Increased drought tolerance through the suppression of ESKMO1 gene and overexpression of CBF-related genes in Arabidopsis

The Triticeae CBF Gene Cluster-To Frost Resistance and Beyond

The pivotal role of CBF/DREB1 transcriptional factors in Triticeae crops involved in the abiotic stress response has been highlighted. The CBFs represent an important hub in the ICE-CBF-COR pathway, which is one of the most relevant mechanisms capable of activating the adaptive response to cold and drought in wheat, barley, and rye. Understanding the intricate mechanisms and regulation of the cluster of CBF genes harbored by the homoeologous chromosome group 5 entails significant potential for the genetic improvement of small grain cereals. Triticeae crops seem to share common mechanisms characterized, however, by some peculiar aspects of the response to stress, highlighting a combined landscape of single-nucleotide variants and copy number variation involving CBF members of subgroup IV. Moreover, while chromosome 5 ploidy appears to confer species-specific levels of resistance, an important involvement of the ICE factor might explain the greater tolerance of rye. By unraveling the genetic basis of abiotic stress tolerance, researchers can develop resilient varieties better equipped to withstand extreme environmental conditions. Hence, advancing our knowledge of CBFs and their interactions represents a promising avenue for improving crop resilience and food security.

Signals and Their Perception for Remodelling, Adjustment and Repair of the Plant Cell Wall

The integrity of the cell wall is important for plant cells. Mechanical or chemical distortions, tension, pH changes in the apoplast, disturbance of the ion homeostasis, leakage of cell compounds into the apoplastic space or breakdown of cell wall polysaccharides activate cellular responses which often occur via plasma membrane-localized receptors. Breakdown products of the cell wall polysaccharides function as damage-associated molecular patterns and derive from cellulose (cello-oligomers), hemicelluloses (mainly xyloglucans and mixed-linkage glucans as well as glucuronoarabinoglucans in Poaceae) and pectins (oligogalacturonides). In addition, several types of channels participate in mechanosensing and convert physical into chemical signals. To establish a proper response, the cell has to integrate information about apoplastic alterations and disturbance of its wall with cell-internal programs which require modifications in the wall architecture due to growth, differentiation or cell division. We summarize recent progress in pattern recognition receptors for plant-derived oligosaccharides, with a focus on malectin domain-containing receptor kinases and their crosstalk with other perception systems and intracellular signaling events.

Genome-Wide Identification of DUF668 Gene Family and Expression Analysis under Drought and Salt Stresses in Sweet Potato [Ipomoea batatas (L.) Lam]

The domain of unknown function 668 (DUF668) is a gene family that plays a vital role in responses to adversity coercion stresses in plant. However, the function of the DUF668 gene family is not fully understood in sweet potato. In this study, bioinformatics methods were used to analyze the number, physicochemical properties, evolution, structure, and promoter cis-acting elements of the IbDUF668 family genes, and RNA-seq and qRT-PCR were performed to detect gene expression and their regulation under hormonal and abiotic stress. A total of 14 IbDUF668 proteins were identified in sweet potato, distributed on nine chromosomes. By phylogenetic analysis, IbDUF668 proteins can be divided into two subfamilies. Transcriptome expression profiling revealed that many genes from DUF668 in sweet potato showed specificity and differential expression under cold, heat, drought, salt and hormones (ABA, GA3 and IAA). Four genes (IbDUF668-6, 7, 11 and 13) of sweet potato were significantly upregulated by qRT-PCR under ABA, drought and NaCl stress. Results suggest that the DUF668 gene family is involved in drought and salt tolerance in sweet potato, and it will further provide the basic information of DUF668 gene mechanisms in plants.

