Both Light-Induced SA Accumulation and ETI Mediators Contribute to the Cell Death Regulated by BAK1 and BKK1

Extracellular perception of multiple novel core effectors from the broad host-range pear anthracnose pathogen Colletotrichum fructicola in the nonhost Nicotiana benthamiana

Colletotrichum fructicola is emerging as a devastating pathogenic fungus causing anthracnose in a wide range of horticultural crops, particularly fruits. Exploitation of nonhost resistance (NHR) represents a robust strategy for plant disease management. Perception of core effectors from phytopathogens frequently leads to hypersensitive cell death and resistance in nonhost plants; however, such core effectors in C. fructicola and their signaling components in non-hosts remain elusive. Here, we found a virulent C. fructicola strain isolated from pear exhibits non-adaptation in the model plant Nicotiana benthamiana. Perception of secreted molecules from C. fructicola appears to be a dominant factor in NHR, and four novel core effectors-CfCE4, CfCE25, CfCE61, and CfCE66-detected by N. benthamiana were, accordingly, identified. These core effectors exhibit cell death-inducing activity in N. benthamiana and accumulate in the apoplast. With a series of CRISPR/Cas9-edited mutants or gene-silenced plants, we found the coreceptor BAK1 and helper NLRs including ADR1, NRG1, and NRCs mediate perceptions of these core effectors in N. benthamiana. Concurrently, multiple N. benthamiana genes encoding cell surface immune receptors and intracellular immune receptors were greatly induced by C. fructicola. This work represents the first characterization of the repertoire of C. fructicola core effectors responsible for NHR. Significantly, the novel core effectors and their signaling components unveiled in this study offered insights into a continuum of layered immunity during NHR and will be helpful for anthracnose disease management in diverse horticultural crops.

BAK-up: the receptor kinase BAK-TO-LIFE 2 enhances immunity when BAK1 is lacking

BRI1-ASSOCIATED KINASE 1 (BAK1/SERK3) and its closest homolog BAK1-LIKE 1 (BKK1/SERK4) are leucine-rich repeat receptor kinases (LRR-RKs) belonging to the SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) family. They act as co-receptors of various other LRR-RKs and participate in multiple signaling events by complexing and transphosphorylating ligand-binding receptors. Initially identified as the brassinosteroid receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) co-receptor, BAK1 also functions in plant immunity by interacting with pattern recognition receptors. Mutations in BAK1 and BKK1 cause severely stunted growth and cell death, characterized as autoimmune cell death. Several factors play a role in this type of cell death, including RKs and components of effector-triggered immunity (ETI) signaling pathways, glycosylation factors, ER quality control components, nuclear trafficking components, ion channels, and Nod-like receptors (NLRs). The Shan lab has recently discovered a novel RK BAK-TO-LIFE 2 (BTL2) that interacts with BAK1 and triggers cell death in the absence of BAK1 and BKK1. This RK compensates for the loss of BAK1-mediated pattern-triggered immunity (PTI) by activating phytocytokine-mediated immune and cell death responses.

A phospho-switch constrains BTL2-mediated phytocytokine signaling in plant immunity

Enabling and constraining immune activation is of fundamental importance in maintaining cellular homeostasis. Depleting BAK1 and SERK4, the co-receptors of multiple pattern recognition receptors (PRRs), abolishes pattern-triggered immunity but triggers intracellular NOD-like receptor (NLR)-mediated autoimmunity with an elusive mechanism. By deploying RNAi-based genetic screens in Arabidopsis, we identified BAK-TO-LIFE 2 (BTL2), an uncharacterized receptor kinase, sensing BAK1/SERK4 integrity. BTL2 induces autoimmunity through activating Ca2+ channel CNGC20 in a kinase-dependent manner when BAK1/SERK4 are perturbed. To compensate for BAK1 deficiency, BTL2 complexes with multiple phytocytokine receptors, leading to potent phytocytokine responses mediated by helper NLR ADR1 family immune receptors, suggesting phytocytokine signaling as a molecular link connecting PRR- and NLR-mediated immunity. Remarkably, BAK1 constrains BTL2 activation via specific phosphorylation to maintain cellular integrity. Thus, BTL2 serves as a surveillance rheostat sensing the perturbation of BAK1/SERK4 immune co-receptors in promoting NLR-mediated phytocytokine signaling to ensure plant immunity.

