Transcriptional regulation of hormone-synthesis and signaling pathways by overexpressing cytokinin-synthesis contributes to improved drought tolerance in creeping bentgrass

Multi-omics insights into the positive role of strigolactone perception in barley drought response

BACKGROUND

Drought is a major environmental stress that affects crop productivity worldwide. Although previous research demonstrated links between strigolactones (SLs) and drought, here we used barley (Hordeum vulgare) SL-insensitive mutant hvd14 (dwarf14) to scrutinize the SL-dependent mechanisms associated with water deficit response.

RESULTS

We have employed a combination of transcriptomics, proteomics, phytohormonomics analyses, and physiological data to unravel differences between wild-type and hvd14 plants under drought. Our research revealed that drought sensitivity of hvd14 is related to weaker induction of abscisic acid-responsive genes/proteins, lower jasmonic acid content, higher reactive oxygen species content, and lower wax biosynthetic and deposition mechanisms than wild-type plants. In addition, we identified a set of transcription factors (TFs) that are exclusively drought-induced in the wild-type barley.

CONCLUSIONS

Critically, we resolved a comprehensive series of interactions between the drought-induced barley transcriptome and proteome responses, allowing us to understand the profound effects of SLs in alleviating water-limiting conditions. Several new avenues have opened for developing barley more resilient to drought through the information provided. Moreover, our study contributes to a better understanding of the complex interplay between genes, proteins, and hormones in response to drought, and underscores the importance of a multidisciplinary approach to studying plant stress response mechanisms.

A novel small open reading frame gene, IbEGF, enhances drought tolerance in transgenic sweet potato

Small open reading frames (sORFs) can encode functional polypeptides or act as cis-translational regulators in stress responses in eukaryotes. Their number and potential importance have only recently become clear in plants. In this study, we identified a novel sORF gene in sweet potato, IbEGF, which encoded the 83-amino acid polypeptide containing an EGF_CA domain. The expression of IbEGF was induced by PEG6000, H₂O₂, abscisic acid (ABA), methyl-jasmonate (MeJA) and brassinosteroid (BR). The IbEGF protein was localized to the nucleus and cell membrane. Under drought stress, overexpression of IbEGF enhanced drought tolerance, promoted the accumulation of ABA, MeJA, BR and proline and upregulated the genes encoding superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in transgenic sweet potato. The IbEGF protein was found to interact with IbCOP9-5α, a regulator in the phytohormone signalling pathways. These results suggest that IbEGF interacting with IbCOP9-5α enhances drought tolerance by regulating phytohormone signalling pathways, increasing proline accumulation and further activating reactive oxygen species (ROS) scavenging system in transgenic sweet potato.

Root exudation of contrasting drought-stressed pearl millet genotypes conveys varying biological nitrification inhibition (BNI) activity

Roots secrete a vast array of low molecular weight compounds into the soil broadly referred to as root exudates. It is a key mechanism by which plants and soil microbes interact in the rhizosphere. The effect of drought stress on the exudation process and composition is rarely studied, especially in cereal crops. This study focuses on comparative metabolic profiling of the exudates from sensitive and tolerant genotypes of pearl millet after a period of drought stress. We employed a combined platform of gas and liquid chromatography coupled to mass spectrometry to cover both primary and secondary metabolites. The results obtained demonstrate that both genotype and drought stress have a significant impact on the concentration and composition of root exudates. The complexity and function of these differential root exudates are discussed. To reveal the potential effect of root exudates on the soil microbial community after a period of drought stress, we also tested for biological nitrification inhibition (BNI) activity. The analysis revealed a genotype-dependent enhancement of BNI activity after a defined period of drought stress. In parallel, we observed a genotype-specific relation of elongated root growth and root exudation under drought stress. These data suggest that the drought stress-dependent change in root exudation can manipulate the microbial soil communities to adapt and survive under harsh conditions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00374-021-01578-w.

