Pathogen-induced autophagy regulates monolignol transport and lignin formation in plant immunity

The JA-to-ABA signaling relay promotes lignin deposition for wound healing in Arabidopsis

Plants are frequently exposed to herbivory and mechanical damage that result in wounding. Two fundamental strategies, regeneration and healing, are employed by plants upon wounding. How plants make different decisions and how wound healing is sustained until the damaged tissues recover are not fully understood. In this study, we found that local auxin accumulation patterns, determined by wounding modes, may activate different recovery programs in wounded tissues. Wounding triggers transient jasmonic acid (JA) signaling that promotes lignin deposition in the first few hours after wounding occurs. This early response is subsequently relayed to ABA signaling via MYC2. The induced JA signaling promotes ABA biosynthesis to maintain the expression of RAP2.6, a key factor for sustained lignin biosynthesis and the later wound-healing process. Our findings provide mechanistic insights into how plants heal from wounding and clarify the molecular mechanisms that underlie the prolonged healing process following wounding.

Extracellular vesicles of Norway spruce contain precursors and enzymes for lignin formation and salicylic acid

Lignin is a phenolic polymer in plants that rigidifies the cell walls of water-conducting tracheary elements and support-providing fibers and stone cells. Different mechanisms have been suggested for the transport of lignin precursors to the site of lignification in the cell wall. Extracellular vesicle (EV)-enriched samples isolated from a lignin-forming cell suspension culture of Norway spruce (Picea abies L. Karst.) contained both phenolic metabolites and enzymes related to lignin biosynthesis. Metabolomic analysis revealed mono-, di-, and oligolignols in the EV isolates, as well as carbohydrates and amino acids. In addition, salicylic acid (SA) and some proteins involved in SA signaling were detected in the EV-enriched samples. A proteomic analysis detected several laccases, peroxidases, β-glucosidases, putative dirigent proteins, and cell wall-modifying enzymes, such as glycosyl hydrolases, transglucosylase/hydrolases, and expansins in EVs. Our findings suggest that EVs are involved in transporting enzymes required for lignin polymerization in Norway spruce, and radical coupling of monolignols can occur in these vesicles.

Stress-Induced Autophagy Is Essential for Microspore Cell Fate Transition to the Initial Cell of Androgenesis

The isolated microspores can be reprogrammed towards embryogenesis via stress treatment during in vitro culture, and produce (doubled) haploid plants as a breeding source of new genetic variability. However, the mechanism underlying the cell fate transition from gametogenesis to embryogenesis remains largely unknown. Here, we report that autophagy plays a key role in cell fate transition for microspore embryogenesis (referred to as androgenesis) in Nicotiana tabacum. Immunofluorescence and transmission electronic microscopy detection unveiled that autophagy was triggered in microspores following exposure to inductive stress, and a transient wave of the numerous autophagy-related genes (ATGs) expression occurred before the initiation of microspore embryogenesis. Suppression or promotion of the original autophagy levels could inhibit microspore embryogenesis, indicating that stress-induced autophagic homeostasis is essential for cell fate transition. Furthermore, quantitative proteomics analysis revealed that autophagy might be involved in lignin biosynthesis and chromatin decondensation for promoting reprogramming for androgenesis initiation. Altogether, we reveal an essential role of autophagy in the microspore cell fate transition and androgenesis initiation, providing novel insight for understanding this critical developmental process.

Serine hydroxymethyltransferase6 is involved in growth and resistance against pathogens via ethylene and lignin production in Arabidopsis

Photorespiratory serine hydroxymethyltransferases (SHMTs) are important enzymes of cellular one-carbon metabolism. In this study, we investigated the potential role of SHMT6 in Arabidopsis thaliana. We found that SHMT6 is localized in the nucleus and expressed in different tissues during development. Interestingly SHMT6 is inducible in response to avirulent, virulent Pseudomonas syringae and to Fusarium oxysporum infection. Overexpression of SHMT6 leads to larger flowers, siliques, seeds, roots, and consequently an enhanced overall biomass. This enhanced growth was accompanied by increased stomatal conductance and photosynthetic capacity as well as ATP, protein, and chlorophyll levels. By contrast, a shmt6 knockout mutant displayed reduced growth. When challenged with Pseudomonas syringae pv tomato (Pst) DC3000 expressing AvrRpm1, SHMT6 overexpression lines displayed a clear hypersensitive response which was characterized by enhanced electrolyte leakage and reduced bacterial growth. In response to virulent Pst DC3000, the shmt6 mutant developed severe disease symptoms and becomes very susceptible, whereas SHMT6 overexpression lines showed enhanced resistance with increased expression of defense pathway associated genes. In response to Fusarium oxysporum, overexpression lines showed a reduction in symptoms. Moreover, SHMT6 overexpression lead to enhanced production of ethylene and lignin, which are important components of the defense response. Collectively, our data revealed that SHMT6 plays an important role in development and defense against pathogens.

