unexplored importance of plant cyanide-resistant respiration in plant photosynthesis, energy conversion and carbon skeleton formation.

An APETALA2/ethylene responsive factor, OsEBP89 knockout enhances adaptation to direct-seeding on wet land and tolerance to drought stress in rice

Water stress is the most important adverse factor limiting rice production. Too much water leads to flood and too little leads to drought. Floods and droughts can severely damage crop at different times of the rice life cycle. So the research on submergence tolerance and drought resistance of rice is particularly urgent. In this study, we reported that OsEBP89 (Oryza sativa Ethylene-responsive element binding protein, clone 89), a member of the AP2/ERF subfamily, is involved in a novel signal transduction associated with the tolerance to drought and submergence stress. OsEBP89 was found to be strongly inhibited by drought stress and promoted by submergence. The OsEBP89 protein was located at the nucleus in the rice protoplast. Loss of OsEBP89 was found to improve the seed germination under submerged conditions and also enhanced the tolerance to drought stress throughout growth stage. Additionally, OsEBP89 knockout rice plants increased the accumulation of proline, improved the ability to scavenge ROS compared to overexpression lines and wild type after PEG treatment. Transcriptome data indicates that knockout of OsEBP89 improved the expression of specific genes in response to adverse factors, such as OsAPX1, OsHsfA3, and OsP5CS. Further results indicate that OsEBP89 can interact with and be phosphorylated by SnRK1α (sucrose non-fermenting-1-related protein kinase-1 gene). These findings provide insight into the mechanism of abiotic stress tolerance, and suggest OsEBP89 as a new genetic engineering resource to improve abiotic stress tolerance in rice.

A point mutation in LTT1 enhances cold tolerance at the booting stage in rice

The cold tolerance of rice at the booting stage is a main factor determining sustainability and regional adaptability. However, relatively few cold tolerance genes have been identified that can be effectively used in breeding programmes. Here, we show that a point mutation in the low-temperature tolerance 1 (LTT1) gene improves cold tolerance by maintaining tapetum degradation and pollen development, by activation of systems that metabolize reactive oxygen species (ROS). Cold-induced ROS accumulation is therefore prevented in the anthers of the ltt1 mutants allowing correct development. In contrast, exposure to cold stress dramatically increases ROS accumulation in the wild type anthers, together with the expression of genes encoding proteins associated with programmed cell death and with the accelerated degradation of the tapetum that ultimately leads to pollen abortion. These results demonstrate that appropriate ROS management is critical for the cold tolerance of rice at the booting stage. Hence, the ltt1 mutation can significantly improve the seed setting ability of cold-sensitive rice varieties under low-temperature stress conditions, with little yield penalty under optimal temperature conditions. This study highlights the importance of a valuable genetic resource that may be applied in rice breeding programmes to enhance cold tolerance.

Microbial inoculation in rice regulates antioxidative reactions and defense related genes to mitigate drought stress

Microbial inoculation in drought challenged rice triggered multipronged steps at enzymatic, non-enzymatic and gene expression level. These multifarious modulations in plants were related to stress tolerance mechanisms. Drought suppressed growth of rice plants but inoculation with Trichoderma, Pseudomonas and their combination minimized the impact of watering regime. Induced PAL gene expression and enzyme activity due to microbial inoculation led to increased accumulation of polyphenolics in plants. Enhanced antioxidant concentration of polyphenolics from microbe inoculated and drought challenged plants showed substantially high values of DPPH, ABTS, Fe-ion reducing power and Fe-ion chelation activity, which established the role of polyphenolic extract as free radical scavengers. Activation of superoxide dismutase that catalyzes superoxide (O2-) and leads to the accumulation of H2O2 was linked with the hypersensitive cell death response in leaves. Microbial inoculation in plants enhanced activity of peroxidase, ascorbate peroxidase, glutathione peroxidase and glutathione reductase enzymes. This has further contributed in reducing ROS burden in plants. Genes of key metabolic pathways including phenylpropanoid (PAL), superoxide dismutation (SODs), H2O2 peroxidation (APX, PO) and oxidative defense response (CAT) were over-expressed due to microbial inoculation. Enhanced expression of OSPiP linked to less-water permeability, drought-adaptation gene DHN and dehydration related stress inducible DREB gene in rice inoculated with microbial inoculants after drought challenge was also reported. The impact of Pseudomonas on gene expression was consistently remained the most prominent. These findings suggested that microbial inoculation directly caused over-expression of genes linked with defense processes in plants challenged with drought stress. Enhanced enzymatic and non-enzymatic antioxidant reactions that helped in minimizing antioxidative load, were the repercussions of enhanced gene expression in microbe inoculated plants. These mechanisms contributed strongly towards stress mitigation. The study demonstrated that microbial inoculants were successful in improving intrinsic biochemical and molecular capabilities of rice plants under stress. Results encouraged us to advocate that the practice of growing plants with microbial inoculants may find strategic place in raising crops under abiotic stressed environments.

