Enhanced antioxidant enzyme activities in developing anther contributes to heat stress alleviation and sustains grain yield in wheat

Physiological and Transcriptome Analyses Reveal the Protective Effect of Exogenous Trehalose in Response to Heat Stress in Tea Plant (Camellia sinensis)

It is well known that application of exogenous trehalose can enhance the heat resistance of plants. To investigate the underlying molecular mechanisms by which exogenous trehalose induces heat resistance in C. sinensis, a combination of physiological and transcriptome analyses was conducted. The findings revealed a significant increase in the activity of superoxide dismutase (SOD) and peroxidase (POD) upon treatment with 5.0 mM trehalose at different time points. Moreover, the contents of proline (PRO), endogenous trehalose, and soluble sugar exhibited a significant increase, while malondialdehyde (MDA) content decreased following treatment with 5.0 mM trehalose under 24 h high-temperature stress (38 °C/29 °C, 12 h/12 h). RNA-seq analysis demonstrated that the differentially expressed genes (DEGs) were significantly enriched in the MAPK pathway, plant hormone signal transduction, phenylpropanoid biosynthesis, flavone and flavonol biosynthesis, flavonoid biosynthesis, and the galactose metabolism pathway. The capability to scavenge free radicals was enhanced, and the expression of a heat shock factor gene (HSFB2B) and two heat shock protein genes (HSP18.1 and HSP26.5) were upregulated in the tea plant. Consequently, it was concluded that exogenous trehalose contributes to alleviating heat stress in C. sinensis. Furthermore, it regulates the expression of genes involved in diverse pathways crucial for C. sinensis under heat-stress conditions. These findings provide novel insights into the molecular mechanisms underlying the alleviation of heat stress in C. sinensis with trehalose.

Early signaling enhance heat tolerance in Arabidopsis through modulating jasmonic acid synthesis mediated by HSFA2

Given the detrimental impact of global warming on crop production, it is particularly important to understand how plants respond and adapt to higher temperatures. Using the non-invasive micro-test technique and laser confocal microscopy, we found that the cascade process of early signals (K+, H2O2, H+, and Ca2+) ultimately resulted in an increase in the cytoplasmic Ca2+ concentration when Arabidopsis was exposed to heat stress. Quantitative real-time PCR demonstrated that heat stress significantly up-regulated the expression of CAM1, CAM3 and HSFA2; however, after CAM1 and CAM3 mutation, the upregulation of HSFA2 was reduced. In addition, heat stress affected the expression of LOX3 and OPR3, which was not observed when HSFA2 was mutated. Luciferase reporter gene expression assay and electrophoretic mobility shift assay showed that HSFA2 regulated the expression of both genes. Determination of jasmonic acid (JA) content showed that JA synthesis was promoted by heat stress, but was damaged when HSFA2 and OPR3 were mutated. Finally, physiological experiments showed that JA reduced the relative electrical conductivity of leaves, enhanced chlorophyll content and relative water content, and improved the survival rate of Arabidopsis under heat stress. Together, our results reveal a new pathway for Arabidopsis to sense and transmit heat signals; HSFA2 is involved in the JA synthesis, which can act as a defensive compound improving Arabidopsis heat tolerance.

Effects of Enhanced Resistance and Transcriptome Analysis of Twig Blight Disease by Exogenous Brassinolide in Myrica rubra

Twig blight disease is the primary disease that affects the production of Myrica rubra in China. It was reported that exogenous brassinolide (BL) can improve disease resistance in plants. Here, we examined the effects of exogenous BL on disease resistance, chlorophyll contents, antioxidant enzyme activity, ROS accumulation, and key gene expression of M. rubra to analyze the mechanism of BR-induced resistance of twig blight disease in M. rubra. The results demonstrated that 2.0 mg·L-1 of BL could significantly lessen the severity of twig blight disease in M. rubra. Exogenous BL increased the contents of chlorophyll a, chlorophyll b, carotenoids, and total chlorophyll. Moreover, exogenous BL also significantly enhanced the activity of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), and decreased malondialdehyde (MDA) content and reactive oxygen species (ROS) accumulation in leaves, such as H2O2 and O2·-. Additionally, exogenous BL dramatically up-regulated the expression of pathogenesis-related (PR) genes such as MrPR1, MrPR2, and MrPR10, as well as important genes such as MrBAK1, MrBRI1, and MrBZR1 involved in brassinosteroid (BR) signaling pathway. The transcriptome analysis revealed that a total of 730 common differentially expressed genes (DEGs) under BL treatment were found, and these DEGs were primarily enriched in four Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. Based on these findings, nine important candidate genes related to the resistance of twig blight disease under BL treatment were further identified. In this study, we elucidated the effects of exogenous BL on enhancing the resistance of M. rubra to twig blight disease and preliminary analyzed the potential mechanism of resistance induction, which will provide a crucial foundation for the management and prevention of twig blight disease in M. rubra.