GmCYB5-4 inhibit SMV proliferation by targeting P3 protein

Soybean mosaic virus (SMV) is a potyvirus found worldwide in soybean (Glycine max). GmCYB5-4 is a strong candidate interactor of P3. In this study, we comprehensively analyzed the GmCYB5 family in soybeans, including its distribution on chromosomes, promoter analysis, conserved motifs, phylogenetic analysis, and expression patterns. We cloned the full-length GmCYB5-4 and examined its interaction with P3 in yeast, which was later confirmed using bimolecular fluorescence complementation (BiFc). We silenced GmCYB5-4 using a bean pottle mosaic viris (BPMV) based system to generate SilCYB5-4 tissues, which surprisingly knocked down four isoforms of GmCYB5s for functional characterization. SilCYB5-4 plants were challenged with the SC3 strain to determine its involvement in SMV infection. Silencing GmCYB5-4 increased SMV accumulation, indicating that GmCYB5-4 inhibited SMV proliferation. However, further experiments are needed to elucidate the mechanism underlying the involvement of GmCYB5-4 in SMV infection.

Structural diversification of fungal cell wall in response to the stress signaling and remodeling during fungal pathogenesis

Fungi are one of the most diverse organisms found in our surroundings. The heterotrophic lifestyle of fungi and the ever-changing external environmental factors pose numerous challenges for their survival. Despite all adversities, fungi continuously develop new survival strategies to secure nutrition and space from their host. During host-pathogen interaction, filamentous phytopathogens in particular, effectively infect their hosts by maintaining polarised growth at the tips of hyphae. The fungal cell wall, being the prime component of host contact, plays a crucial role in fortifying the intracellular environment against the harsh external environment. Structurally, the fungal cell wall is a highly dynamic yet rigid component, responsible for maintaining cellular morphology. Filamentous pathogens actively maintain their dynamic cell wall to compensate rapid growth on the host. Additionally, they secrete effectors to dampen the sophisticated mechanisms of plant defense and initiate various downstream signaling cascades to repair the damage inflicted by the host. Thus, the fungal cell wall serves as a key modulator of fungal pathogenicity. The fungal cell wall with their associated signaling mechanisms emerge as intriguing targets for host immunity. This review comprehensively examines and summarizes the multifaceted findings of various research groups regarding the dynamics of the cell wall in filamentous fungal pathogens during host invasion.

Autophagy maintains endosperm quality during seed storage to preserve germination ability in Arabidopsis

To preserve germination ability, plant seeds must be protected from environmental stresses during the storage period. Here, we demonstrate that autophagy, an intracellular degradation system, maintains seed germination ability in Arabidopsis thaliana. The germination ability of long-term (>5 years) stored dry seeds of autophagy-defective (atg) mutant and wild-type (WT) plants was compared. Long-term stored (old) seeds of atg mutants showed lower germination ability than WT seeds, although short-term stored (new) seeds of atg mutants did not show such a phenotype. After removal of the seed coat and endosperm from old atg mutant seeds, the embryos developed into seedlings. Autophagic flux was maintained in endosperm cells during the storage period, and autophagy defect resulted in the accumulation of oxidized proteins and accelerated endosperm cell death. Consistent with these findings, the transcripts of genes, ENDO-β-MANNANASE 7 and EXPANSIN 2, which are responsible for degradation/remodeling of the endosperm cell wall during germination, were reduced in old atg mutant seeds. We conclude that autophagy maintains endosperm quality during seed storage by suppressing aging-dependent oxidative damage and cell death, which allows the endosperm to perform optimal functions during germination, i.e., cell wall degradation/remodeling, even after long-term storage.

New insights into plant autophagy: molecular mechanisms and roles in development and stress responses

Autophagy is an evolutionarily conserved eukaryotic intracellular degradation process. Although the molecular mechanisms of plant autophagy share similarities with those in yeast and mammals, certain unique mechanisms have been identified. Recent studies have highlighted the importance of autophagy during vegetative growth stages as well as in plant-specific developmental processes, such as seed development, germination, flowering, and somatic reprogramming. Autophagy enables plants to adapt to and manage severe environmental conditions, such as nutrient starvation, high-intensity light stress, and heat stress, leading to intracellular remodeling and physiological changes in response to stress. In the past, plant autophagy research lagged behind similar studies in yeast and mammals; however, recent advances have greatly expanded our understanding of plant-specific autophagy mechanisms and functions. This review summarizes current knowledge and latest research findings on the mechanisms and roles of plant autophagy with the objective of improving our understanding of this vital process in plants.

