Mediator tail module subunits MED16 and MED25 differentially regulate abscisic acid signaling in Arabidopsis

Mediator kinase subunit CDK8 phosphorylates transcription factor TCP15 during tomato pollen development

Pollen development in flowering plants has strong implications for reproductive success. Pollen DNA can be targeted to improve plant traits for yield and stress tolerance. In this study, we demonstrated that the Mediator subunit CYCLIN-DEPENDENT KINASE 8 (CDK8) is a key modulator of pollen development in tomato (Solanum lycopersicum). SlCDK8 knockout led to significant decreases in pollen viability, fruit yield, and fruit seed number. We also found that SlCDK8 directly interacts with transcription factor TEOSINTE BRANCHED1-CYCLOIDEA-PCF15 (SlTCP15) using yeast two-hybrid screens. We subsequently showed that SlCDK8 phosphorylates Ser 187 of SlTCP15 to promote SlTCP15 stability. Phosphorylated TCP15 directly bound to the TGGGCY sequence in the promoters of DYSFUNCTIONAL TAPETUM 1 (SlDYT1) and MYB DOMAIN PROTEIN 103 (SlMYB103), which are responsible for pollen development. Consistently, disruption of SlTCP15 resembled slcdk8 tomato mutants. In sum, our work identified a new substrate of Mediator CDK8 and revealed an important regulatory role of SlCDK8 in pollen development via cooperation with SlTCP15.

Apple E3 ligase MdPUB23 mediates ubiquitin-dependent degradation of MdABI5 to delay ABA-triggered leaf senescence

ABSCISIC ACID-INSENSITIVE5 (ABI5) is a core regulatory factor that mediates the ABA signaling response and leaf senescence. However, the molecular mechanism underlying the synergistic regulation of leaf senescence by ABI5 with interacting partners and the homeostasis of ABI5 in the ABA signaling response remain to be further investigated. In this study, we found that the accelerated effect of MdABI5 on leaf senescence is partly dependent on MdbHLH93, an activator of leaf senescence in apple. MdABI5 directly interacted with MdbHLH93 and improved the transcriptional activation of the senescence-associated gene MdSAG18 by MdbHLH93. MdPUB23, a U-box E3 ubiquitin ligase, physically interacted with MdABI5 and delayed ABA-triggered leaf senescence. Genetic and biochemical analyses suggest that MdPUB23 inhibited MdABI5-promoted leaf premature senescence by targeting MdABI5 for ubiquitin-dependent degradation. In conclusion, our results verify that MdABI5 accelerates leaf senescence through the MdABI5-MdbHLH93-MdSAG18 regulatory module, and MdPUB23 is responsible for the dynamic regulation of ABA-triggered leaf senescence by modulating the homeostasis of MdABI5.

Comparative phylogenetic analysis of the mediator complex subunit in asparagus bean (Vigna unguiculata ssp. sesquipedialis) and its expression profile under cold stress

BACKGROUND

The mediator complex subunits (MED) constitutes a multiprotein complex, with each subunit intricately involved in crucial aspects of plant growth, development, and responses to stress. Nevertheless, scant reports pertain to the VunMED gene within the context of asparagus bean (Vigna unguiculata ssp. sesquipedialis). Establishing the identification and exploring the responsiveness of VunMED to cold stress forms a robust foundation for the cultivation of cold-tolerant asparagus bean cultivars.

RESULTS

Within this study, a comprehensive genome-wide identification of VunMED genes was executed in the asparagus bean cultivar 'Ningjiang3', resulting in the discovery of 36 distinct VunMED genes. A phylogenetic analysis encompassing 232 MED genes from diverse species, including Arabidopsis, tomatoes, soybeans, mung beans, cowpeas, and asparagus beans, underscored the highly conserved nature of MED gene sequences. Throughout evolutionary processes, each VunMED gene underwent purification and neutral selection, with the exception of VunMED19a. Notably, VunMED9/10b/12/13/17/23 exhibited structural variations discernible across four cowpea species. Divergent patterns of temporal and spatial expression were evident among VunMED genes, with a prominent role attributed to most genes during early fruit development. Additionally, an analysis of promoter cis-acting elements was performed, followed by qRT-PCR assessments on roots, stems, and leaves to gauge relative expression after exposure to cold stress and subsequent recovery. Both treatments induced transcriptional alterations in VunMED genes, with particularly pronounced effects observed in root-based genes following cold stress. Elucidating the interrelationships between subunits involved a preliminary understanding facilitated by correlation and principal component analyses.