Plant Cell Wall Integrity Perturbations and Priming for Defense

A plant cell wall is a highly complex structure consisting of networks of polysaccharides, proteins, and polyphenols that dynamically change during growth and development in various tissues. The cell wall not only acts as a physical barrier but also dynamically responds to disturbances caused by biotic and abiotic stresses. Plants have well-established surveillance mechanisms to detect any cell wall perturbations. Specific immune signaling pathways are triggered to contrast biotic or abiotic forces, including cascades dedicated to reinforcing the cell wall structure. This review summarizes the recent developments in molecular mechanisms underlying maintenance of cell wall integrity in plant-pathogen and parasitic interactions. Subjects such as the effect of altered expression of endogenous plant cell-wall-related genes or apoplastic expression of microbial cell-wall-modifying enzymes on cell wall integrity are covered. Targeted genetic modifications as a tool to study the potential of cell wall elicitors, priming of signaling pathways, and the outcome of disease resistance phenotypes are also discussed. The prime importance of understanding the intricate details and complete picture of plant immunity emerges, ultimately to engineer new strategies to improve crop productivity and sustainability.

Coexpression of Fungal Cell Wall-Modifying Enzymes Reveals Their Additive Impact on Arabidopsis Resistance to the Fungal Pathogen, Botrytis cinerea

Simple Summary

In the present study, we created transgenic Arabidopsis plants overexpressing two fungal acetylesterases and a fungal feruloylesterase that acts on cell wall polysaccharides and studied their possible complementary additive effects on host defense reactions against the fungal pathogen, Botrytis cinerea. Our results showed that the Arabidopsis plants overexpressing two acetylesterases together contributed significantly higher resistance to B. cinerea in comparison with single protein expression. Conversely, coexpression of either of the acetyl esterases together with feruloylesterase compensates the latter’s negative impact on plant resistance. The results also provided evidence that combinatorial coexpression of some cell wall polysaccharide-modifying enzymes might exert an additive effect on plant immune response by constitutively priming plant defense pathways even before pathogen invasion. These findings have potential uses in protecting valuable crops against pathogens.

Abstract

The plant cell wall (CW) is an outer cell skeleton that plays an important role in plant growth and protection against both biotic and abiotic stresses. Signals and molecules produced during host–pathogen interactions have been proven to be involved in plant stress responses initiating signal pathways. Based on our previous research findings, the present study explored the possibility of additively or synergistically increasing plant stress resistance by stacking beneficial genes. In order to prove our hypothesis, we generated transgenic Arabidopsis plants constitutively overexpressing three different Aspergillus nidulans CW-modifying enzymes: a xylan acetylesterase, a rhamnogalacturonan acetylesterase and a feruloylesterase. The two acetylesterases were expressed either together or in combination with the feruloylesterase to study the effect of CW polysaccharide deacetylation and deferuloylation on Arabidopsis defense reactions against a fungal pathogen, Botrytis cinerea. The transgenic Arabidopsis plants expressing two acetylesterases together showed higher CW deacetylation and increased resistance to B. cinerea in comparison to wild-type (WT) Col-0 and plants expressing single acetylesterases. While the expression of feruloylesterase alone compromised plant resistance, coexpression of feruloylesterase together with either one of the two acetylesterases restored plant resistance to the pathogen. These CW modifications induced several defense-related genes in uninfected healthy plants, confirming their impact on plant resistance. These results demonstrated that coexpression of complementary CW-modifying enzymes in different combinations have an additive effect on plant stress response by constitutively priming the plant defense pathways. These findings might be useful for generating valuable crops with higher protections against biotic stresses.

Crosstalk between Light- and Temperature-Mediated Processes under Cold and Heat Stress Conditions in Plants

Extreme temperatures are among the most important stressors limiting plant growth and development. Results indicate that light substantially influences the acclimation processes to both low and high temperatures, and it may affect the level of stress injury. The interaction between light and temperature in the regulation of stress acclimation mechanisms is complex, and both light intensity and spectral composition play an important role. Higher light intensities may lead to overexcitation of the photosynthetic electron transport chain; while different wavelengths may act through different photoreceptors. These may induce various stress signalling processes, leading to regulation of stomatal movement, antioxidant and osmoregulation capacities, hormonal actions, and other stress-related pathways. In recent years, we have significantly expanded our knowledge in both light and temperature sensing and signalling. The present review provides a synthesis of results for understanding how light influences the acclimation of plants to extreme low or high temperatures, including the sensing mechanisms and molecular crosstalk processes.