Transcriptional regulation and stress-defensive key genes induced by γ-aminobutyric acid in association with tolerance to water stress in creeping bentgrass

γ-Aminobutyric acid (GABA) acts as an important regulator involved in the mediation of cell signal transduction and stress tolerance in plants. However, the function of GABA in transcriptional regulation is not fully understood in plants under water stress. The creeping bentgrass (Agrostis stolonifera) was pretreated with or without GABA (0.5 mM) for 24 hours before being exposed to 5 days of water stress. Physiological analysis showed that GABA-treated plants maintained significantly higher endogenous GABA content, leaf relative water content, net photosynthetic rate, and lower osmotic potential than untreated plants under water stress. The GABA application also significantly alleviated stress-induced increases in superoxide anion (O2.-) content, hydrogen peroxide (H2O2) content, and electrolyte leakage through enhancing total antioxidant capacity, superoxide dismutase (SOD) activity, and peroxidase (POD) activity in response to water stress. The transcriptomic analysis demonstrated that the GABA-induced changes in differentially expressed genes (DEGs) involved in carbohydrates, amino acids, and secondary metabolism helped to maintain better osmotic adjustment, energy supply, and metabolic homeostasis when creeping bentgrass suffers from water stress. The GABA triggered Ca2+-dependent protein kinase (CDPK) signaling and improved transcript levels of DREB1/2 and WRKY1/24/41 that could be associated with the upregulation of stress-related functional genes such as POD, DHNs, and HSP70 largely contributing to improved tolerance to water stress in relation to the antioxidant, prevention of cell dehydration, and protein protection in leaves.

A R2R3-MYB Transcription Factor Gene, BpMYB123, Regulates BpLEA14 to Improve Drought Tolerance in Betula platyphylla

Drought stress causes various negative impacts on plant growth and crop production. R2R3-MYB transcription factors (TFs) play crucial roles in the response to abiotic stress. However, their functions in Betula platyphylla haven't been fully investigated. In this study, a R2R3 MYB transcription factor gene, BpMYB123, was identified from Betula platyphylla and reveals its significant role in drought stress. Overexpression of BpMYB123 enhances tolerance to drought stress in contrast to repression of BpMYB123 by RNA interference (RNAi) in transgenic experiment. The overexpression lines increased peroxidase (POD) and superoxide dismatase (SOD) activities, while decreased hydrogen peroxide (H₂O₂), superoxide radicals (O₂ ^-), electrolyte leakage (EL) and malondialdehyde (MDA) contents. Our study showed that overexpression of BpMYB123 increased BpLEA14 gene expression up to 20-fold due to BpMYB123 directly binding to the MYB1AT element of BpLEA14 promoter. These results indicate that BpMYB123 acts as a regulator via regulating BpLEA14 to improve drought tolerance in birch.

Diverse Roles of MAX1 Homologues in Rice

Cytochrome P450 enzymes encoded by MORE AXILLARY GROWTH1 (MAX1)-like genes produce most of the structural diversity of strigolactones during the final steps of strigolactone biosynthesis. The diverse copies of MAX1 in Oryza sativa provide a resource to investigate why plants produce such a wide range of strigolactones. Here we performed in silico analyses of transcription factors and microRNAs that may regulate each rice MAX1, and compared the results with available data about MAX1 expression profiles and genes co-expressed with MAX1 genes. Data suggest that distinct mechanisms regulate the expression of each MAX1. Moreover, there may be novel functions for MAX1 homologues, such as the regulation of flower development or responses to heavy metals. In addition, individual MAX1s could be involved in specific functions, such as the regulation of seed development or wax synthesis in rice. Our analysis reveals potential new avenues of strigolactone research that may otherwise not be obvious.