Grass lignin: biosynthesis, biological roles, and industrial applications

Lignin is a phenolic heteropolymer found in most terrestrial plants that contributes an essential role in plant growth, abiotic stress tolerance, and biotic stress resistance. Recent research in grass lignin biosynthesis has found differences compared to dicots such as Arabidopsis thaliana. For example, the prolific incorporation of hydroxycinnamic acids into grass secondary cell walls improve the structural integrity of vascular and structural elements via covalent crosslinking. Conversely, fundamental monolignol chemistry conserves the mechanisms of monolignol translocation and polymerization across the plant phylum. Emerging evidence suggests grass lignin compositions contribute to abiotic stress tolerance, and periods of biotic stress often alter cereal lignin compositions to hinder pathogenesis. This same recalcitrance also inhibits industrial valorization of plant biomass, making lignin alterations and reductions a prolific field of research. This review presents an update of grass lignin biosynthesis, translocation, and polymerization, highlights how lignified grass cell walls contribute to plant development and stress responses, and briefly addresses genetic engineering strategies that may benefit industrial applications.

Integrated transcriptomic and metabolomic analysis reveals the potential mechanisms underlying indium-induced inhibition of root elongation in wheat plants

Soil contamination by indium, an emerging contaminant from electronics, has a negative impact on crop growth. Inhibition of root growth serves as a valuable biomarker for predicting indium phytotoxicity. Therefore, elucidating the molecular mechanisms underlying indium-induced root damage is essential for developing strategies to mitigate its harmful effects. Our transcriptomic findings revealed that indium affects the expression of numerous genes related to cell wall composition and metabolism in wheat roots. Morphological and compositional analysis revealed that indium induced a 2.9-fold thickening and a 17.5 % increase in the content of cell walls in wheat roots. Untargeted metabolomics indicated a substantial upregulation of the phenylpropanoid biosynthesis pathway. As the major end product of phenylpropanoid metabolism, lignin significantly accumulated in root cell walls after indium exposure. Together with increased lignin precursors, enhanced activity of lignin biosynthesis-related enzymes was observed. Moreover, analysis of the monomeric content and composition of lignin revealed a significant enrichment of p-hydroxyphenyl (H) and syringyl (S) units in root cell walls under indium stress. The present study contributes to the existing knowledge of indium toxicity. It provides valuable insights for developing sustainable solutions to address the challenges posed by electronic waste and indium contamination on agroecosystems.

Transcriptomic-Proteomic Analysis Revealed the Regulatory Mechanism of Peanut in Response to Fusarium oxysporum

Peanut Fusarium rot, which is widely observed in the main peanut-producing areas in China, has become a significant factor that has limited the yield and quality in recent years. It is highly urgent and significant to clarify the regulatory mechanism of peanuts in response to Fusarium oxysporum. In this study, transcriptome and proteome profiling were combined to provide new insights into the molecular mechanisms of peanut stems after F. oxysporums infection. A total of 3746 differentially expressed genes (DEGs) and 305 differentially expressed proteins (DEPs) were screened. The upregulated DEGs and DEPs were primarily enriched in flavonoid biosynthesis, circadian rhythm-plant, and plant-pathogen interaction pathways. Then, qRT-PCR analysis revealed that the expression levels of phenylalanine ammonia-lyase (PAL), chalcone isomerase (CHI), and cinnamic acid-4-hydroxylase (C4H) genes increased after F. oxysporums infection. Moreover, the expressions of these genes varied in different peanut tissues. All the results revealed that many metabolic pathways in peanut were activated by improving key gene expressions and the contents of key enzymes, which play critical roles in preventing fungi infection. Importantly, this research provides the foundation of biological and chemical analysis for peanut disease resistance mechanisms.

Pour some sugar on me: The diverse functions of phenylpropanoid glycosylation

The phenylpropanoid metabolism is the source of a vast array of specialized metabolites that play diverse functions in plant growth and development and contribute to all aspects of plant interactions with their surrounding environment. These compounds protect plants from damaging ultraviolet radiation and reactive oxygen species, provide mechanical support for the plants to stand upright, and mediate plant-plant and plant-microorganism communications. The enormous metabolic diversity of phenylpropanoids is further expanded by chemical modifications known as "decorative reactions", including hydroxylation, methylation, glycosylation, and acylation. Among these modifications, glycosylation is the major driving force of phenylpropanoid structural diversification, also contributing to the expansion of their properties. Phenylpropanoid glycosylation is catalyzed by regioselective uridine diphosphate (UDP)-dependent glycosyltransferases (UGTs), whereas glycosyl hydrolases known as β-glucosidases are the major players in deglycosylation. In this article, we review how the glycosylation process affects key physicochemical properties of phenylpropanoids, such as molecular stability and solubility, as well as metabolite compartmentalization/storage and biological activity/toxicity. We also summarize the recent knowledge on the functional implications of glycosylation of different classes of phenylpropanoid compounds. A balance of glycosylation/deglycosylation might represent an essential molecular mechanism to regulate phenylpropanoid homeostasis, allowing plants to dynamically respond to diverse environmental signals.