Major viral diseases in grain legumes: designing disease resistant legumes from plant breeding and OMICS integration

Grain legumes play a crucial role in human nutrition and as a staple crop for low-income farmers in developing and underdeveloped nations, contributing to overall food security and agroecosystem services. Viral diseases are major biotic stresses that severely challenge global grain legume production. In this review, we discuss how exploring naturally resistant grain legume genotypes within germplasm, landraces, and crop wild relatives could be used as promising, economically viable, and eco-environmentally friendly solution to reduce yield losses. Studies based on Mendelian and classical genetics have enhanced our understanding of key genetic determinants that govern resistance to various viral diseases in grain legumes. Recent advances in molecular marker technology and genomic resources have enabled us to identify genomic regions controlling viral disease resistance in various grain legumes using techniques such as QTL mapping, genome-wide association studies, whole-genome resequencing, pangenome and 'omics' approaches. These comprehensive genomic resources have expedited the adoption of genomics-assisted breeding for developing virus-resistant grain legumes. Concurrently, progress in functional genomics, especially transcriptomics, has helped unravel underlying candidate gene(s) and their roles in viral disease resistance in legumes. This review also examines the progress in genetic engineering-based strategies, including RNA interference, and the potential of synthetic biology techniques, such as synthetic promoters and synthetic transcription factors, for creating viral-resistant grain legumes. It also elaborates on the prospects and limitations of cutting-edge breeding technologies and emerging biotechnological tools (e.g., genomic selection, rapid generation advances, and CRISPR/Cas9-based genome editing tool) in developing virus-disease-resistant grain legumes to ensure global food security.

Allelic variation in the Arabidopsis TNL CHS3/CSA1 immune receptor pair reveals two functional cell-death regulatory modes

Some plant NLR immune receptors are encoded in head-to-head "sensor-executor" pairs that function together. Alleles of the NLR pair CHS3/CSA1 form three clades. The clade 1 sensor CHS3 contains an integrated domain (ID) with homology to regulatory domains, which is lacking in clades 2 and 3. In this study, we defined two cell-death regulatory modes for CHS3/CSA1 pairs. One is mediated by ID domain on clade 1 CHS3, and the other relies on CHS3/CSA1 pairs from all clades detecting perturbation of an associated pattern-recognition receptor (PRR) co-receptor. Our data support the hypothesis that an ancestral Arabidopsis CHS3/CSA1 pair gained a second recognition specificity and regulatory mechanism through ID acquisition while retaining its original specificity as a "guard" against PRR co-receptor perturbation. This likely comes with a cost, since both ID and non-ID alleles of the pair persist in diverse Arabidopsis populations through balancing selection.

Guarding the central regulator of extracellular perception in plants - A job for two

BAK1 is a central regulator of extracellular receptor proteins, essential in plant development and defense. In this issue of Cell Host & Microbe, dual reports (Schultze et al. and Yang et al.) describe how intracellular NLR immune receptors guard BAK1, with implications for extracellular perception and immune receptor engineering.

The Arabidopsis TIR-NBS-LRR protein CSA1 guards BAK1-BIR3 homeostasis and mediates convergence of pattern- and effector-induced immune responses

Arabidopsis BAK1/SERK3, a co-receptor of leucine-rich repeat pattern recognition receptors (PRRs), mediates pattern-triggered immunity (PTI). Genetic inactivation of BAK1 or BAK1-interacting receptor-like kinases (BIRs) causes cell death, but the direct mechanisms leading to such deregulation remains unclear. Here, we found that the TIR-NBS-LRR protein CONSTITUTIVE SHADE AVOIDANCE 1 (CSA1) physically interacts with BIR3, but not with BAK1. CSA1 mediates cell death in bak1-4 and bak1-4 bir3-2 mutants via components of effector-triggered immunity-(ETI) pathways. Effector HopB1-mediated perturbation of BAK1 also results in CSA1-dependent cell death. Likewise, microbial pattern pg23-induced cell death, but not PTI responses, requires CSA1. Thus, we show that CSA1 guards BIR3 BAK1 homeostasis and integrates pattern- and effector-mediated cell death pathways downstream of BAK1. De-repression of CSA1 in the absence of intact BAK1 and BIR3 triggers ETI cell death. This suggests that PTI and ETI pathways are activated downstream of BAK1 for efficient plant immunity.

