Signal Integration by Cyclin-Dependent Kinase 8 (CDK8) Module and Other Mediator Subunits in Biotic and Abiotic Stress Responses

Salt stress activates the CDK8-AHL10-SUVH2/9 module to dynamically regulate salt tolerance in Arabidopsis

Salt stress has devastating effects on agriculture, yet the key regulators modulating the transcriptional dynamics of salt-responsive genes remain largely elusive in plants. Here, we discover that salt stress substantially induces the kinase activity of Mediator cyclin-dependent kinase 8 (CDK8), which is essential for its positive role in regulating salt tolerance. CDK8 is identified to phosphorylate AT-hook motif nuclear-localized protein 10 (AHL10) at serine 314, leading to its degradation under salt stress. Consistently, AHL10 is found to negatively regulate salt tolerance. Transcriptome analysis further indicates that CDK8 regulates over 20% of salt-responsive genes, half of which are co-regulated by AHL10. Moreover, AHL10 is revealed to recruit SU(VAR)3-9 homologs (SUVH2/9) to AT-rich DNA sequences in the nuclear matrix-attachment regions (MARs) of salt-responsive gene promoters, facilitating H3K9me2 deposition and repressing salt-responsive genes. Our study thereby has identified the CDK8-AHL10-SUVH2/9 module as a key molecular switch controlling transcriptional dynamics in response to salt stress.

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.

Isolation of the AccCDK8 gene of Apis cerana cerana and its functional analysis under pesticide and heavy metal stress

Environmental pollution has gained negative attention in recent years. The pesticides and heavy metals are top list of environmental toxicants directly endangering the survival and development of Apis cerana cerana. Cyclin-dependent kinases (CDKs) are heteromeric serine/threonine kinases that participate in cell cycle regulation and have a vital role in pesticide and heavy metal stress in Apis cerana cerana. In this experiment, we filtered out CDK8 gene from Apis cerana cerana (AccCDK8) and investigated its functions of pesticide and heavy metals resistance. Sequence analysis indicated that AccCDK8 is highly homologous to multiple CDK8s and contains a highly conserved CDK active site sequence. Phylogenetic analysis showed that AmCDK8 and AccCDK8 were closely related evolutionarily in Apis mellifera. Transcriptome analysis revealed that AccCDK8 expression was differentially affected after exposure to pesticide and heavy metal stresses. This indicates that AccCDK8 has a significant role in the resistance of Apis cerana cerana to pesticide and heavy metal stresses. It has implications for studying the function of CDK in other insects in response to stress.

CDK8 inhibitors antagonize HIV-1 reactivation and promote provirus latency in T cells

Latent HIV-1 provirus represents the barrier toward a cure for infection and is dependent upon the host RNA Polymerase (Pol) II machinery for reemergence. Here, we find that inhibitors of the RNA Pol II mediator kinases CDK8/19, Senexin A and BRD6989, inhibit induction of HIV-1 expression in response to latency-reversing agents and T cell signaling agonists. These inhibitors were found to impair recruitment of RNA Pol II to the HIV-1 LTR. Furthermore, HIV-1 expression in response to several latency reversal agents was impaired upon disruption of CDK8 by shRNA or gene knockout. However, the effects of CDK8 depletion did not entirely mimic CDK8/19 kinase inhibition suggesting that the mediator kinases are not functionally redundant. Additionally, treatment of CD4+ peripheral blood mononuclear cells isolated from people living with HIV-1 and who are receiving antiretroviral therapy with Senexin A inhibited induction of viral replication in response to T cell stimulation by PMA and ionomycin. These observations indicate that the mediator kinases, CDK8 and CDK19, play a significant role for regulation of HIV-1 transcription and that small molecule inhibitors of these enzymes may contribute to therapies designed to promote deep latency involving the durable suppression of provirus expression. IMPORTANCE A cure for HIV-1 infection will require novel therapies that can force elimination of cells that contain copies of the virus genome inserted into the cell chromosome, but which is shut off, or silenced. These are known as latently-infected cells, which represent the main reason why current treatment for HIV/AIDS cannot cure the infection because the virus in these cells is unaffected by current drugs. Our results indicate that chemical inhibitors of Cdk8 also inhibit the expression of latent HIV provirus. Cdk8 is an important enzyme that regulates the expression of genes in response to signals to which cells need to respond and which is produced by a gene that is frequently mutated in cancers. Our observations indicate that Cdk8 inhibitors may be employed in novel therapies to prevent expression from latent provirus, which might eventually enable infected individuals to cease treatment with antiretroviral drugs.

