Modes of Brassinosteroid Activity in Cold Stress Tolerance

Cold-stress induced metabolomic and transcriptomic changes in leaves of three mango varieties with different cold tolerance

BACKGROUND

Mango (Mangifera indica L.) is grown in Hainan, Guangdong, Yunnan, Sichuan, and Fujian provinces and Guanxi autonomous region of China. However, trees growing in these areas suffer severe cold stress during winter, which affects the yield. To this regard, data on global metabolome and transcriptome profiles of leaves are limited. Here, we used combined metabolome and transcriptome analyses of leaves of three mango cultivars with different cold stress tolerance, i.e. Jinhuang (J)-tolerant, Tainung (T) and Guiremang No. 82 (G)-susceptible, after 24 (LF), 48 (MF) and 72 (HF) hours of cold.

RESULTS

A total of 1,323 metabolites belonging to 12 compound classes were detected. Of these, amino acids and derivatives, nucleotides and derivatives, and lipids accumulated in higher quantities after cold stress exposure in the three cultivars. Notably, Jinhuang leaves showed increasing accumulation trends of flavonoids, terpenoids, lignans and coumarins, and alkaloids with exposure time. Among the phytohormones, jasmonic acid and abscisic acid levels decreased, while N6-isopentenyladenine increased with cold stress time. Transcriptome analysis led to the identification of 22,526 differentially expressed genes. Many genes enriched in photosynthesis, antenna proteins, flavonoid, terpenoid (di- and sesquiterpenoids) and alkaloid biosynthesis pathways were upregulated in Jihuang leaves. Moreover, expression changes related to phytohormones, MAPK (including calcium and H2O2), and the ICE-CBF-COR signalling cascade indicate involvement of these pathways in cold stress responses.

CONCLUSION

Cold stress tolerance in mango leaves is associated with regulation of primary and secondary metabolite biosynthesis pathways. Jasmonic acid, abscisic acid, and cytokinins are potential regulators of cold stress responses in mango leaves.

The BAS chromatin remodeler determines brassinosteroid-induced transcriptional activation and plant growth in Arabidopsis

Brassinosteroid (BR) signaling leads to the nuclear accumulation of the BRASSINAZOLE-RESISTANT 1 (BZR1) transcription factor, which plays dual roles in activating or repressing the expression of thousands of genes. BZR1 represses gene expression by recruiting histone deacetylases, but how it activates transcription of BR-induced genes remains unclear. Here, we show that BR reshapes the genome-wide chromatin accessibility landscape, increasing the accessibility of BR-induced genes and reducing the accessibility of BR-repressed genes in Arabidopsis. BZR1 physically interacts with the BRAHMA-associated SWI/SNF (BAS)-chromatin-remodeling complex on the genome and selectively recruits the BAS complex to BR-activated genes. Depletion of BAS abrogates the capacities of BZR1 to increase chromatin accessibility, activate gene expression, and promote cell elongation without affecting BZR1's ability to reduce chromatin accessibility and expression of BR-repressed genes. Together, these data identify that BZR1 recruits the BAS complex to open chromatin and to mediate BR-induced transcriptional activation of growth-promoting genes.

Experimental investigation of freeze injury temperatures in trees and their contributing factors based on electrical impedance spectroscopy

In trees, injuries resulting from subfreezing temperatures can cause damage to the cellular biofilm system, metabolic functions, and fibrous reticulum, and even cell death. Investigating the occurrence of freezing damage and its contributing factors could help understand the mechanisms underlying freezing injury and prevent the subsequent damage in trees. To achieve this, a laboratory experiment was conducted using cut wood samples from Korean pine (Pinus koraiensis Siebold & Zucc) and Simon poplar (Populus simonii Carr.), and the effects of environmental freezing factors, including freezing temperatures, freezing duration, and cooling rate, on the temperature at which freezing injuries occur were examined using the electrical impedance spectroscopy (EIS) method. The semi-lethal temperature (LT50), as an indicator of freezing injury in wood tissue, was theoretically deduced based on the measured extracellular resistance (r e) using EIS. The contributory factors to changes in LT50 were determined and their relationship was established. The results revealed that all freezing factors exhibited significant effects on electrical impedance characteristics (r e, r i, and τ), significantly influencing the LT50 of the wood. Random forest (RF) and support vector machine (SVM) models were used to assess the contribution of the freezing factors and moisture content (MC). Among the factors examined, freezing duration had the greatest impact on LT50, followed by the MC, whereas the contribution of the cooling rate was minimal. The model accuracies were 0.89 and 0.86 for Korean pine and Simon poplar, respectively. The findings of our study illustrate that the occurrence of freezing injury in trees is primarily influenced by the duration of freezing at specific subzero temperatures. Slow cooling combined with prolonged freezing at low subzero temperatures leads to earlier and more severe freezing damage.