CONCLUSIONS

This study elucidates the pivotal contribution of VunMED genes to the growth, development, and response to cold stress in asparagus beans. Furthermore, it offers a valuable point of reference regarding the individual roles of MED subunits.

Analysis of the molecular mechanisms regulating how ZmEREB24 improves drought tolerance in maize (Zea mays) seedlings

Drought stress is one of the most limiting factors of maize productivity and can lead to a sharp reduction in the total biomass when it occurs at the seedling stage. Improving drought tolerance at the seedling stage is of great importance for maize breeding. The AP2/ERF transcription factor family plays a critical role in plant response to abiotic stresses. Here, we used a preliminary previously-generated ranscriptomic dataset to identify a highly drought-stress-responsive AP2 gene, i.e., ZmEREB24. Compared to the wild type, the overexpression of ZmEREB24 in maize significantly promotes drought tolerance of transgenic plants at the seedling stage. CRISPR/Cas9-based ZmEREB24-knockout mutants showed a drought-sensitive phenotype. RNA-seq analysis and EMSA assay revealed AATGG.CT and GTG.T.GCC motifs as the main binding sites of ZmEREB24 to the promoters of downstream target genes. DAP-seq identified four novel target genes involved in proline and sugar metabolism and hormone signal transduction of ZmEREB24. Our data indicate that ZmEREB24 plays important biological functions in regulating drought tolerance by binding to the promoters of drought stress genes and modulating their expression. The results further suggest a role of ZmEREB24 in regulating drought adaptation in maize, indicating its potential importance for employing molecular breeding in the development of high-yield drought-tolerant maize cultivars.

Molecular mechanism analysis of ZmRL6 positively regulating drought stress tolerance in maize

MYB-related genes, a subclass of MYB transcription factor family, have been documented to play important roles in biological processes such as secondary metabolism and stress responses that affect plant growth and development. However, the regulatory roles of MYB-related genes in drought stress response remain unclear in maize. In this study, we discovered that a 1R-MYB gene, ZmRL6, encodes a 96-amino acid protein and is highly drought-inducible. We also found that it is conserved in both barley (Hordeum vulgare L.) and Aegilops tauschii. Furthermore, we observed that overexpression of ZmRL6 can enhance drought tolerance while knock-out of ZmRL6 by CRISPR-Cas9 results in drought hypersensitivity. DAP-seq analyses additionally revealed the ZmRL6 target genes mainly contain ACCGTT, TTACCAAAC and AGCCCGAG motifs in their promoters. By combining RNA-seq and DAP-seq results together, we subsequently identified eight novel target genes of ZmRL6 that are involved in maize's hormone signal transduction, sugar metabolism, lignin synthesis, and redox signaling/oxidative stress. Collectively, our data provided insights into the roles of ZmRL6 in maize's drought response.

ABI5-FLZ13 module transcriptionally represses growth-related genes to delay seed germination in response to ABA

The bZIP transcription factor ABSCISIC ACID INSENSITIVE5 (ABI5) is a master regulator of seed germination and post-germinative growth in response to abscisic acid (ABA), but the detailed molecular mechanism by which it represses plant growth remains unclear. In this study, we used proximity labeling to map the neighboring proteome of ABI5 and identified FCS-LIKE ZINC FINGER PROTEIN 13 (FLZ13) as a novel ABI5 interaction partner. Phenotypic analysis of flz13 mutants and FLZ13-overexpressing lines demonstrated that FLZ13 acts as a positive regulator of ABA signaling. Transcriptomic analysis revealed that both FLZ13 and ABI5 downregulate the expression of ABA-repressed and growth-related genes involved in chlorophyll biosynthesis, photosynthesis, and cell wall organization, thereby repressing seed germination and seedling establishment in response to ABA. Further genetic analysis showed that FLZ13 and ABI5 function together to regulate seed germination. Collectively, our findings reveal a previously uncharacterized transcriptional regulatory mechanism by which ABA mediates inhibition of seed germination and seedling establishment.