Reactive oxygen species induce cyanide-resistant respiration in potato infected by Erwinia carotovora subsp. Carotovora

Studies have shown that pathogenic bacteria infections induce the overproduction of reactive oxygen species (ROS) in plants. Cyanide-resistant respiration, an energy-dissipating pathway in plants, has also been induced by a pathogenic bacteria infection. However, it is unknown whether the induction of cyanide-resistant respiration under the pathogenic bacteria infection was caused by ROS. In this study, two pathogenic Erwinia strains were used to infect potato tuber, and membrane lipid peroxidation levels and the cyanide-resistant respiration capacity were determined. In addition, StAOX expression and regulation by ROS in potato tuber were analyzed. Moreover, the role of the Ca²⁺ pathway in regulating cyanide-resistant respiration was determined. The results showed that ROS induced cyanide-resistant respiration in potato tuber infected by Erwinia. Cyanide-resistant respiration inhibited the production of H₂O₂. Intracellular Ca²⁺ regulated the expression of calcium-dependent protein kinase (StCDPK1, StCDPK4, and StCDPK5) in potato, which indirectly controlled intracellular ROS levels. These results indicate that Ca²⁺ metabolism is involved in ROS-induced cyanide-resistant respiration.

Overexpression of Rice Expansin7 (Osexpa7) Confers Enhanced Tolerance to Salt Stress in Rice

Expansins are key regulators of cell-wall extension and are also involved in the abiotic stress response. In this study, we evaluated the function of OsEXPA7 involved in salt stress tolerance. Phenotypic analysis showed that OsEXPA7 overexpression remarkably enhanced tolerance to salt stress. OsEXPA7 was highly expressed in the shoot apical meristem, root, and the leaf sheath. Promoter activity of OsEXPA7:GUS was mainly observed in vascular tissues of roots and leaves. Morphological analysis revealed structural alterations in the root and leaf vasculature of OsEXPA7 overexpressing (OX) lines. OsEXPA7 overexpression resulted in decreased sodium ion (Na⁺) and accumulated potassium ion (K⁺) in the leaves and roots. Under salt stress, higher antioxidant activity was also observed in the OsEXPA7-OX lines, as indicated by lower reactive oxygen species (ROS) accumulation and increased antioxidant activity, when compared with the wild-type (WT) plants. In addition, transcriptional analysis using RNA-seq and RT-PCR revealed that genes involved in cation exchange, auxin signaling, cell-wall modification, and transcription were differentially expressed between the OX and WT lines. Notably, salt overly sensitive 1, which is a sodium transporter, was highly upregulated in the OX lines. These results suggest that OsEXPA7 plays an important role in increasing salt stress tolerance by coordinating sodium transport, ROS scavenging, and cell-wall loosening.

Regulation of carbon metabolism in two maize sister lines contrasted for chilling tolerance

Maize can grow in cool temperate climates but is often exposed to spring chilling temperatures that can affect early seedling growth. Here, we used two sister double-haploid lines displaying a contrasted tolerance to chilling to identify major determinants of long-term chilling tolerance. The chilling-sensitive (CS) and the chilling-tolerant (CT) lines were grown at 14 °C day/10 °C night for 60 d. CS plants displayed a strong reduction in growth and aerial biomass compared with CT plants. Photosynthetic efficiency was affected with an increase in energy dissipation in both lines. Chilling tolerance in CT plants was associated with higher chlorophyll content, glucose-6-phosphate dehydrogenase activity, and higher sucrose to starch ratio. Few changes in cell wall composition were observed in both genotypes. There was no obvious correlation between nucleotide sugar content and cell wall polysaccharide composition. Our findings suggest that the central starch-sucrose metabolism is one major determinant of the response to low temperature, and its modulation accounts for the ability of CT plants to cope with low temperature. This modulation seemed to be linked to a strong alteration in the biosynthesis of nucleotide sugars that, at a high level, could reflect the remobilization of carbon in response to chilling.