Melatonin-Induced Water Stress Tolerance in Plants: Recent Advances

Water stress (drought and waterlogging) is severe abiotic stress to plant growth and development. Melatonin, a bioactive plant hormone, has been widely tested in drought situations in diverse plant species, while few studies on the role of melatonin in waterlogging stress conditions have been published. In the current review, we analyze the biostimulatory functions of melatonin on plants under both drought and waterlogging stresses. Melatonin controls the levels of reactive oxygen and nitrogen species and positively changes the molecular defense to improve plant tolerance against water stress. Moreover, the crosstalk of melatonin and other phytohormones is a key element of plant survival under drought stress, while this relationship needs further investigation under waterlogging stress. In this review, we draw the complete story of water stress on both sides-drought and waterlogging-through discussing the previous critical studies under both conditions. Moreover, we suggest several research directions, especially for waterlogging, which remains a big and vague piece of the melatonin and water stress puzzle.

Melatonin-Induced Water Stress Tolerance in Plants: Recent Advances

Water stress (drought and waterlogging) is severe abiotic stress to plant growth and development. Melatonin, a bioactive plant hormone, has been widely tested in drought situations in diverse plant species, while few studies on the role of melatonin in waterlogging stress conditions have been published. In the current review, we analyze the biostimulatory functions of melatonin on plants under both drought and waterlogging stresses. Melatonin controls the levels of reactive oxygen and nitrogen species and positively changes the molecular defense to improve plant tolerance against water stress. Moreover, the crosstalk of melatonin and other phytohormones is a key element of plant survival under drought stress, while this relationship needs further investigation under waterlogging stress. In this review, we draw the complete story of water stress on both sides-drought and waterlogging-through discussing the previous critical studies under both conditions. Moreover, we suggest several research directions, especially for waterlogging, which remains a big and vague piece of the melatonin and water stress puzzle.

Cytokinin-induced protein synthesis suppresses growth and osmotic stress tolerance

Cytokinins control critical aspects of plant development and environmental responses. Perception of cytokinin ultimately leads to the activation of proteins belonging to the type-B Response Regulator family of cytokinin response activators. In Arabidopsis thaliana, ARR1 is one of the most abundantly expressed type-B Response Regulators. We investigated the link between cytokinin signaling, protein synthesis, plant growth and osmotic stress tolerance. We show that the increased cytokinin signaling in ARR1 gain-of-function transgenic lines is associated with increased rates of protein synthesis, which lead to growth inhibition and hypersensitivity to osmotic stress. Cytokinin-induced growth inhibition and osmotic stress hypersensitivity were rescued by treatments with ABA, a hormone known to inhibit protein synthesis. We also demonstrate that cytokinin-induced protein synthesis requires isoforms of the ribosomal protein L4 encoded by the cytokinin-inducible genes RPL4A and RPL4D, and that RPL4 loss-of-function increases osmotic stress tolerance and decreases sensitivity to cytokinin-induced growth inhibition. These findings reveal that an increase in protein synthesis negatively impacts growth and osmotic stress tolerance and explain some of the adverse effects of elevated cytokinin action on plant development and stress physiology.

Distinctive phytohormonal and metabolic profiles of Arabidopsis thaliana and Eutrema salsugineum under similar soil drying

Arabidopsis and Eutrema show similar stomatal sensitivity to drying soil. In Arabidopsis, larger metabolic adjustments than in Eutrema occurred, with considerable differences in the phytohormonal responses of the two species. Although plants respond to soil drying via a series of concurrent physiological and molecular events, drought tolerance differs greatly within the plant kingdom. While Eutrema salsugineum (formerly Thellungiella salsuginea) is regarded as more stress tolerant than its close relative Arabidopsis thaliana, their responses to soil water deficit have not previously been directly compared. To ensure a similar rate of soil drying for the two species, daily soil water depletion was controlled to 5-10% of the soil water content. While partial stomatal closure occurred earlier in Arabidopsis (Day 4) than Eutrema (from Day 6 onwards), thereafter both species showed similar stomatal sensitivity to drying soil. However, both targeted and untargeted metabolite analysis revealed greater response to drought in Arabidopsis than Eutrema. Early peaks in foliar phytohormone concentrations and different sugar profiles between species were accompanied by opposing patterns in the bioactive cytokinin profiles. Untargeted analysis showed greater metabolic adjustment in Arabidopsis with more statistically significant changes in both early and severe drought stress. The distinct metabolic responses of each species during early drought, which occurred prior to leaf water status declining, seemed independent of later stomatal closure in response to drought. The two species also showed distinct water usage, with earlier reduction in water consumption in Eutrema (Day 3) than Arabidopsis (Day 6), likely reflecting temporal differences in growth responses. We propose Arabidopsis as a promising model to evaluate the mechanisms responsible for stress-induced growth inhibition under the mild/moderate soil drying that crop plants are typically exposed to.