CycC1;1-WRKY75 complex-mediated transcriptional regulation of SOS1 controls salt stress tolerance in Arabidopsis

SALT OVERLY SENSITIVE1 (SOS1) is a key component of plant salt tolerance. However, how SOS1 transcription is dynamically regulated in plant response to different salinity conditions remains elusive. Here, we report that C-type Cyclin1;1 (CycC1;1) negatively regulates salt tolerance by interfering with WRKY75-mediated transcriptional activation of SOS1 in Arabidopsis (Arabidopsis thaliana). Disruption of CycC1;1 promotes SOS1 expression and salt tolerance in Arabidopsis because CycC1;1 interferes with RNA polymerase II recruitment by occupying the SOS1 promoter. Enhanced salt tolerance of the cycc1;1 mutant was completely compromised by an SOS1 mutation. Moreover, CycC1;1 physically interacts with the transcription factor WRKY75, which can bind to the SOS1 promoter and activate SOS1 expression. In contrast to the cycc1;1 mutant, the wrky75 mutant has attenuated SOS1 expression and salt tolerance, whereas overexpression of SOS1 rescues the salt sensitivity of wrky75. Intriguingly, CycC1;1 inhibits WRKY75-mediated transcriptional activation of SOS1 via their interaction. Thus, increased SOS1 expression and salt tolerance in cycc1;1 were abolished by WRKY75 mutation. Our findings demonstrate that CycC1;1 forms a complex with WRKY75 to inactivate SOS1 transcription under low salinity conditions. By contrast, under high salinity conditions, SOS1 transcription and plant salt tolerance are activated at least partially by increased WRKY75 expression but decreased CycC1;1 expression.

A natural promoter variation of SlBBX31 confers enhanced cold tolerance during tomato domestication

Cold stress affects crop growth and productivity worldwide. Understanding the genetic basis of cold tolerance in germplasms is critical for crop improvement. Plants can coordinate environmental stimuli of light and temperature to regulate cold tolerance. However, it remains unknown which gene in germplasms could have such function. Here, we utilized genome-wide association study (GWAS) to investigate the cold tolerance of wild and cultivated tomato accessions and discovered that increased cold tolerance is accompanied with tomato domestication. We further identified a 27-bp InDel in the promoter of the CONSTANS-like transcription factor (TF) SlBBX31 is significantly linked with cold tolerance. Coincidentally, a key regulator of light signalling, SlHY5, can directly bind to the SlBBX31 promoter to activate SlBBX31 transcription while the 27-bp InDel can prevent S1HY5 from transactivating SlBBX31. Parallel to these findings, we observed that the loss of function of SlBBX31 results in impaired tomato cold tolerance. SlBBX31 can also modulate the cold-induced expression of several ERF TFs including CBF2 and DREBs. Therefore, our study has uncovered that SlBBX31 is possibly selected during tomato domestication for cold tolerance regulation, providing valuable insights for the development of hardy tomato varieties.

Do Opposites Attract? Auxin-Abscisic Acid Crosstalk: New Perspectives

Plants are constantly exposed to a variety of different environmental stresses, including drought, salinity, and elevated temperatures. These stress cues are assumed to intensify in the future driven by the global climate change scenario which we are currently experiencing. These stressors have largely detrimental effects on plant growth and development and, therefore, put global food security in jeopardy. For this reason, it is necessary to expand our understanding of the underlying mechanisms by which plants respond to abiotic stresses. Especially boosting our insight into the ways by which plants balance their growth and their defense programs appear to be of paramount importance, as this may lead to novel perspectives that can pave the way to increase agricultural productivity in a sustainable manner. In this review, our aim was to present a detailed overview of different facets of the crosstalk between the antagonistic plant hormones abscisic acid (ABA) and auxin, two phytohormones that are the main drivers of plant stress responses, on the one hand, and plant growth, on the other.