Effect of Brassinolide on Soil Microorganisms in Millet Field Polluted by Tribenuron-Methyl

Tribenuron-methyl is used to control broad-leaved weeds and has a promising application prospect in millet fields. However, its negative impact on soil ecology cannot be ignored. Brassinosteroids have been widely reported to enhance plant resistance to stress, but information on brassinosteroids for the remediation of pesticide-contaminated soils is limited. Under field conditions, brassinosteroids were applied to explore their effects on the residues of tribenuron-methyl, soil enzyme activity, soil microbiol community, and millet yield. After applying brassinosteroids according to the dose of 150 mL hm^-2, the degradation rate of tribenuron-methyl accelerated. Brassinolide stimulated the activities of catalase and dehydrogenase, while the activities of sucrase and alkaline phosphatase were inhibited. The results of high-throughput sequencing showed that brassinosteroids inhibited the growth of Verrucomicrobia, Ascomycota, and Mortierellomycota and promoted the abundance of cyanobacteria. Additionally, brassinosteroids could also significantly increase the diversity index and change the community structure of soil bacteria and fungi. Further, the predicted function results indicated that brassinosteroids changed some metabolic-related ecological functions of the soil. We also found that brassinolide could increase millet yields by 2.4% and 13.6%. This study provides a theoretical basis for the safe use of tribenuron-methyl in millet fields and a new idea for the treatment of pesticide residues in soil.

Contents of endogenous brassinosteroids and the response to drought and/or exogenously applied 24-epibrassinolide in two different maize leaves

Exogenously applied brassinosteroids (BRs) improve plant response to drought. However, many important aspects of this process, such as the potential differences caused by different developmental stages of analyzed organs at the beginning of drought, or by BR application before or during drought, remain still unexplored. The same applies for the response of different endogenous BRs belonging to the C₂₇, C₂₈-and C₂₉- structural groups to drought and/or exogenous BRs. This study examines the physiological response of two different leaves (younger and older) of maize plants exposed to drought and treated with 24-epibrassinolide (epiBL), together with the contents of several C₂₇, C₂₈-and C₂₉-BRs. Two timepoints of epiBL application (prior to and during drought) were utilized to ascertain how this could affect plant drought response and the contents of endogenous BRs. Marked differences in the contents of individual BRs between younger and older maize leaves were found: the younger leaves diverted their BR biosynthesis from C₂₈-BRs to C₂₉-BRs, probably at the very early biosynthetic steps, as the levels of C₂₈-BR precursors were very low in these leaves, whereas C₂₉-BR levels vere extremely high. Drought also apparently negatively affected contents of C₂₈-BRs (particularly in the older leaves) and C₂₉-BRs (particularly in the younger leaves) but not C₂₇-BRs. The response of these two types of leaves to the combination of drought exposure and the application of exogenous epiBL differed in some aspects. The older leaves showed accelerated senescence under such conditions reflected in their reduced chlorophyll content and diminished efficiency of the primary photosynthetic processes. In contrast, the younger leaves of well-watered plants showed at first a reduction of proline levels in response to epiBL treatment, whereas in drought-stressed, epiBL pre-treated plants they were subsequently characterized by elevated amounts of proline. The contents of C₂₉- and C₂₇-BRs in plants treated with exogenous epiBL depended on the length of time between this treatment and the BR analysis regardless of plant water supply; they were more pronounced in plants subjected to the later epiBL treatment. The application of epiBL before or during drought did not result in any differences of plant response to this stressor.

Phytohormones regulate the non-redundant response of ω-3 fatty acid desaturases to low temperatures in Chorispora bungeana

To explore the contributions of ω-3 fatty acid desaturases (FADs) to cold stress response in a special cryophyte, Chorispora bungeana, two plastidial ω-3 desaturase genes (CbFAD7, CbFAD8) were cloned and verified in an Arabidopsis fad7fad8 mutant, before being compared with the microsomal ω-3 desaturase gene (CbFAD3). Though these genes were expressed in all tested tissues of C. bungeana, CbFAD7 and CbFAD8 have the highest expression in leaves, while CbFAD3 was mostly expressed in suspension-cultured cells. Low temperatures resulted in significant increases in trienoic fatty acids (TAs), corresponding to the cooperation of CbFAD3 and CbFAD8 in cultured cells, and the coordination of CbFAD7 and CbFAD8 in leaves. Furthermore, the cold induction of CbFAD8 in the two systems were increased with decreasing temperature and independently contributed to TAs accumulation at subfreezing temperature. A series of experiments revealed that jasmonie acid and brassinosteroids participated in the cold-responsive expression of ω-3 CbFAD genes in both C. bungeana cells and leaves, while the phytohormone regulation in leaves was complex with the participation of abscisic acid and gibberellin. These results point to the hormone-regulated non-redundant contributions of ω-3 CbFADs to maintain appropriate level of TAs under low temperatures, which help C. bungeana survive in cold environments.