Exogenous abscisic acid improves grain filling capacity under heat stress by enhancing antioxidative defense capability in rice

BACKGROUND

Heat stress is a major restrictive factor that causes yield loss in rice. We previously reported the priming effect of abscisic acid (ABA) on rice for enhanced thermotolerance at the germination, seedling and heading stages. In the present study, we aimed to understand the priming effect and mechanism of ABA on grain filling capacity in rice under heat stress.

RESULTS

Rice plants were pretreated with distilled water, 50 μM ABA and 10 μM fluridone by leaf spraying at 8 d or 15 d after initial heading (AIH) stage and then were subjected to heat stress conditions of 38 °C day/30 °C night for 7 days, respectively. Exogenous ABA pretreatment significantly super-activated the ABA signaling pathway and improved the SOD, POD, CAT and APX enzyme activity levels, as well as upregulated the ROS-scavenging genes; and decreased the heat stress-induced ROS content (O2- and H2O2) by 15.0-25.5% in rice grain under heat stress. ABA pretreatment also increased starch synthetase activities in rice grain under heat stress. Furthermore, ABA pretreatment significantly improved yield component indices and grain yield by 14.4-16.5% under heat stress. ABA pretreatment improved the milling quality and the quality of appearance and decreased the incidence of chalky kernels and chalkiness in rice grain and improved the rice grain cooking quality by improving starch content and gel consistence and decreasing the amylose percentage under heat stress. The application of paraquat caused overaccumulation of ROS, decreased starch synthetase activities and ultimately decreased starch content and grain yield. Exogenous antioxidants decreased ROS overaccumulation and increased starch content and grain yield under heat stress.

CONCLUSION

Taken together, these results suggest that exogenous ABA has a potential priming effect for enhancing rice grain filling capacity under heat stress at grain filling stage mainly by inhibiting ROS overaccumulation and improving starch synthetase activities in rice grain.

Integrated application of plant bioregulator and micronutrients improves crop physiology, productivity and grain biofortification of delayed sown wheat

Delay sowing of wheat is a common problem in Punjab that exacerbates serious yield loss. To reduce yield loss and improve yield, different combinations of foliar-applied bioregulator and micronutrients, control (CK), zinc (Zn), boron (B), thiourea (TU), Zn + B (ZnB), Zn + TU (ZnTU), B + TU (BTU), Zn + B + TU (ZnBTU) were applied at booting and grain filling stages in delayed sown wheat in 2017-18 and 2018-19. The results showed that ZnBTU treatment significantly increased leaf area index by 25.06% and 23.21%, spike length by 15.11% and 19.65% in 2017 and 2018, respectively, compared to CK. The ZnBTU treatment also increased 1000-grain weight by 21.96% and 22.01% in 2017 and 2018, respectively, compared to CK. Similarly, higher Zn, B and N contents in straw and grain were recoded for ZnBTU treatment which was statistically similar to ZnB and ZnTU treatments. Overall, ZnBTU treatment also increased the photosynthetic rate, transpiration rate, stomatal conductance by 46.67%, 26.03%, 76.25% and decreased internal CO2 by 28.18%, compared to CK, respectively. Moreover, ZnBTU also recorded the highest grain yield in 2017-18 (25.05%) and 2018-19 (28.49%) than CK. In conclusion, foliar application of ZnBTU at the booting and grain filling stages of delayed sown wheat could be a promising strategy to increase grain yield.