Identification and characterization of the first cytokinin glycosyltransferase from rice

BACKGROUND

Cytokinins are one of the five major hormones families in plants and are important for their normal growth and environmental adaptability. In plants, cytokinins are mostly present as glycosides in plants, and their glycosylation modifications are catalyzed by family 1 glycosyltransferases. Current research on cytokinin glycosylation has focused on the biochemical identification of enzymes and the analysis of metabolites in Arabidopsis. There are few studies that examine how cytokinin glycosylation affects its synthesis and accumulation in plants. It is particularly important to understand these processes in food crops such as rice (Oryza sativa); however, to date, cytokinin glycosyltransferase genes in rice have not been reported.

RESULTS

In this study, we identified eight rice genes that were functionally homologous to an Arabidopsis cytokinin glycosyltransferase gene. These genes were cloned and expressed in a prokaryotic system to obtain their purified proteins. Through enzymatic analysis and liquid chromatography-mass spectrometry, a single rice glycosyltransferase, Os6, was identified that glycosylated cytokinin in vitro. Os6 was overexpressed in Arabidopsis, and the extraction of cytokinin glycosides showed that Os6 is functionally active in planta.

CONCLUSIONS

The identification and characterization of the first cytokinin glycosyltransferase from rice is important for future studies on the cytokinin metabolic pathway in rice. An improved understanding of rice cytokinin glycosyltransferases may facilitate genetic improvements in rice quality.

Biosynthesis and Signal Transduction of ABA, JA, and BRs in Response to Drought Stress of Kentucky Bluegrass

Kentucky bluegrass (KB, Poa pratensis) is one of the most widely used cool-season turfgrass species, but it is sensitive to drought stress. Molecular studies in KB are hindered by its large and complex genome structure. In this study, a comparative transcriptomic study was conducted between a short and long period of water deficiency. Three transcriptome libraries were constructed and then sequenced by using leaf RNA samples of plants at 0, 2, and 16 h after PEG6000 treatment. A total of 199,083 differentially expressed genes (DEGs) were found. The Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation revealed that DEGs were enriched in "Plant hormone signal transduction" and "MAPK signaling pathway-Plant". Some key up-regulated genes, including PYL, JAZ, and BSK, were involved in hormone signaling transduction of abscisic acid, jasmonic acid, and brassinosteroid and possibly these genes play important roles in coping with drought stress in KB. Furthermore, our results showed that the concentrations of ABA, JA and BR increased significantly with the extension of the drought period. The specific DEGs encoding functional proteins, kinase and transcription factors, could be valuable information for genetic manipulation to promote drought tolerance of KB in the future.

Biosynthesis and Signal Transduction of ABA, JA, and BRs in Response to Drought Stress of Kentucky Bluegrass

Kentucky bluegrass (KB, Poa pratensis) is one of the most widely used cool-season turfgrass species, but it is sensitive to drought stress. Molecular studies in KB are hindered by its large and complex genome structure. In this study, a comparative transcriptomic study was conducted between a short and long period of water deficiency. Three transcriptome libraries were constructed and then sequenced by using leaf RNA samples of plants at 0, 2, and 16 h after PEG6000 treatment. A total of 199,083 differentially expressed genes (DEGs) were found. The Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation revealed that DEGs were enriched in "Plant hormone signal transduction" and "MAPK signaling pathway-Plant". Some key up-regulated genes, including PYL, JAZ, and BSK, were involved in hormone signaling transduction of abscisic acid, jasmonic acid, and brassinosteroid and possibly these genes play important roles in coping with drought stress in KB. Furthermore, our results showed that the concentrations of ABA, JA and BR increased significantly with the extension of the drought period. The specific DEGs encoding functional proteins, kinase and transcription factors, could be valuable information for genetic manipulation to promote drought tolerance of KB in the future.