Role of pathogen's effectors in understanding host-pathogen interaction

Pathogens can pose challenges to plant growth and development at various stages of their life cycle. Two interconnected defense strategies prevent the growth of pathogens in plants, i.e., molecular patterns triggered immunity (PTI) and pathogenic effector-triggered immunity (ETI) that often provides resistance when PTI no longer functions as a result of pathogenic effectors. Plants may trigger an ETI defense response by directly or indirectly detecting pathogen effectors via their resistance proteins. A typical resistance protein is a nucleotide-binding receptor with leucine-rich sequences (NLRs) that undergo structural changes as they recognize their effectors and form associations with other NLRs. As a result of dimerization or oligomerization, downstream components activate "helper" NLRs, resulting in a response to ETI. It was thought that ETI is highly dependent on PTI. However, recent studies have found that ETI and PTI have symbiotic crosstalk, and both work together to create a robust system of plant defense. In this article, we have summarized the recent advances in understanding the plant's early immune response, its components, and how they cooperate in innate defense mechanisms. Moreover, we have provided the current perspective on engineering strategies for crop protection based on up-to-date knowledge.

An Overview of PRR- and NLR-Mediated Immunities: Conserved Signaling Components across the Plant Kingdom That Communicate Both Pathways

Cell-surface-localized pattern recognition receptors (PRRs) and intracellular nucleotide-binding domain and leucine-rich repeat receptors (NLRs) are plant immune proteins that trigger an orchestrated downstream signaling in response to molecules of microbial origin or host plant origin. Historically, PRRs have been associated with pattern-triggered immunity (PTI), whereas NLRs have been involved with effector-triggered immunity (ETI). However, recent studies reveal that such binary distinction is far from being applicable to the real world. Although the perception of plant pathogens and the final mounting response are achieved by different means, central hubs involved in signaling are shared between PTI and ETI, blurring the zig-zag model of plant immunity. In this review, we not only summarize our current understanding of PRR- and NLR-mediated immunities in plants, but also highlight those signaling components that are evolutionarily conserved across the plant kingdom. Altogether, we attempt to offer an overview of how plants mediate and integrate the induction of the defense responses that comprise PTI and ETI, emphasizing the need for more evolutionary molecular plant-microbe interactions (EvoMPMI) studies that will pave the way to a better understanding of the emergence of the core molecular machinery involved in the so-called evolutionary arms race between plants and microbes.

Molecular Mechanisms of Tumor Immunomodulation in the Microenvironment of Colorectal Cancer

Colorectal cancer remains one of the most important health challenges in our society. The development of cancer immunotherapies has fostered the need to better understand the anti-tumor immune mechanisms at play in the tumor microenvironment and the strategies by which the tumor escapes them. In this review, we provide an overview of the molecular interactions that regulate tumor inflammation. We particularly discuss immunomodulatory cell-cell interactions, cell-soluble factor interactions, cell-extracellular matrix interactions and cell-microbiome interactions. While doing so, we highlight relevant examples of tumor immunomodulation in colorectal cancer.

A novel leucine-rich repeat receptor-like kinase MRK1 regulates resistance to multiple stresses in tomato

Leucine-rich repeat receptor-like kinases (LRR-RLKs) are ubiquitous in higher plants, which act as receptors of extracellular signals to trigger multiple physiological processes. However, the functions of the majority of LRR-RLKs remain largely unknown, especially in tomato (Solanum lycopersicum L.). Here, we found that MRK1 (Multiple resistance-associated kinase 1), encoding a novel tomato LRR-RLK, was significantly induced either by temperature stresses or bacterial pathogen attacks. Knocking out MRK1 impaired the tolerance to both cold and heat stress, accompanied with the decrease in transcripts of master regulators C-repeat binding factor 1 (CBF1) and Heat shock transcription factor a-1a (HsfA1a), respectively. Additionally, mrk1 mutants were hypersensitive to Pseudomonas syringae pv. tomato DC3000 and Ralstonia solanacearum and compromised pattern-triggered immunity (PTI) responses as evidenced by decreased reactive oxygen species production and reduced upregulation of the PTI marker genes. Moreover, bimolecular fluorescence complementation, split-luciferase assay and coimmunoprecipitation supported the existence of complex formation between the MRK1, FLS2 and Somatic embryogenesis receptor kinase (SERK3A/SERK3B) in a ligand-independent manner. This work demonstrates that tomato MRK1 as a novel positive regulator of multiple stresses, which might be a potential breeding target to improve crop stress resistance.