Overexpression of Arabidopsis ICE1 enhances yield and multiple abiotic stress tolerance in indica rice

ICE1 (Inducer of CBF Expression 1), a MYC-type bHLH transcription factor, is a regulator of cold tolerance in Arabidopsis. Indica rice, which occupies the major rice cultivated area, is highly sensitive to cold stress. Hence in this study, Arabidopsis ICE1 (AtICE1) was overexpressed in indica rice to analyze its role in reproductive stage cold and other abiotic stress tolerance to indica rice. AtICE1 was overexpressed by using stress inducible AtRD29A promoter in mega rice cv. MTU1010. Under cold stress conditions, AtICE1 overexpression lines showed lower accumulation of MDA and H₂O₂, higher membrane stability, and thus higher seedling survival rate than the WT plants. Expression levels of OsDREB1A, OsMYB3R2, and OsTPP1 were significantly higher in transgenics as compared with WT under cold stress conditions. AtICE1 transgenic rice plants produced 44-60% higher grain yield as compared with WT plants under control conditions in three independent experiments. Of the three AtICE1 overexpression lines, two lines produced significantly higher grain yield as compared with WT plants after recovery from cold, salt and drought stresses. AtICE1 overexpression lines showed significantly higher stomatal density and conductance under non-stress conditions. qRT-PCR analysis showed that expression levels of stomatal pathway genes viz., OsSPCH1, OsSPCH2, OsSCR1, OsSCRM1, OsSCRM2 and OsMUTE were significantly higher in AtICE1 transgenics as compared with WT plants. The components of water use viz., stomatal conductance, photosynthesis, and instantaneous WUE were higher in transgenics as compared with WT plants. The results showed that AtICE1 confers multiple stress tolerance to indica rice, and the role of ICE1 in stress tolerance and stomatal development is conserved across species.

Arabinoxylan-Oligosaccharides Act as Damage Associated Molecular Patterns in Plants Regulating Disease Resistance

Immune responses in plants can be triggered by damage/microbe-associated molecular patterns (DAMPs/MAMPs) upon recognition by plant pattern recognition receptors (PRRs). DAMPs are signaling molecules synthesized by plants or released from host cellular structures (e.g., plant cell walls) upon pathogen infection or wounding. Despite the hypothesized important role of plant cell wall-derived DAMPs in plant-pathogen interactions, a very limited number of these DAMPs are well characterized. Recent work demonstrated that pectin-enriched cell wall fractions extracted from the cell wall mutant impaired in Arabidopsis Response Regulator 6 (arr6), that showed altered disease resistance to several pathogens, triggered more intense immune responses than those activated by similar cell wall fractions from wild-type plants. It was hypothesized that arr6 cell wall fractions could be differentially enriched in DAMPs. In this work, we describe the characterization of the previous immune-active fractions of arr6 showing the highest triggering capacities upon further fractionation by chromatographic means. These analyses pointed to a role of pentose-based oligosaccharides triggering plant immune responses. The characterization of several pentose-based oligosaccharide structures revealed that β-1,4-xylooligosaccharides of specific degrees of polymerization and carrying arabinose decorations are sensed as DAMPs by plants. Moreover, the pentasaccharide 33-α-L-arabinofuranosyl-xylotetraose (XA3XX) was found as a highly active DAMP structure triggering strong immune responses in Arabidopsis thaliana and enhancing crop disease resistance.