Reproductive-Stage Heat Stress in Cereals: Impact, Plant Responses and Strategies for Tolerance Improvement

Reproductive-stage heat stress (RSHS) poses a major constraint to cereal crop production by damaging main plant reproductive structures and hampering reproductive processes, including pollen and stigma viability, pollination, fertilization, grain setting and grain filling. Despite this well-recognized fact, research on crop heat stress (HS) is relatively recent compared to other abiotic stresses, such as drought and salinity, and in particular, RSHS studies in cereals are considerably few in comparison with seedling-stage and vegetative-stage-centered studies. Meanwhile, climate change-exacerbated HS, independently or synergistically with drought, will have huge implications on crop performance and future global food security. Fortunately, due to their sedentary nature, crop plants have evolved complex and diverse transient and long-term mechanisms to perceive, transduce, respond and adapt to HS at the molecular, cell, physiological and whole plant levels. Therefore, uncovering the molecular and physiological mechanisms governing plant response and tolerance to RSHS facilitates the designing of effective strategies to improve HS tolerance in cereal crops. In this review, we update our understanding of several aspects of RSHS in cereals, particularly impacts on physiological processes and yield; HS signal perception and transduction; and transcriptional regulation by heat shock factors and heat stress-responsive genes. We also discuss the epigenetic, post-translational modification and HS memory mechanisms modulating plant HS tolerance. Moreover, we offer a critical set of strategies (encompassing genomics and plant breeding, transgenesis, omics and agronomy) that could accelerate the development of RSHS-resilient cereal crop cultivars. We underline that a judicious combination of all of these strategies offers the best foot forward in RSHS tolerance improvement in cereals. Further, we highlight critical shortcomings to RSHS tolerance investigations in cereals and propositions for their circumvention, as well as some knowledge gaps, which should guide future research priorities. Overall, our review furthers our understanding of HS tolerance in plants and supports the rational designing of RSHS-tolerant cereal crop cultivars for the warming climate.

Exploring the significant contribution of silicon in regulation of cellular redox homeostasis for conferring stress tolerance in plants

Silicon (Si), a bioactive metalloid is beneficial for plant growth and development. It also plays a key role in the amelioration of different abiotic and biotic stresses. Extensive studies have elucidated the morpho-physiological, biochemical and molecular background of Si-mediated stress tolerance in plants. However, the mechanism acquired by Si to enhance stress tolerance in plants is still unheeded. Present review summarized the prospective mechanisms of Si in acquisition of stress tolerance with emphasis on its interactions with secondary messengers. Silicon usually modulates the different gene expressions in plants under stress conditions rather than acting as a direct signal or secondary messengers. Silicon regulates the production and accumulation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in plants under stress conditions. Furthermore, Si also activates the antioxidant defence system in plants; thereby, maintaining the cellular redox homeostasis and preventing the oxidative damage of cells. Silicon also up-regulates the synthesis of hydrogen sulfide (H₂S) or acts synergistically with nitric oxide (NO), consequently conferring stress tolerance in plants. Overall, the review may provide a progressive understanding of the role of Si in conservation of the redox homeostasis in plants.

Redox imbalance impedes photosynthetic activity in rice by disrupting cellular membrane integrity and induces programmed cell death under submergence

Climate change negatively impacts the global hydrological resources leading to detrimental flood events. Submergence impedes the cellular membrane integrity, consequently affecting the membrane fluidity. Different abiotic stresses influence membrane lipid composition. Therefore, the remodeling of membrane lipids plays a major role in stress adaptation. Submergence-induced membrane lipid peroxidation is well established in plants. However, dynamic changes in lipid composition for regulating submergence tolerance in rice remain so far unexplored. The present study explored the effect of submergence on the lipidomic profile of the Sub1 near-isogenic lines (NILs) of rice, viz. Swarna, and Swarna Sub1 with contrasting submergence tolerance. The study also examined the association of lipidomic alteration with the membrane integrity and submergence tolerance. Submergence caused increased accumulation of reactive oxygen species (ROS), which was significantly higher in Swarna than Swarna Sub1. The lipid profile was also considerably altered under submergence. Following submergence, Swarna exhibited a significant decrease in phospholipid content accompanied by increased lipid peroxidation and electrolyte leakage. Furthermore, the disintegration of the thylakoid membrane resulted in a significant decrease in the chlorophyll content and photosynthesis rate under submergence. Submergence-induced hypoxic condition also promoted starch depletion to fulfill the energy requirement. In contrast, submergence acclimation in Swarna Sub1 was associated with the shift to anaerobic respiration mediated by increased alcohol dehydrogenase (ADH) activity. Effective ROS detoxification in Swarna Sub1 facilitated by increased antioxidant enzyme activities contributed to the submergence tolerance by maintaining membrane integrity and photosynthetic activity. The present study established the direct association of lipid remodeling with membrane integrity, cell viability, and photosynthesis and also devised a crop model to reveal the molecular background of submergence tolerance in plants.