Transcriptional Coactivators: Driving Force of Plant Immunity

Salicylic acid (SA) is a plant defense signal that mediates local and systemic immune responses against pathogen invasion. However, the underlying mechanism of SA-mediated defense is very complex due to the involvement of various positive and negative regulators to fine-tune its signaling in diverse pathosystems. Upon pathogen infections, elevated level of SA promotes massive transcriptional reprogramming in which Non-expresser of PR genes 1 (NPR1) acts as a central hub and transcriptional coactivator in defense responses. Recent findings show that Enhanced Disease Susceptibility 1 (EDS1) also functions as a transcriptional coactivator and stimulates the expression of PR1 in the presence of NPR1 and SA. Furthermore, EDS1 stabilizes NPR1 protein level, while NPR1 sustains EDS1 expression during pathogenic infection. The interaction of NPR1 and EDS1 coactivators initiates transcriptional reprogramming by recruiting cyclin-dependent kinase 8 in the Mediator complex to control immune responses. In this review, we highlight the recent breakthroughs that considerably advance our understanding on how transcriptional coactivators interact with their functional partners to trigger distinct pathways to facilitate immune responses, and how SA accumulation induces dynamic changes in NPR1 structure for transcriptional reprogramming. In addition, the functions of different Mediator subunits in SA-mediated plant immunity are also discussed in light of recent discoveries. Taken together, the available evidence suggests that transcriptional coactivators are essential and potent regulators of plant defense pathways and play crucial roles in coordinating plant immune responses during plant-pathogen interactions.

The kinase CIPK14 functions as a negative regulator of plant immune responses to Pseudomonas syringae in Arabidopsis

As a critical second messenger in plants, Ca²⁺ is involved in numerous biological processes including biotic and abiotic stress responses. The CBL-interacting protein kinases, known as CIPKs, are essential components in Ca²⁺-mediated signal transduction pathways. Here, we found that CIPK14 plays a role in the process of regulating immune response in Arabidopsis. The CIPK14 loss-of-function mutants exhibited enhanced resistance to the P. syringae, whereas CIPK14 overexpression plants were more susceptible to bacterial pathogen. Enhanced resistance in cipk14 mutants were accompanied by increased accumulation of SA and elevated expression of defense marker genes (PR1, EDS1, EDS5, ICS1). Overexpression of CIPK14 suppressed Pst DC3000, Pst DC3000 hrcC and flg22 induced generation of ROS and callose deposition. As compared with wild type plants, the expression levels of MPK3/6-dependent PTI marker genes (FRK1, CYP81F2, WAK2, FOX) were up-regulated in cipk14 mutants but down-regulated in CIPK14 overexpression plants after flg22 and elf18 treatment. Additionally, both loss-of-function and gain-of-function of CIPK14 significantly altered the phosphorylation status of MPK3/6 under flg22 treatment, suggesting that CIPK14 is a general modulator of plant immunity at both transcriptional and post-transcriptional level. Taken together, our results uncover that CIPK14 acts as a negative regulator in plant immune response.

Effectors of Puccinia striiformis f. sp. tritici Suppressing the Pathogenic-Associated Molecular Pattern-Triggered Immune Response Were Screened by Transient Expression of Wheat Protoplasts

Puccinia striiformis f. sp. tritici (Pst) is an important pathogen of wheat (Triticum aestivum L.) stripe rust, and the effector protein secreted by haustoria is a very important component involved in the pathogenic process. Although the candidate effector proteins secreted by Pst haustoria have been predicted to be abundant, few have been functionally validated. Our study confirmed that chitin and flg22 could be used as elicitors of the pathogenic-associated molecular pattern-triggered immune (PTI) reaction in wheat leaves and that TaPr-1-14 could be used as a marker gene to detect the PTI reaction. In addition, the experimental results were consistent in wheat protoplasts. A rapid and efficient method for screening and identifying the effector proteins of Pst was established by using the wheat protoplast transient expression system. Thirty-nine Pst haustorial effector genes were successfully cloned and screened for expression in the protoplast. We identified three haustorial effector proteins, PSEC2, PSEC17, and PSEC45, that may inhibit the response of wheat to PTI. These proteins are localized in the somatic cytoplasm and nucleus of wheat protoplasts and are highly expressed during the infection and parasitism of wheat.