Arabidopsis Response Regulator 6 (ARR6) Modulates Plant Cell-Wall Composition and Disease Resistance

The cytokinin signaling pathway, which is mediated by Arabidopsis response regulator (ARR) proteins, has been involved in the modulation of some disease-resistance responses. Here, we describe novel functions of ARR6 in the control of plant disease-resistance and cell-wall composition. Plants impaired in ARR6 function (arr6) were more resistant and susceptible, respectively, to the necrotrophic fungus Plectosphaerella cucumerina and to the vascular bacterium Ralstonia solanacearum, whereas Arabidopsis plants that overexpress ARR6 showed the opposite phenotypes, which further support a role of ARR6 in the modulation of disease-resistance responses against these pathogens. Transcriptomics and metabolomics analyses revealed that, in arr6 plants, canonical disease-resistance pathways, like those activated by defensive phytohormones, were not altered, whereas immune responses triggered by microbe-associated molecular patterns were slightly enhanced. Cell-wall composition of arr6 plants was found to be severely altered compared with that of wild-type plants. Remarkably, pectin-enriched cell-wall fractions extracted from arr6 walls triggered more intense immune responses than those activated by similar wall fractions from wild-type plants, suggesting that arr6 pectin fraction is enriched in wall-related damage-associated molecular patterns, which trigger immune responses. This work supports a novel function of ARR6 in the control of cell-wall composition and disease resistance and reinforces the role of the plant cell wall in the modulation of specific immune responses.

Cell Wall Acetylation in Hybrid Aspen Affects Field Performance, Foliar Phenolic Composition and Resistance to Biological Stress Factors in a Construct-Dependent Fashion

The production of biofuels and "green" chemicals from the lignocellulose of fast-growing hardwood species is hampered by extensive acetylation of xylan. Different strategies have been implemented to reduce xylan acetylation, resulting in transgenic plants that show good growth in the greenhouse, improved saccharification and fermentation, but the field performance of such plants has not yet been reported. The aim of this study was to evaluate the impact of reduced acetylation on field productivity and identify the best strategies for decreasing acetylation. Growth and biological stress data were evaluated for 18 hybrid aspen lines with 10-20% reductions in the cell wall acetyl content from a five year field experiment in Southern Sweden. The reduction in acetyl content was achieved either by suppressing the process of acetylation in the Golgi by reducing expression of REDUCED WALL ACETYLATION (RWA) genes, or by post-synthetic acetyl removal by fungal acetyl xylan esterases (AXEs) from two different families, CE1 and CE5, targeting them to cell walls. Transgene expression was regulated by either a constitutive promoter (35S) or a wood-specific promoter (WP). For the majority of transgenic lines, growth was either similar to that in WT and transgenic control (WP:GUS) plants, or slightly reduced. The slight reduction was observed in the AXE-expressing lines regulated by the 35S promoter, not those with the WP promoter which limits expression to cells developing secondary walls. Expressing AXEs regulated by the 35S promoter resulted in increased foliar arthropod chewing, and altered condensed tannins and salicinoid phenolic glucosides (SPGs) profiles. Greater growth inhibition was observed in the case of CE5 than with CE1 AXE, and it was associated with increased foliar necrosis and distinct SPG profiles, suggesting that CE5 AXE could be recognized by the pathogen-associated molecular pattern system. For each of three different constructs, there was a line with dwarfism and growth abnormalities, suggesting random genetic/epigenetic changes. This high frequency of dwarfism (17%) is suggestive of a link between acetyl metabolism and chromatin function. These data represent the first evaluation of acetyl-reduced plants from the field, indicating some possible pitfalls, and identifying the best strategies, when developing highly productive acetyl-reduced feedstocks.

Isolation and functional characterization of the SpCBF1 gene from Solanum pinnatisectum