CycC1;1 negatively modulates ABA signaling by interacting with and inhibiting ABI5 during seed germination

Regulation of seed germination is important for plant survival and propagation. ABSCISIC ACID (ABA) INSENSITIVE5 (ABI5), the central transcription factor in the ABA signaling pathway, plays a fundamental role in the regulation of ABA-responsive gene expression during seed germination; however, how ABI5 transcriptional activation activity is regulated remains to be elucidated. Here, we report that C-type Cyclin1;1 (CycC1;1) is an ABI5-interacting partner affecting the ABA response and seed germination in Arabidopsis (Arabidopsis thaliana). The CycC1;1 loss-of-function mutant is hypersensitive to ABA, and this phenotype was rescued by mutation of ABI5. Moreover, CycC1;1 suppresses ABI5 transcriptional activation activity for ABI5-targeted genes including ABI5 itself by occupying their promoters and disrupting RNA polymerase II recruitment; thus the cycc1;1 mutant shows increased expression of ABI5 and genes downstream of ABI5. Furthermore, ABA reduces the interaction between CycC1;1 and ABI5, while phospho-mimic but not phospho-dead mutation of serine-42 in ABI5 abolishes CycC1;1 interaction with ABI5 and relieves CycC1;1 inhibition of ABI5-mediated transcriptional activation of downstream target genes. Together, our study illustrates that CycC1;1 negatively modulates the ABA response by interacting with and inhibiting ABI5, while ABA relieves the CycC1;1 interaction with and inhibition of ABI5 to activate ABI5 activity for the ABA response, thereby inhibiting seed germination.

Integration of ABA, GA, and light signaling in seed germination through the regulation of ABI5

Seed germination is precisely controlled by a variety of signals, among which light signals and the phytohormones abscisic acid (ABA) and gibberellin (GA) play crucial roles. New findings have greatly increased our understanding of the mechanisms by which these three signals regulate seed germination and the close connections between them. Although much work has been devoted to ABA, GA, and light signal interactions, there is still no systematic description of their combination, especially in seed germination. In this review, we integrate ABA, GA, and light signaling in seed germination through the direct and indirect regulation of ABSCISIC ACID INSENSITIVE5 (ABI5), the core transcription factor that represses seed germination in ABA signaling, into our current understanding of the regulatory mechanism of seed germination.

The Mediator Complex: A Central Coordinator of Plant Adaptive Responses to Environmental Stresses

As sessile organisms, plants are constantly exposed to a variety of environmental stresses and have evolved adaptive mechanisms, including transcriptional reprogramming, in order to survive or acclimate under adverse conditions. Over the past several decades, a large number of gene-specific transcription factors have been identified in the transcriptional regulation of plant adaptive responses. The Mediator complex plays a key role in transducing signals from gene-specific transcription factors to the transcription machinery to activate or repress target gene expression. Since its first purification about 15 years ago, plant Mediator complex has been extensively analyzed for its composition and biological functions. Mutants of many plant Mediator subunits are not lethal but are compromised in growth, development and response to biotic and abiotic stress, underscoring a particularly important role in plant adaptive responses. Plant Mediator subunits also interact with partners other than transcription factors and components of the transcription machinery, indicating the complexity of the regulation of gene expression by plant Mediator complex. Here, we present a comprehensive discussion of recent analyses of the structure and function of plant Mediator complex, with a particular focus on its roles in plant adaptive responses to a wide spectrum of environmental stresses and associated biological processes.