Recent Advances in Effector-Triggered Immunity in Plants: New Pieces in the Puzzle Create a Different Paradigm

Plants rely on multiple immune systems to protect themselves from pathogens. When pattern-triggered immunity (PTI)-the first layer of the immune response-is no longer effective as a result of pathogenic effectors, effector-triggered immunity (ETI) often provides resistance. In ETI, host plants directly or indirectly perceive pathogen effectors via resistance proteins and launch a more robust and rapid defense response. Resistance proteins are typically found in the form of nucleotide-binding and leucine-rich-repeat-containing receptors (NLRs). Upon effector recognition, an NLR undergoes structural change and associates with other NLRs. The dimerization or oligomerization of NLRs signals to downstream components, activates "helper" NLRs, and culminates in the ETI response. Originally, PTI was thought to contribute little to ETI. However, most recent studies revealed crosstalk and cooperation between ETI and PTI. Here, we summarize recent advancements in our understanding of the ETI response and its components, as well as how these components cooperate in the innate immune signaling pathways. Based on up-to-date accumulated knowledge, this review provides our current perspective of potential engineering strategies for crop protection.

H2O2 signaling modulates Glycoprotein-1 induced programmed cell death in tobacco suspension cells

Glycoprotein (GP)-1 is a glycoprotein elicitor with antiviral activity found in Streptomyces kanasensis zx01. GP-1 can induce programmed cell death (PCD) in vitro; however, the underlying mechanism is unclear. In the present study, we demonstrated that GP-1 induced PCD in tobacco suspension cells, which was modulated by hydrogen peroxide (H2O2). GP-1 participated in and modulated biologically relevant signaling in plant cells. GP-1 induced tobacco cell death in a dose- and time-dependent manner; affected the expression of BRI1-associated receptor kinase 1 (BAK1) and the accumulation of salicylic acid (SA), which are related to PCD; and enzymatic activities and mitochondrial functions. In conclusion, GP-1-induced PCD in tobacco may be mediated by H2O2 which alters BAK1 and SA levels, as well as mitochondrial and gene function. This cell signal cascade played an important role in the process of GP-1 induced plant disease resistance.

Loss of the common immune coreceptor BAK1 leads to NLR-dependent cell death

Plants utilize a two-tiered immune system consisting of pattern recognition receptor (PRR)-triggered immunity (PTI) and effector-triggered immunity (ETI) to defend themselves against pathogenic microbes. The receptor protein kinase BAK1 plays a central role in multiple PTI signaling pathways in Arabidopsis However, double mutants made by BAK1 and its closest paralog BKK1 exhibit autoimmune phenotypes, including cell death resembling a typical nucleotide-binding leucine-rich repeat protein (NLR)-mediated ETI response. The molecular mechanisms of the cell death caused by the depletion of BAK1 and BKK1 are poorly understood. Here, we show that the cell-death phenotype of bak1 bkk1 is suppressed when a group of NLRs, ADR1s, are mutated, indicating the cell-death of bak1 bkk1 is the consequence of NLR activation. Furthermore, introduction of a Pseudomonas syringae effector HopB1, which proteolytically cleaves activated BAK1 and its paralogs via either gene transformation or bacterium-delivery, results in a cell-death phenotype in an ADR1s-dependent manner. Our study thus pinpoints that BAK1 and its paralogs are likely guarded by NLRs.

TaRPM1 Positively Regulates Wheat High-Temperature Seedling-Plant Resistance to Puccinia striiformis f. sp. tritici

RPM1 is a CC-NBS-LRR protein that was first shown to be required for resistance to Pseudomonas syringae pv. maculicola in Arabidopsis thaliana. Our previous study showed that TaRPM1 gene in wheat was upregulated about six times following infection by Puccinia striiformis f. sp. tritici (Pst) under high temperature, compared with normal temperature. To study the function of TaRPM1 in wheat high-temperature seedling-plant (HTSP) resistance to Pst, the full length of TaRPM1 was cloned, with three copies each located on chromosomes 1A, 1B, and 1D. Transient expression of the TaRPM1-GFP fusion protein in Nicotiana benthamiana indicated that TaRPM1 localizes in the cytoplasm and nucleus. Profiling TaRPM1 expression indicated that TaRPM1 transcription was rapidly upregulated upon Pst inoculation under high temperature. In addition, TaRPM1 was induced by exogenous salicylic acid hormone application. Silencing TaRPM1 in wheat cultivar Xiaoyan 6 (XY 6) resulted in reduced HTSP resistance to Pst in terms of reduced number of necrotic cells and increased uredinial length, whereas no obvious phenotypic changes were observed in TaRPM1-silenced leaves under normal temperature. Related defense genes TaPR1 and TaPR2 were downregulated in TaRPM1-silenced plants under high temperature. We conclude that TaRPM1 is involved in HTSP resistance to Pst in XY 6.