Beyond stress response: OST1 opening doors for plants to grow

The sucrose non-fermenting 1 (SNF1)-related protein kinase 2 (SnRK2) family members have been discovered to regulate abiotic stress response via the abscisic acid (ABA)-independent and dependent signaling pathways. SnRK2.6, also known as Open Stomata 1 (OST1), is a serine/threonine protein kinase that plays critical roles in linking ABA receptor complexes and downstream components such as transcription factors and anion channels to regulate stress response. Asides from its well-known regulatory roles in stomatal movement and cold stress response, OST1 has also been demonstrated recently to modulate major developmental roles of flowering and growth in plants. In this review, we will discuss about the various roles of OST1 as well as the 'doors' that OST1 can 'open' to help plants perform stress adaptation. Therefore, we will address how OST1 can regulate stomata apertures, cold stress tolerance as well as other aspects of its emerging roles such as balancing flowering and root growth in response to drought.

Simple, Fast and Efficient Methods for Analysing the Structural, Ultrastructural and Cellular Components of the Cell Wall

Plant cell walls are a fundamental component of plant biology and play an essential role in plant growth and development. The metabolic components of the cell wall can be investigated in a fast, simple, and highly efficient manner using various and distinct microscopy techniques. Here, we report implementing a flowchart to analyse tobacco plants’ structural, ultrastructural, and metabolic components supplemented with far-red light. In addition, biochemical components, such as lignin, cellulose, phenolic compounds, and reducing sugars, present in the plant cell walls were quantified using light, fluorescence, and electron microscopy. Our data were generated from samples prepared via tissue fixation, incorporation in resins, and slicing using microtomes. Moreover, we have used routine staining and contrast techniques to characterise plant cell walls. Here, we describe several protocols that use classic and modern techniques as well as qualitative and quantitative analytical methods to study cell walls, enabling the plant research community to understand and select the most suitable methods for the microscopic analysis of metabolic components. Finally, we discuss specific ideas aimed at new students of plant anatomy and microscopy. This research not only described the structural, ultrastructural, and metabolic components of the plant cell wall, but also explained the strategies for understanding cellular development.

Effects of Low Temperature Stress on Source-Sink Organs in Wheat and Phosphorus Mitigation Strategies

The 21st century presents many challenges to mankind, including climate change, fast growing human population, and serious concerns over food security. Wheat is a leading cereal crop that largely fulfills the global food needs. Low temperature stress accompanied by nutrient-starved soils is badly disrupting the source-sink relationship of wheat, thus causing an acute decline in final yield and deteriorating the grain quality. This review paper aimed to understand how low temperature stress affects wheat source-sink organs (i.e., leaves, roots, and spikes) and how phosphorus application reliefs in alleviating its harmful consequences. Also, we discussed mitigation strategies to enhance wheat capacity to adapt to varying temperature extremes and made rational recommendations based on modern agronomic and breeding approaches. Therefore, this study is likely to establish a solid foundation for improving the tolerance to low temperature stress and to improve its phosphorus utilization efficiency in wheat.

Plant CDKs-Driving the Cell Cycle through Climate Change

In a growing population, producing enough food has become a challenge in the face of the dramatic increase in climate change. Plants, during their evolution as sessile organisms, developed countless mechanisms to better adapt to the environment and its fluctuations. One important way is through the plasticity of their body and their forms, which are modulated during plant growth by accurate control of cell divisions. A family of serine/threonine kinases called cyclin-dependent kinases (CDK) is a key regulator of cell divisions by controlling cell cycle progression. In this review, we compile information on the primary response of plants in the regulation of the cell cycle in response to environmental stresses and show how the cell cycle proteins (mainly the cyclin-dependent kinases) involved in this regulation can act as components of environmental response signaling cascades, triggering adaptive responses to drive the cycle through climate fluctuations. Understanding the roles of CDKs and their regulators in the face of adversity may be crucial to meeting the challenge of increasing agricultural productivity in a new climate.

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.