A Novel Sweetpotato Transcription Factor Gene IbMYB116 Enhances Drought Tolerance in Transgenic Arabidopsis

Several members of the MYB transcription factor family have been found to regulate growth, developmental processes, metabolism, and biotic and abiotic stress responses in plants. However, the role of MYB116 in plants is still unclear. In this study, a MYB transcription factor gene IbMYB116 was cloned and characterized from the sweetpotato [Ipomoea batatas (L.) Lam.] line Xushu55-2, a line that is considered to be drought resistant. We show here that IbMYB116 is a nuclear protein and that it possesses a transactivation domain at the C terminus. This gene exhibited a high expression level in the leaf tissues of Xushu55-2 and was strongly induced by PEG6000 and methyl-jasmonate (MeJA). The IbMYB116-overexpressing Arabidopsis plants showed significantly enhanced drought tolerance, increased MeJA content, and a decreased H₂O₂ level under drought stress. The overexpression of IbMYB116 in Arabidopsis systematically upregulated jasmonic acid (JA) biosynthesis genes and activated the JA signaling pathway as well as reactive oxygen species (ROS)-scavenging system genes under drought stress conditions. The overall results suggest that the IbMYB116 gene might enhance drought tolerance by activating a ROS-scavenging system through the JA signaling pathway in transgenic Arabidopsis. These findings reveal, for the first time, the crucial role of IbMYB116 in the drought tolerance of plants.

Comparative transcriptomic analysis reveals common molecular factors responsive to heat and drought stress in Agrostis stolonifera

Heat and drought stress are primary abiotic stresses confining growth of cool-season grass species during summer. The objective of this study was to identify common molecular factors and metabolic pathways associated with heat and drought responses in creeping bentgrass (Agrostis stolonifera) by comparative analysis of transcriptomic profiles between plants exposed to heat and drought stress. Plants were exposed to heat stress (35/30 °C day/night temperature) or drought stress by withholding irrigation for 21 d in growth chambers. Transcriptomic profiling by RNA-seq in A. stolonifera (cv. 'Penncross') found 670 commonly up-regulated and 812 commonly down-regulated genes by heat and drought stress. Transcriptional up-regulations of differentially expressed genes (DEGs) due to heat and drought stress include genes that were highly enriched in oxylipin biosynthetic process and proline biosynthetic process. Transcriptional down-regulations of genes under heat and drought stress were highly enriched and involved in thiamine metabolic process and calcium sensing receptor. These commonly-regulated genes by heat and drought stress identified in A. stolonifera suggested that drought and heat responses shared such common molecular factors and pathways, which could be potential candidate genes for genetic modification of improving plant tolerance to the combined heat and drought stress.

Root exudate metabolomes change under drought and show limited capacity for recovery

Root exudates comprise a large variety of compounds released by plants into the rhizosphere, including low-molecular-weight primary metabolites (particularly saccharides, amino acids and organic acids) and secondary metabolites (phenolics, flavonoids and terpenoids). Changes in exudate composition could have impacts on the plant itself, on other plants, on soil properties (e.g. amount of soil organic matter), and on soil organisms. The effects of drought on the composition of root exudates, however, have been rarely studied. We used an ecometabolomics approach to identify the compounds in the exudates of Quercus ilex (holm oak) under an experimental drought gradient and subsequent recovery. Increasing drought stress strongly affected the composition of the exudate metabolome. Plant exudates under drought consisted mainly of secondary metabolites (71% of total metabolites) associated with plant responses to drought stress, whereas the metabolite composition under recovery shifted towards a dominance of primary metabolites (81% of total metabolites). These results strongly suggested that roots exude the most abundant root metabolites. The exudates were changed irreversibly by the lack of water under extreme drought conditions, and the plants could not recover.