Temporal regulation of cold transcriptional response in switchgrass

Switchgrass low-land ecotypes have significantly higher biomass but lower cold tolerance compared to up-land ecotypes. Understanding the molecular mechanisms underlying cold response, including the ones at transcriptional level, can contribute to improving tolerance of high-yield switchgrass under chilling and freezing environmental conditions. Here, by analyzing an existing switchgrass transcriptome dataset, the temporal cis-regulatory basis of switchgrass transcriptional response to cold is dissected computationally. We found that the number of cold-responsive genes and enriched Gene Ontology terms increased as duration of cold treatment increased from 30 min to 24 hours, suggesting an amplified response/cascading effect in cold-responsive gene expression. To identify genomic sequences likely important for regulating cold response, machine learning models predictive of cold response were established using k-mer sequences enriched in the genic and flanking regions of cold-responsive genes but not non-responsive genes. These k-mers, referred to as putative cis-regulatory elements (pCREs) are likely regulatory sequences of cold response in switchgrass. There are in total 655 pCREs where 54 are important in all cold treatment time points. Consistent with this, eight of 35 known cold-responsive CREs were similar to top-ranked pCREs in the models and only these eight were important for predicting temporal cold response. More importantly, most of the top-ranked pCREs were novel sequences in cold regulation. Our findings suggest additional sequence elements important for cold-responsive regulation previously not known that warrant further studies.

Physiological and Transcriptomic Analysis provide Molecular Insight into 24-Epibrassinolide mediated Cr(VI)-Toxicity Tolerance in Pepper Plants

The ever-increasing industrial activities over the decades have generated high toxic metals such as chromium (Cr) that hampers plant growth and development. To counter Cr-toxicity, plants have evolved complex defensive systems including hormonal crosstalk with various signaling pathways. 24-epibrassinolide (24-EBR) lowers oxidative stress and alleviates Cr(VI)-toxicity in plants. In this study, the concealed BR-mediated influences on Cr(VI)-stress tolerance were explored by transcriptome analysis in the Capsicum annuum. Results revealed a linkage between plant development under Cr(VI)-stress and the mitigating effect of 24-epibrassinolide and brassinazole. Growth inhibition, chlorophyll degradation, and a significant rise of malondialdehyde (MDA) were observed after 40 mg/L Cr(VI) treatment in Brz supplemented seedlings, whereas 24-EBR supplemented seedlings exhibited commendatory effect. Comparative transcriptome analysis showed that the expression levels of 6687 genes changed (3846 up-regulated and 2841 downregulated) under Cr(VI)-stress with Brz supplementation. Whereas the expression levels of only 1872 genes changed under Cr(VI)-stress with 24-EBR supplementation (1223 up-regulated and 649 downregulated). The functional categories of the differentially expressed genes (DEGs) by gene ontology (GO) revealed that drug transport, defense responses, and drug catabolic process were the considerable enrichments between 24-EBR and Brz supplemented seedlings under Cr(VI)-stress. Furthermore, auxin signaling, glutathione metabolism, ABC transporters, MAPK pathway, and 36 heavy metal-related genes were significantly differentially expressed components between Cr(VI)-stress, 24-EBR, and Brz supplemented seedlings. Overall, our data demonstrate that employing 24-EBR can commendably act as a growth stimulant in plants subjected to Cr(VI)-stress by modulating the physiological and defense regulatory system.

SlCESTA Is a Brassinosteroid-Regulated bHLH Transcription Factor of Tomato That Promotes Chilling Tolerance and Fruit Growth When Over-Expressed

Brassinosteroids (BRs) are required for various aspects of plant growth and development, but also participate in stress responses. The hormones convey their activity through transcriptional regulation and posttranslational modification of transcription factors and one class are basic helix-loop-helix (bHLH) proteins of the BR Enhanced Expression (BEE) subfamily, which in Arabidopsis thaliana include BEE1-3 and CESTA (CES). CES and the BEEs promote the expression of different BR-responsive genes, including genes encoding gibberellin (GA) biosynthetic and catabolizing enzymes, as well as cold-responsive genes. Interestingly, in terms of an application, CES could promote both fruit growth and cold stress tolerance when over-expressed in A. thaliana and here it was investigated, if this function is conserved in the fruit crop Solanum lycopersicum (cultivated tomato). Based on amino acid sequence similarity and the presence of regulatory motifs, a CES orthologue of S. lycopersicum, SlCES, was identified and the effects of its over-expression were analysed in tomato. This showed that SlCES, like AtCES, was re-localized to nuclear bodies in response to BR signaling activation and that it effected GA homeostasis, with related phenotypes, when over-expressed. In addition, over-expression lines showed an increased chilling tolerance and had altered fruit characteristics. The possibilities and potential limitations of a gain of SlCES function as a breeding strategy for tomato are discussed.