BAKing up to Survive a Battle: Functional Dynamics of BAK1 in Plant Programmed Cell Death

In plants, programmed cell death (PCD) has diverse, essential roles in vegetative and reproductive development, and in the responses to abiotic and biotic stresses. Despite the rapid progress in understanding the occurrence and functions of the diverse forms of PCD in plants, the signaling components and molecular mechanisms underlying the core PCD machinery remain a mystery. The roles of BAK1 (BRASSINOSTEROID INSENSITIVE 1-associated receptor kinase 1), an essential co-receptor of multiple receptor complexes, in the regulation of immunity and development- and defense-related PCD have been well characterized. However, the ways in which BAK1 functions in mediating PCD need to be further explored. In this review, different forms of PCD in both plants and mammals are discussed. Moreover, we mainly summarize recent advances in elucidating the functions and possible mechanisms of BAK1 in controlling diverse forms of PCD. We also highlight the involvement of post-translational modifications (PTMs) of multiple signaling component proteins in BAK1-mediated PCD.

Arabidopsis GAAP1 to GAAP3 Play Redundant Role in Cell Death Inhibition by Suppressing the Upregulation of Salicylic Acid Pathway Under Endoplasmic Reticulum Stress

The unfolded protein response (UPR) is activated to sustain cell survival by reducing misfolded protein accumulation in the endoplasmic reticulum (ER). The UPR also promotes cell death when the ER stress is severe. However, the underlying molecular mechanisms of UPR activity regulation and cell death transition are less understood in plants. Arabidopsis GAAP1 and GAAP3 are involved in the regulation of UPR and cell death. Five GAAP gene members are found in Arabidopsis. Here, we analyzed the function of GAAP2 in addition to GAAP1 and GAAP3 in ER stress response using single, double, and triple mutants. Results showed that single or double or triple mutants reduced plant survival and enhanced cell death under ER stress. And the sensitivity increased with the number of mutation genes increase. Quantitative real-time polymerase chain reaction analysis showed that mutation in triple genes promoted UPR signaling when confronted with mild ER stress, advanced SA target genes upregulation when confronted with severe stress. Moreover, Quantitative detection by UPLC-ESI-MS/MS showed that ER stress upregulated salicylic acid (SA) content in plants. These data suggest that GAAP1 to GAAP3 played redundant roles in cell death resistance and fine tuning UPR activation. And the anti-cell death function of GAAPs might be achieved by impairing the up-regulation of the SA pathway under ER stress.

Combined QTL mapping, physiological and transcriptomic analyses to identify candidate genes involved in Brassica napus seed aging

Seed aging is an inevitable problem in the germplasm conservation of oil crops. Thus, clarifying the genetic mechanism of seed aging is important for rapeseed breeding. In this study, Brassica napus seeds were exposed to an artificial aging environment (40 °C and 90% relative humidity). Using a population of 172 recombinant inbred lines, 13 QTLs were detected on 8 chromosomes, which explained ~ 9.05% of the total phenotypic variation. The QTLs q2015AGIA-C08 and q2016AGI-C08-2 identified in the two environments were considered the same QTL. After artificial aging, lower germination index, increased relative electrical conductivity, malondialdehyde and proline content, and reduced soluble sugar, protein content and antioxidant enzyme activities were detected. Furthermore, seeds of extreme lines that were either left untreated (R0 and S0) or subjected to 15 days of artificial aging (R15 and S15) were used for transcriptome sequencing. In total, 2843, 1084, 429 and 1055 differentially expressed genes were identified in R15 vs. R0, S15 vs. S0, R0 vs. S0 and R15 vs. S15, respectively. Through integrated QTL mapping and RNA-sequencing analyses, seven genes, such as BnaA03g37460D, encoding heat shock transcription factor C1, and BnaA03g40360D, encoding phosphofructokinase 4, were screened as candidate genes involved in seed aging. Further researches on these candidate genes could broaden our understanding of the regulatory mechanisms of seed aging.