Nitro-oxidative stress induces the formation of roots' cortical aerenchyma in rice under osmotic stress

Drought stress induces the formation of cortical aerenchyma in roots, providing drought tolerance by reducing respiration. However, unrestricted aerenchyma formation impedes the radial transport of water through the root's central cylinder; thereby decreasing the water uptake under drought stress. Therefore, exploring the root architectural and anatomical alterations in rice under drought is essential for targeting crop improvement. Drought stress-induced accumulation of reactive oxygen species (ROS) plays a key role in the lysigenous aerenchyma development. However, the influence of nitric oxide (NO) and reactive nitrogen species (RNS) in the development of lysigenous aerenchyma under drought has never been studied in rice. The present study examined the effect of ROS and RNS, generated by progressive drought stress, on the lysigenous aerenchyma formation in the roots of contrasting rice genotypes of the Eastern Indo-Gangetic plains (EIGP). As expected, the PEG-induced drought stress stimulated the expression of NADPH oxidase (NOX), thereby promoting the ROS generation in roots of the rice seedlings. Excessive ROS and RNS accumulations in roots affected the membrane lipids, promoting the tissue-specific programmed cell death (PCD) in rice. The activation of the antioxidant defense system played a major role in the ROS and RNS detoxification, thereby restricting the root aerenchyma formation in rice under drought stress. The results also displayed that drought tolerance in rice is associated with the formation of the Casparian strip, which limits the apoplastic flow of water in the water-deficient roots. Overall, our study revealed the association of nitro-oxidative metabolism with PCD and lysigenous aerenchyma formation in the cortical cells of root under drought stress in rice.

Elevated CO2 modulates the effect of heat stress responses in Triticum aestivum by differential expression of isoflavone reductase-like (IRL) gene

Two wheat genotypes forming high and low biomass (HB and LB), exhibiting differential expression of an isoflavone reductase-like (IRL) gene, and resulting in contrasting grain yield under heat stress field conditions, were analyzed in detail for their responses under controlled heat and elevated CO2 conditions. Significant differences in IRL expression between the two lines were hypothesized to be the basis of their differential performance under the tested conditions and their stress tolerance potential. By a holistic approach integrating advanced cell physiological phenotyping of the antioxidative and phytohormone system in spikes and leaves with measurements of ecophysiological and agronomic traits, the genetic differences of the genotypes in IRL expression were assessed. In response to heat and elevated CO2, the two genotypes showed opposite regulation of IRL expression, which was associated with cytokinin concentration, total flavonoid contents, activity of superoxide dismutase, antioxidant capacity and photosynthetic rate in leaves and cytokinin concentration and ascorbate peroxidase activity in spikes. Our study showed that IRL expression is associated with wheat yield performance under heat stress at anthesis, mediated by diverse physiological mechanisms. Hence, based on our results, the IRL gene is a promising candidate for developing genetic markers for breeding heat-tolerant wheat.

Glycinebetaine mitigated the photoinhibition of photosystem II at high temperature in transgenic tomato plants

Photosystem II (PSII), especially the D1 protein, is highly sensitive to the detrimental impact of heat stress. Photoinhibition always occurs when the rate of photodamage exceeds the rate of D1 protein repair. Here, genetically engineered codA-tomato with the capability to accumulate glycinebetaine (GB) was established. After photoinhibition treatment at high temperature, the transgenic lines displayed more thermotolerance to heat-induced photoinhibition than the control line. GB maintained high expression of LeFtsHs and LeDegs and degraded the damaged D1 protein in time. Meanwhile, the increased transcription of synthesis-related genes accelerated the de novo synthesis of D1 protein. Low ROS accumulation reduced the inhibition of D1 protein translation in the transgenic plants, thereby reducing protein damage. The increased D1 protein content and decreased phosphorylated D1 protein (pD1) in the transgenic plants compared with control plants imply that GB may minimize photodamage and maximize D1 protein stability. As D1 protein exhibits a high turnover, PSII maybe repaired rapidly and efficiently in transgenic plants under photoinhibition treatment at high temperature, with the resultant mitigation of photoinhibition of PSII.