Metabolic profile analysis and identification of key metabolites during rice seed germination under low-temperature stress

The metabolic profile of rice (Oryza sativa) during germination under low temperature (LT) has not been reported. In this study, the rice varieties 02428 (japonica) and YZX (indica) were subjected to experiments consisting of treatments including LT, normal temperature (NT) and a transition from LT to NT, and tissues were sampled at different time points during germination. A total of 730 metabolites were detected by a liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based widely targeted metabolomics method. On the basis of the screening criteria of increased contents under LT and decreased contents under NT, we identified 35 different metabolites that responded to LT stress among the 730 metabolites. Furthermore, the content differences of the 35 metabolites were compared when the samples were transferred from LT to NT. According to a fold change <0.5 or a variable importance in projection (VIP) score>1 at the transition point, 7 out of the 35 metabolites responded significantly to LT stress and were defined as key metabolites. A partial least squares (PLS) regression model of seven key metabolites with seedling length (SL), seedling area (SSA), and seedling volume (SV) was constructed, and the fitting effect was good. These seven key metabolites participate in the biosynthesis of amino acids and phenylpropanoids and in the metabolism of glutathione and inositol phosphate. This study laid a foundation for an improved understanding of the LT-germination mechanism of rice seeds.

Isolation and functional characterization of three abiotic stress-inducible (Apx, Dhn and Hsc70) promoters from pearl millet (Pennisetum glaucum L.)

Pearl millet is a C4 cereal crop that grows in arid and semi-arid climatic conditions with the remarkable abiotic stress tolerance. It contributed to the understanding of stress tolerance not only at the physiological level but also at the genetic level. In the present study, we functionally cloned and characterized three abiotic stress-inducible promoters namely cytoplasmic Apx1 (Ascorbate peroxidase), Dhn (Dehydrin), and Hsc70 (Heat shock cognate) from pearl millet. Sequence analysis revealed that all three promoters have several cis-acting elements specific for temporal and spatial expression. PgApx pro, PgDhn pro and PgHsc70 pro were fused with uidA gene in Gateway-based plant transformation pMDC164 vector and transferred into tobacco through leaf-disc method. While PgApx pro and PgDhn pro were active in seedling stages, PgHsc70 pro was active in stem and root tissues of the T2 transgenic tobacco plants under control conditions. Higher activity was observed under high temperature and drought, and less in salt and cold stress conditions. Further, all three promoters displayed higher GUS gene expression in the stem, moderate expression in roots, and less expression in leaves under similar conditions. While RT-qPCR data showed that PgApx pro and PgDhn pro were expressed highly in high temperature, salt and drought, PgHsc70 pro was fairly expressed during high temperature stress only. Histochemical and RT-qPCR assays showed that all three promoters are inducible under abiotic stress conditions. Thus, these promoters appear to be immediate candidates for developing abiotic stress tolerant crops as these promoter-driven transgenics confer high degree of tolerance in comparison with the wild-type (WT) plants.

Stress-induced changes in the expression of antioxidant system genes for rice (Oryza sativa L.) and bread wheat (Triticum aestivum L.)

BACKGROUND

Plant cell metabolism inevitably forms reactive oxygen species (ROS), which can damage cells or lead to their death. The antioxidant system (AOS) evolved to eliminate a high concentration of ROS. For plants, this system consists of the seven classes of antioxidant enzymes and antioxidant compounds. Each enzymatic class contains a various number of genes which may vary from species to species. In such a multi-copy genetic system, the integration of evolutionary characteristics and expression data makes it possible to effectively predict promising breeding targets for the design of highly-yielding cultivars. In the plant cells, ROS production can increase as a result of abiotic stresses. Accordingly, AOS responds to stress by altering the expression of the genes of its components. Expression profiles of AOS enzymes, including their changes under stress, remains incomplete. A comprehensive study of the system behavior in response to stress for different species gives the key to identify the general mechanisms of AOS regulation. In this article, we studied stress-induced changes in the expression of AOS genes in photosynthetic tissues for rice and bread wheat.