The tomato OST1-VOZ1 module regulates drought-mediated flowering

Flowering is a critical agricultural trait that substantially affects tomato fruit yield. Although drought stress influences flowering time, the molecular mechanism underlying drought-regulated flowering in tomato remains elusive. In this study, we demonstrated that loss of function of tomato OPEN STOMATA 1 (SlOST1), a protein kinase essential for abscisic acid (ABA) signaling and abiotic stress responses, lowers the tolerance of tomato plants to drought stress. slost1 mutants also exhibited a late flowering phenotype under both normal and drought stress conditions. We also established that SlOST1 directly interacts with and phosphorylates the NAC (NAM, ATAF and CUC)-type transcription factor VASCULAR PLANT ONE-ZINC FINGER 1 (SlVOZ1), at residue serine 67, thereby enhancing its stability and nuclear translocation in an ABA-dependent manner. Moreover, we uncovered several SlVOZ1 binding motifs from DNA affinity purification sequencing analyses and revealed that SlVOZ1 can directly bind to the promoter of the major flowering-integrator gene SINGLE FLOWER TRUSS to promote tomato flowering transition in response to drought. Collectively, our data uncover the essential role of the SlOST1-SlVOZ1 module in regulating flowering in response to drought stress in tomato and offer insights into a novel strategy to balance drought stress response and flowering.

The MIEL1-ABI5/MYB30 regulatory module fine tunes abscisic acid signaling during seed germination

The transcription factor ABSCISIC ACID INSENSITIVE5 (ABI5) plays a crucial role in abscisic acid (ABA) signaling during seed germination. However, how ABI5 is regulated during this process is poorly understood. Here, we report that the ubiquitin E3 ligase MIEL1 and its target transcription factor MYB30 modulate ABA responses in Arabidopsis thaliana during seed germination and seedling establishment via the precise regulation of ABI5. MIEL1 interacts with and ubiquitinates ABI5 to facilitate its degradation during germination. The transcription factor MYB30, whose turnover is mediated by MIEL1 during seed germination, also interacts with ABI5 to interfere with its transcriptional activity. MYB30 functions downstream of MIEL1 in the ABA response, and both are epistatic to ABI5 in ABA-mediated inhibition of seed germination and postgerminative growth. ABA treatment induces the degradation of MIEL1 and represses the interaction between MIEL1 and ABI5/MYB30, thus releasing both ABI5 and MYB30. Our results demonstrate that MIEL1 directly mediates the proteasomal degradation of ABI5 and inhibits its activity via the release of its target protein MYB30, thus ensuring precise ABA signaling during seed germination and seedling establishment.

The Arabidopsis circadian clock protein PRR5 interacts with and stimulates ABI5 to modulate abscisic acid signaling during seed germination

Seed germination and postgerminative growth require the precise coordination of multiple intrinsic and environmental signals. The phytohormone abscisic acid (ABA) suppresses these processes in Arabidopsis thaliana and the circadian clock contributes to the regulation of ABA signaling. However, the molecular mechanism underlying circadian clock-mediated ABA signaling remains largely unknown. Here, we found that the core circadian clock proteins PSEUDO-RESPONSE REGULATOR5 (PRR5) and PRR7 physically associate with ABSCISIC ACID-INSENSITIVE5 (ABI5), a crucial transcription factor of ABA signaling. PRR5 and PRR7 positively modulate ABA signaling redundantly during seed germination. Disrupting PRR5 and PRR7 simultaneously rendered germinating seeds hyposensitive to ABA, whereas the overexpression of PRR5 enhanced ABA signaling to inhibit seed germination. Consistent with this, the expression of several ABA-responsive genes is upregulated by PRR proteins. Genetic analysis demonstrated that PRR5 promotes ABA signaling mainly dependently on ABI5. Further mechanistic investigation revealed that PRR5 stimulates the transcriptional function of ABI5 without affecting its stability. Collectively, our results indicate that these PRR proteins function synergistically with ABI5 to activate ABA responses during seed germination, thus providing a mechanistic understanding of how ABA signaling and the circadian clock are directly integrated through a transcriptional complex involving ABI5 and central circadian clock components.