Low temperature causes a negative impact on plant growth and development, but plants evolve a series of mechanisms to respond to chilling stress, and one of them is CBF [C-repeat (CRT)/dehydration-responsive element (DRE) binding factor] gene family which has been well studied in different crops. In this paper, a new CBF1 gene, named as SpCBF1, was isolated from frost-tolerant Solanum pinnatisectum by PCR and analyzed for its function in cold-tolerance by over-expression technique. The ORF of SpCBF1 was 666 bp long and encoded a protein of 221 amino acids with a predicted molecular mass 24.5821 kDa and theoretically isoelectric point 5.0. SpCBF1 protein contained a highly conserved specific AP2/ERF domain. SpCBF1 was expressed in all tested tissues with the highest level in tuber and the lowest in root, and induced by chilling stress (0 °C). Under natural low temperature condition (1-10 °C), plants over-expressing SpCBF1 (OE) exhibited slighter necrotic lesion and lower necrotic injury, compared with untransformed Solanum tuberosum cv. Désirée (WT) and antisense-StCBF1 control lines. Over-expression of CBF1 increased the level of COR (cold-regulated) gene transcripts in OE lines, and the physiological indexes related to cold tolerance like the contents of SOD, soluble protein, MDA, proline and soluble sugar were higher in OE lines than in WT except RWC which was lower. All these results indicated that SpCBF1 gene plays a promoting role in potato responding to cold stress.

Growth- and stress-related defects associated with wall hypoacetylation are strigolactone-dependent

Mutants affected in the Arabidopsis TBL29/ESK1 xylan O-acetyltransferase display a strong reduction in total wall O-acetylation accompanied by a dwarfed plant stature, collapsed xylem morphology, and enhanced freezing tolerance. A newly identified tbl29/esk1 suppressor mutation reduces the expression of the MAX4 gene, affecting the biosynthesis of methyl carlactonoate (MeCLA), an active strigolactone (SL). Genetic and biochemical evidence suggests that blocking the biosynthesis of this SL is sufficient to recover all developmental and stress-related defects associated with the TBL29/ESK1 loss of function without affecting its direct effect-reduced wall O-acetylation. Altered levels of the MAX4 SL biosynthetic gene, reduced branch number, and higher levels of MeCLA, were also found in tbl29/esk1 plants consistent with a constitutive activation of the SL pathway. These results suggest that the reduction in O-acetyl substituents in xylan is not directly responsible for the observed tbl29/esk1 phenotypes. Alternatively, plants may perceive defects in the structure of wall polymers and/or wall architecture activating the SL hormonal pathway as a compensatory mechanism.

Plant cell wall-mediated immunity: cell wall changes trigger disease resistance responses

Plants have evolved a repertoire of monitoring systems to sense plant morphogenesis and to face environmental changes and threats caused by different attackers. These systems integrate different signals into overreaching triggering pathways which coordinate developmental and defence-associated responses. The plant cell wall, a dynamic and complex structure surrounding every plant cell, has emerged recently as an essential component of plant monitoring systems, thus expanding its function as a passive defensive barrier. Plants have a dedicated mechanism for maintaining cell wall integrity (CWI) which comprises a diverse set of plasma membrane-resident sensors and pattern recognition receptors (PRRs). The PRRs perceive plant-derived ligands, such as peptides or wall glycans, known as damage-associated molecular patterns (DAMPs). These DAMPs function as 'danger' alert signals activating DAMP-triggered immunity (DTI), which shares signalling components and responses with the immune pathways triggered by non-self microbe-associated molecular patterns that mediate disease resistance. Alteration of CWI by impairment of the expression or activity of proteins involved in cell wall biosynthesis and/or remodelling, as occurs in some plant cell wall mutants, or by wall damage due to colonization by pathogens/pests, activates specific defensive and growth responses. Our current understanding of how these alterations of CWI are perceived by the wall monitoring systems is scarce and few plant sensors/PRRs and DAMPs have been characterized. The identification of these CWI sensors and PRR-DAMP pairs will help us to understand the immune functions of the wall monitoring system, and might allow the breeding of crop varieties and the design of agricultural strategies that would enhance crop disease resistance.