METHODS

A meta-analysis of genome-wide transcriptome data on stress-induced changes in expression profiles of antioxidant genes using microarray and next generation sequencing (NGS) experiments from the GEO NCBI database for rice and bread wheat was carried out. Experimental study of expression changes in short (6 h) and prolonged (24 h) cold stress responses for selected AOS genes of bread wheat cultivars Saratovskaya29 and Yanetzkis Probat was conducted using qPCR.

RESULTS

The large-scale meta-transcriptome and complementary experimental analysis revealed a summary of fold changes in the AOS gene expression in response to cold and water deficiency for rice and bread wheat.

Cloning and overexpression of the ascorbate peroxidase gene from the yam (Dioscorea alata) enhances chilling and flood tolerance in transgenic Arabidopsis

Minghuai 1 (MH1) is a yam (Dioscorea alata) cultivar with high tolerance to flooding but sensitivity to chilling. MH1 responded differently to chilling and flooding according to various physiological parameters and antioxidant enzymes. Flooding led to an increase in ascorbate peroxidase (APX) activity in both roots and leaves, while chilling did not affect APX activity. The full length DaAPX ORF sequence from MH1 (750 bp) was then cloned. Phylogenetic analysis showed that plant cytosolic APXs into four major clusters and DaAPX was closely related to Oncidium. The DaAPX gene driven by a 35S promoter was transferred into Arabidopsis. The gene expression and enzyme activity of APX in the DaAPX transgenic lines 1-3 were significantly higher than in wild type (WT) plants. Compared to WT plants, seedling growth characteristics were significantly better in all transgenic lines under chilling, flooding, and oxidative stresses, indicating that the overexpression of DaAPX in Arabidopsis enhanced tolerance to several abiotic stresses. MH1 plants supplied with H₂O₂ presented an increase in the activity of APX leading to enhanced tolerance to chilling. Functional characterization of the APX gene should improve our understanding of the chilling- and flood-response mechanism in the yam.

Knockout of OsPRP1, a gene encoding proline-rich protein, confers enhanced cold sensitivity in rice (Oryza sativa L.) at the seedling stage

Proline-rich proteins (PRPs) play multiple physiological and biochemical roles in plant growth and stress response. In this study, we reported that the knockout of OsPRP1 induced cold sensitivity in rice. Mutant plants were generated by CRISPR/Cas9 technology to investigate the role of OsPRP1 in cold stress and 26 mutant plants were obtained in T₀ generation with the mutation rate of 85% including 15% bi-allelic, 53.3% homozygous, and 16.7% heterozygous and 16 T-DNA-free lines in T₁ generation. The conserved amino acid sequence was changed and the expression level of OsPRP1 was reduced in mutant plants. The OsPRP1 mutant plants displayed more sensitivity to cold stress and showed low survival rate with decreased root biomass than wild-type (WT) and homozygous mutant line with large fragment deletion was more sensitive to low temperature. Mutant lines accumulated less antioxidant enzyme activity and lower levels of proline, chlorophyll, abscisic acid (ABA), and ascorbic acid (AsA) content relative to WT under low-temperature stress. The changes of antioxidant enzymes were examined in the leaves and roots with exogenous salicylic acid (SA) treatment which resulted in increased activity of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) under cold stress, while enzyme antioxidant activity was lower in untreated seedlings which showed that exogenous SA pretreatment could alleviate the low-temperature stress in rice. Furthermore, the expression of three genes encoding antioxidant enzyme activities (SOD4, POX1, and OsCAT3) was significantly down-regulated in the mutant lines as compared to WT. These results suggested that OsPRP1 enhances cold tolerance by modulating antioxidants and maintaining cross talk through signaling pathways. Therefore, OsPRP1 gene could be exploited for improving cold tolerance in rice and CRISPR/Cas9 technology is helpful to study the function of a gene by analyzing the phenotypes of knockout mutants generated.