Differential physiological response to heat and cold stress of tomato plants and its implication on fruit quality

The upcoming climate change presents a great challenge for plant growth and development being extremes temperatures among the major environmental limitations to crop productivity. Understanding the repercussions of these extreme temperatures is of high importance to elaborate future strategies to confront crop damages. Tomato plants (Solanum lycopersicum L.) are one of the most cultivated crops and their fruits are consumed worldwide standing out for their organoleptic characteristics and nutritional value. Tomato plants are sensitive to temperatures below 12 °C and above 32 °C. In this study, Micro-Tom cultivar was used to evaluate the effects of extreme temperatures on the plant of tomato and the fruit productivity and quality from the stressed plants, either exposed to cold (4 °C for three nights per week) or heat (32 °C during the day, seven days per week) treatments. Total productivity and the percentage of ripe fruits per plant were evaluated together with foliar stress markers and the contents of photosynthetic pigments and tocochromanols. Fruit quality was also assessed determining lycopene contents, total soluble solids, total acidity and ascorbate contents. High temperatures altered multiple physiological parameters indicating a moderate stress, particularly decreasing fruit yield. As a response to this stress, plants enhanced their antioxidant contents both at leaf and fruit level. Low temperatures did not negatively affect the physiology of plants with similar yields as compared to controls, suggesting chilling acclimation. Both high and low temperatures, but most particularly the former, increased total soluble solids contents indicating that temperature control may be used as a strategy to modulate fruit quality.

Cold stress in maize (Zea mays) is alleviated by the over-expression of Phytoglobin 1 (ZmPgb1.1)

Maize (Zea mays) plants over-expressing or suppressing the class 1 Phytoglobin (ZmPgb1.1) were evaluated for their ability to cope with low temperature stress. Cold treatment (10 °C day/4 °C night) depressed several gas exchange parameters including photosynthetic rate, stomatal conductance and transpiration, while elevated the levels of reactive oxygen species (ROS) and ROS-induced damage. These effects were attenuated by the over-expression of ZmPgb1.1, and aggravated when the level of the same gene was suppressed. Combination of transcriptomic and pharmacological studies revealed that over-expression of ZmPgb1.1 suppressed the level of nitric oxide (NO), which lowers the transcription of several Brassinosteroid (BR) biosynthetic and response genes. Cellular BR was required to induce the expression of ZmMPK5, a component of the mitogen-activated protein kinase (MAPK) cascade, which is known to be involved in the regulation of ROS-producing pathways. Experimental reduction of NO content, suppression of BR or inhibition of ZmMPK5 reverted the beneficial effects of ZmPgb1.1 over-expression, and increased plant susceptibility to cold stress through accumulation of ROS. Conversely, tolerance to cold was augmented in the ZmPgb1.1 down-regulating line when the levels of NO or BR were elevated. Together, this study demonstrates a novel role of ZmPgb1.1 in modulating plant performance to cold stress, and integrates the ZmPgb1.1 response in a model requiring NO and BR to alleviate oxidative stress through ZmMPK5.

Role of Reactive Oxygen Species and Hormones in Plant Responses to Temperature Changes

Temperature stress is one of the major abiotic stresses that adversely affect agricultural productivity worldwide. Temperatures beyond a plant's physiological optimum can trigger significant physiological and biochemical perturbations, reducing plant growth and tolerance to stress. Improving a plant's tolerance to these temperature fluctuations requires a deep understanding of its responses to environmental change. To adapt to temperature fluctuations, plants tailor their acclimatory signal transduction events, and specifically, cellular redox state, that are governed by plant hormones, reactive oxygen species (ROS) regulatory systems, and other molecular components. The role of ROS in plants as important signaling molecules during stress acclimation has recently been established. Here, hormone-triggered ROS produced by NADPH oxidases, feedback regulation, and integrated signaling events during temperature stress activate stress-response pathways and induce acclimation or defense mechanisms. At the other extreme, excess ROS accumulation, following temperature-induced oxidative stress, can have negative consequences on plant growth and stress acclimation. The excessive ROS is regulated by the ROS scavenging system, which subsequently promotes plant tolerance. All these signaling events, including crosstalk between hormones and ROS, modify the plant's transcriptomic, metabolomic, and biochemical states and promote plant acclimation, tolerance, and survival. Here, we provide a comprehensive review of the ROS, hormones, and their joint role in shaping a plant's responses to high and low temperatures, and we conclude by outlining hormone/ROS-regulated plant responsive strategies for developing stress-tolerant crops to combat temperature changes.