Oil palm EgCBF3 conferred stress tolerance in transgenic tomato plants through modulation of the ethylene signaling pathway

CBF/DREB1 is a group of transcription factors that are mainly involved in abiotic stress tolerance in plants. They belong to the AP2/ERF superfamily of plant-specific transcription factors. A gene encoding a new member of this group was isolated from ripening oil palm fruit and designated as EgCBF3. The oil palm fruit demonstrates the characteristics of a climacteric fruit like tomato, in which ethylene has a major impact on the ripening process. A transgenic approach was used for functional characterization of the EgCBF3, using tomato as the model plant. The effects of ectopic expression of EgCBF3 were analyzed based on expression profiling of the ethylene biosynthesis-related genes, anti-freeze proteins (AFPs), abiotic stress tolerance and plant growth and development. The EgCBF3 tomatoes demonstrated altered phenotypes compared to the wild type tomatoes. Delayed leaf senescence and flowering, increased chlorophyll content and abnormal flowering were the consequences of overexpression of EgCBF3 in the transgenic tomatoes. The EgCBF3 tomatoes demonstrated enhanced abiotic stress tolerance under in vitro conditions. Further, transcript levels of ethylene biosynthesis-related genes, including three SlACSs and two SlACOs, were altered in the transgenic plants' leaves and roots compared to that in the wild type tomato plant. Among the eight AFPs studied in the wounded leaves of the EgCBF3 tomato plants, transcript levels of SlOSM-L, SlNP24, SlPR5L and SlTSRF1 decreased, while expression of the other four, SlCHI3, SlPR1, SlPR-P2 and SlLAP2, were up-regulated. These findings indicate the possible functions of EgCBF3 in plant growth and development as a regulator of ethylene biosynthesis-related and AFP genes, and as a stimulator of abiotic stress tolerance.

The NF-YA transcription factor OsNF-YA7 confers drought stress tolerance of rice in an abscisic acid independent manner

The mechanisms of plant response and adaptation to drought stress require the regulation of transcriptional networks via the induction of drought-responsive transcription factors. Nuclear Factor Y (NF-Y) transcription factors have aroused interest in roles of plant drought stress responses. However, the molecular mechanism of the NF-Y-induced drought tolerance is not well understood. Here, we functionally analyzed two rice NF-YA genes, OsNF-YA7 and OsNF-YA4. Expression of OsNF-YA7 was induced by drought stress and its overexpression in transgenic rice plants improved their drought tolerance. In contrast, OsNF-YA4 expression was not increased by drought stress and its overexpression in transgenic rice plants did not affect their sensitivity to drought stress. OsNF-YA4 expression was highly induced by the stress-related hormone abscisic acid (ABA), while OsNF-YA7 was not, indicating that OsNF-YA7 mediates drought tolerance in an ABA-independent manner. Analysis of the OsNF-YA7 promoter revealed three ABA-independent DRE/CTR elements and RNA-seq analysis identified 48 genes downstream of OsNFYA7 action putatively involved in the OsNF-YA7-mediated drought tolerance pathway. Taken together, our results suggest an important role for OsNF-YA7 in rice drought stress tolerance.

INDUCER OF CBF EXPRESSION 1 integrates cold signals into FLOWERING LOCUS C-mediated flowering pathways in Arabidopsis

Plants constantly monitor changes in photoperiod and temperature throughout the year to synchronize flowering with optimal environmental conditions. In the temperate zones, both photoperiod and temperature fluctuate in a somewhat predictable manner through the seasons, although a transient shift to low temperature is also encountered during changing seasons, such as early spring. Although low temperatures are known to delay flowering by inducing the floral repressor FLOWERING LOCUS C (FLC), it is not fully understood how temperature signals are coordinated with photoperiodic signals in the timing of seasonal flowering. Here, we show that the cold signaling activator INDUCER OF CBF EXPRESSION 1 (ICE1), FLC and the floral promoter SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) constitute an elaborate signaling network that integrates cold signals into flowering pathways. The cold-activated ICE1 directly induces the gene encoding FLC, which represses SOC1 expression, resulting in delayed flowering. In contrast, under floral promotive conditions, SOC1 inhibits the binding of ICE1 to the promoters of the FLC gene, inducing flowering with a reduction of freezing tolerance. These observations indicate that the ICE1-FLC-SOC1 signaling network contributes to the fine-tuning of flowering during changing seasons.