Cold tolerance response mechanisms revealed through comparative analysis of gene and protein expression in multiple rice genotypes

Due to its tropical origin and adaptation, rice is significantly impacted by cold stress, and consequently sustains large losses in growth and productivity. Currently, rice is the second most consumed cereal in the world and production losses caused by extreme temperature events in the context of "major climatic changes" can have major impacts on the world economy. We report here an analysis of rice genotypes in response to low-temperature stress, studied through physiological gas-exchange parameters, biochemical changes in photosynthetic pigments and antioxidants, and at the level of gene and protein expression, towards an understanding and identification of multiple low-temperature tolerance mechanisms. The first effects of cold stress were observed on photosynthesis among all genotypes. However, the tropical japonica genotypes Secano do Brazil and Cypress had a greater reduction in gas exchange parameters like photosynthesis and water use efficiency in comparison to the temperate japonica Nipponbare and M202 genotypes. The analysis of biochemical profiles showed that despite the impacts of low temperature on tolerant plants, they quickly adjusted to maintain their cellular homeostasis by an accumulation of antioxidants and osmolytes like phenolic compounds and proline. The cold tolerant and sensitive genotypes showed a clear difference in gene expression at the transcript level for OsGH3-2, OsSRO1a, OsZFP245, and OsTPP1, as well as for expression at the protein level for LRR-RLKs, bHLH, GLYI, and LTP1 proteins. This study exemplifies the cold tolerant features of the temperate japonica Nipponbare and M202 genotypes, as observed through the analysis of physiological and biochemical responses and the associated changes in gene and protein expression patterns. The genes and proteins showing differential expression response are notable candidates towards understanding the biological pathways affected in rice and for engineering cold tolerance, to generate cultivars capable of maintaining growth, development, and reproduction under cold stress. We also propose that the mechanisms of action of the genes analyzed are associated with the tolerance response.

Effect of Rhizobium Symbiosis on Low-Temperature Tolerance and Antioxidant Response in Alfalfa (Medicago sativa L.)

Low temperature-induced stress is a major environmental factor limiting the growth and development of plants. Alfalfa (Medicago sativa L.) is a legume well known for its tolerance of extreme environments. In this study, we sought to experimentally investigate the role of rhizobium symbiosis in alfalfa's performance under a low-temperature stress condition. To do this, alfalfa "Ladak+" plants carrying active nodules (AN), inactive nodules (IN), or no nodules (NN) were exposed to an imposed low temperature stress and their survivorship calculated. The antioxidant defense responses, the accumulation of osmotic regulation substances, the cell membrane damage, and the expression of low temperature stress-related genes were determined in both the roots and the shoots of alfalfa plants. We found that more plants with AN survived than those with IN or NN under the same low temperature-stress condition. Greater activity of oxidation protective enzymes was observed in the AN and IN groups, conferring higher tolerance to low temperature in these plants. In addition, rhizobia nodulation also enhanced alfalfa's ability to tolerate low temperature by altering the expression of regulatory and metabolism-associated genes, which resulted in the accumulation of soluble proteins and sugars in the nodulated plants. Taken together, the findings of this study indicate that rhizobium inoculation offers a practical way to promote the persistence and growth potential of alfalfa "Ladak+" in cold areas.

PGL3 is required for chlorophyll synthesis and impacts leaf senescence in rice

Rice leaf color mutants play a great role in research about the formation and development of chloroplasts and the genetic mechanism of the chlorophyll (Chl) metabolism pathway. pgl3 is a rice leaf color mutant derived from Xiushui11 (Oryza sativa L. spp. japonica), treated with ethyl methane sulfonate (EMS). The mutant exhibited a pale-green leaf (pgl) phenotype throughout the whole development as well as reduced grain quality. Map-based cloning of PGL3 revealed that it encodes the chloroplast signal recognition particle 43 kDa protein (cpSRP43). PGL3 affected the Chl synthesis by regulating the expression levels of the Chl synthesis-associated genes. Considerable reactive oxygen species were accumulated in the leaves of pgl3, and the transcription levels of its scavenging genes were down-regulated, indicating that pgl3 can accelerate senescence. In addition, high temperatures could inhibit the plant's growth and facilitate the process of senescence in pgl3.