The cotton MAPK kinase GhMPK20 negatively regulates resistance to Fusarium oxysporum by mediating the MKK4-MPK20-WRKY40 cascade

Fusarium wilt is one of the most serious diseases affecting cotton. However, the pathogenesis and mechanism by which Fusarium oxysporum overcomes plant defence responses are unclear. Here, a new group D mitogen-activated protein kinase (MAPK) gene, GhMPK20, was identified and functionally analysed in cotton. GhMPK20 expression was significantly induced by F. oxysporum. Virus-induced gene silencing (VIGS) of GhMPK20 in cotton increased the tolerance to F. oxysporum, whereas ectopic GhMPK20 overexpression in Nicotiana benthamiana reduced F. oxysporum resistance via disruption of the salicylic acid (SA)-mediated defence pathway. More importantly, an F. oxysporum-induced MAPK cascade pathway composed of GhMKK4, GhMPK20 and GhWRKY40 was identified. VIGS of GhMKK4 and GhWRKY40 also enhanced F. oxysporum resistance in cotton, and the function of GhMKK4-GhMPK20 was shown to be essential for F. oxysporum-induced GhWRKY40 expression. Together, our results indicate that the GhMKK4-GhMPK20-GhWRKY40 cascade in cotton plays an important role in the pathogenesis of F. oxysporum. This research broadens our knowledge of the negative role of the MAPK cascade in disease resistance in cotton and provides an important scientific basis for the formulation of Fusarium wilt prevention strategies.

SlBIR3 Negatively Regulates PAMP Responses and Cell Death in Tomato

Bri1-associated kinase 1 (BAK1)-interacting receptor-like kinase (BIR) proteins have been shown to play important roles in regulating growth and development, pathogen associated molecular pattern (PAMP)-triggered immunity (PTI) responses, and cell death in the model plant, Arabidopsis thaliana. We identified four BIR family members in tomato (Solanum lycopersicum), including SlBIR3, an ortholog of AtBIR3 from A. thaliana. SlBIR3 is predicted to encode a membrane localized non-arginine-aspartate (non-RD) kinase that, based on protein sequence, does not have autophosphorylation activity but that can be phosphorylated in vivo. We established that SlBIR3 interacts with SlBAK1 and AtBAK1 using yeast two-hybrid assays and co-immunoprecipitation and maltose-binding protein pull down assays. We observed that SlBIR3 overexpression in tomato (cv. micro-tom) and A. thaliana has weak effect on growth and development through brassinosteroid (BR) signaling. SlBIR3 overexpression in A. thaliana suppressed flg22-induced defense responses, but did not affect infection with the bacterial pathogen Pseudomonas syringae (PstDC3000). This result was confirmed using virus-induced gene silencing (VIGS) in tomato in conjunction with PstDC3000 infection. Overexpression of SlBIR3 in tomato (cv. micro-tom) and A. thaliana resulted in enhanced susceptibility to the necrotrophic fungus Botrytis cinerea. In addition, co-silencing SlBIR3 with SlSERK3A or SlSERK3B using VIGS and the tobacco rattle virus (TRV)-RNA2 vector containing fragments of both the SlSERK3 and SlBIR3 genes induced spontaneous cell death, indicating a cooperation between the two proteins in this process. In conclusion, our study revealed that SlBIR3 is the ortholog of AtBIR3 and that it participates in BR, PTI, and cell death signaling pathways.

ERECTA Regulates Cell Elongation by Activating Auxin Biosynthesis in Arabidopsis thaliana

The ERECTA family genes, ERECTA (ER), ERECTA-LIKE1 (ERL1), and ERECTA-LIKE2 (ERL2), encode leucine-rich repeat receptor-like kinases in Arabidopsis thaliana. Knocking out these three genes can cause severe phenotypes, which indicates that they play significant roles in plant growth and development. However, the molecular mechanism within remains unclear. Here we show that the short hypocotyl phenotypes of er erl1 erl2 mutants are mainly due to the defects of cell elongation rather than the cell division. In contrast, in the ERECTA overexpression transgenic plants, the hypocotyl length is increased with elongated cells. Moreover, we show that the er erl1 erl2 triple mutant contains a low level of auxin, and the expression levels of the key auxin biosynthesis genes are significantly reduced. Consistent with this observation, increasing exogenous or endogenous auxin levels could partially rescue the cell elongation defects of the er erl1 erl2 triple mutant. Therefore, our results provide a molecular basis for auxin mediated ERECTA control of the hypocotyl length in Arabidopsis thaliana.