Brassica napus Mediator Subunit16 Induces BnMED25- and BnWRKY33-Activated Defense Signaling to Confer Sclerotinia sclerotiorum Resistance

The plant mediator is a highly conserved protein complex that interacts with transcription factors (TFs) and RNA polymerase II (RNAP II) to relay regulatory information during transcription. Plant immune response is one of the biological processes that is orchestrated by this regulatory mechanism. Brassica napus, an important oil crop, is severely attacked by a devastating disease Sclerotinia stem rot. Here, we explored broad-spectrum disease resistant roles of B. napus mediator subunit 16 (BnMED16) and its host defense mechanism against fugal pathogen Sclerotinia sclerotiorum. We found that BnMED16 expression was significantly increased by S. sclerotiorum infection, and its homologous overexpression resulted in rapid and comprehensive defense responses from the beginning to the end. This affected signal transduction with multiple channels including pathogen recognition, intracellular Ca²⁺ concentration, reactive oxygen species (ROS) accumulation and clearance, and activation of mitogen-activated protein kinase (MAPK) signaling cascades initially. Subsequently, pathogen-/defense-related genes and hormone-responsive pathways were highly activated, which resulted in enhanced cell wall and secretion of defense proteases. Furthermore, the biochemical analysis showed that BnMED16 interacts with BnMED25 and BnWRKY33. Additionally, BnMED25 also interacts with TFs BnMYC2, BnCOI1, and BnEIN3 of the JA/ET signal transduction pathway. Taken together, we proposed a hypothetical model that BnMED16 confers S. sclerotiorum resistance by enhancing BnMED25-mediated JA/ET defense pathways and BnWRKY33-activated defense signaling in B. napus. The BnMED16 overexpressing lines with enhanced broad-spectrum disease resistance could be useful for breeding Sclerotinia-resistant oilseed rape varieties, as well as serving as basis for further strategy development in resistance breeding.

Interconnection of iron and osmotic stress signalling in plants: is FIT a regulatory hub to cross-connect abscisic acid responses?

Osmotic stresses, such as salinity and drought, have deleterious effects on uptake and translocation of essential mineral nutrients. Iron (Fe) is an important micronutrient that regulates many processes in plants. Plants have adopted various molecular and physiological strategies for Fe acquisition from soil and transport to and within plants. Dynamic Fe signalling in plants tightly regulates Fe uptake and homeostasis. In this way, Fe nutrition is adjusted to growth and stress conditions, and Fe deficiency-regulated transcription factors, such as FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT), act as regulatory hubs in these responses. Here, we review and analyse expression of the various components of the Fe signalling during osmotic stresses. We discuss common players in the Fe and osmotic stress signalling. Furthermore, this review focuses on exploring a novel and exciting direct connection of regulatory mechanisms of Fe intake and acquisition with ABA-mediated environmental stress cues, like salt/drought. We propose a model that discuss how environmental stress affects Fe uptake and acquisition and vice versa at molecular-physiological levels in plants.

Functional Analysis of OsMED16 and OsMED25 in Response to Biotic and Abiotic Stresses in Rice

Mediator complex is a multiprotein complex that regulates RNA polymerase II-mediated transcription. Moreover, it functions in several signaling pathways, including those involved in response to biotic and abiotic stresses. We used virus-induced gene silencing (VIGS) to study the functions of two genes, namely OsMED16 and OsMED25 in response to biotic and abiotic stresses in rice. Both genes were differentially induced by Magnaporthe grisea (M. grisea), the causative agent of blast disease, hormone treatment, and abiotic stress. We found that both BMV: OsMED16- and BMV: OsMED25-infiltrated seedlings reduced the resistance to M. grisea by regulating the accumulation of H₂O₂ and expression of defense-related genes. Furthermore, BMV: OsMED16-infiltrated seedlings decreased the tolerance to cold by increasing the malondialdehyde (MDA) content and reducing the expression of cold-responsive genes.