Genome-wide identification and expression patterns of the laccase gene family in response to kiwifruit bacterial canker infection

BACKGROUND

Kiwifruit bacterial canker, caused by Pseudomonas syringae pv. actinidiae (Psa), is a destructive disease worldwide. Resistance genes that respond to Psa infection urgently need to be identified for controlling this disease. Laccase is mainly involved in the synthesis of lignin in the plant cell wall and plays a prominent role in plant growth and resistance to pathogen infection. However, the role of laccase in kiwifruit has not been reported, and whether laccase is pivotal in the response to Psa infection remains unclear.

RESULTS

We conducted a bioinformatics analysis to identify 55 laccase genes (AcLAC1-AcLAC55) in the kiwifruit genome. These genes were classified into five cluster groups (I-V) based on phylogenetic analysis, with cluster groups I and II having the highest number of members. Analysis of the exon-intron structure revealed that the number of exons varied from 1 to 8, with an average of 5 introns. Our evolutionary analysis indicated that fragment duplication played a key role in the expansion of kiwifruit laccase genes. Furthermore, evolutionary pressure analysis suggested that AcLAC genes were under purifying selection. We also performed a cis-acting element analysis and found that AcLAC genes contained multiple hormone (337) and stress signal (36) elements in their promoter regions. Additionally, we investigated the expression pattern of laccase genes in kiwifruit stems and leaves infected with Psa. Our findings revealed that laccase gene expression levels in the stems were higher than those in the leaves 5 days after inoculation with Psa. Notably, AcLAC2, AcLAC4, AcLAC17, AcLAC18, AcLAC26, and AcLAC42 showed significantly higher expression levels (p < 0.001) compared to the non-inoculated control (0 d), suggesting their potential role in resisting Psa infection. Moreover, our prediction indicated that 21 kiwifruit laccase genes are regulated by miRNA397, they could potentially act as negative regulators of lignin biosynthesis.

CONCLUSIONS

These results are valuable for further analysis of the resistance function and molecular mechanism of laccases in kiwifruit.

VAMP726 from maize and Arabidopsis confers pollen resistance to heat and UV radiation by influencing lignin content of sporopollenin

Sporopollenin in the pollen cell wall protects male gametophytes from stresses. Phenylpropanoid derivatives, including guaiacyl (G) lignin units, are known to be structural components of sporopollenin, but the exact composition of sporopollenin remains to be fully resolved. We analyzed the phenylpropanoid derivatives in sporopollenin from maize and Arabidopsis by thioacidolysis coupled with nuclear magnetic resonance (NMR) and gas chromatography-mass spectrometry (GC-MS). The NMR and GC-MS results confirmed the presence of p-hydroxyphenyl (H), G, and syringyl (S) lignin units in sporopollenin from maize and Arabidopsis. Strikingly, H units account for the majority of lignin monomers in sporopollenin from these species. We next performed a genome-wide association study to explore the genetic basis of maize sporopollenin composition and identified a vesicle-associated membrane protein (ZmVAMP726) that is strongly associated with lignin monomer composition of maize sporopollenin. Genetic manipulation of VAMP726 affected not only lignin monomer composition in sporopollenin but also pollen resistance to heat and UV radiation in maize and Arabidopsis, indicating that VAMP726 is functionally conserved in monocot and dicot plants. Our work provides new insight into the lignin monomers that serve as structural components of sporopollenin and characterizes VAMP726, which affects sporopollenin composition and stress resistance in pollen.

The Hypersensitive Response to Plant Viruses

Plant proteins with domains rich in leucine repeats play important roles in detecting pathogens and triggering defense reactions, both at the cellular surface for pattern-triggered immunity and in the cell to ensure effector-triggered immunity. As intracellular parasites, viruses are mostly detected intracellularly by proteins with a nucleotide binding site and leucine-rich repeats but receptor-like kinases with leucine-rich repeats, known to localize at the cell surface, have also been involved in response to viruses. In the present review we report on the progress that has been achieved in the last decade on the role of these leucine-rich proteins in antiviral immunity, with a special focus on our current understanding of the hypersensitive response.

Water-Soluble Alkali Lignin as a Natural Radical Scavenger and Anticancer Alternative

Several phytochemicals, which display antioxidant activity and inhibit cancer cell phenotypes, could be used for cancer treatment and prevention. Lignin, as a part of plant biomass, is the second most abundant natural biopolymer worldwide, and represents approximately 30% of the total organic carbon content of the biosphere. Historically, lignin-based products have been viewed as waste materials of limited industrial usefulness, but modern technologies highlight the applicability of lignin in a variety of industrial branches, including biomedicine. The aims of our preliminary study were to compare the antioxidant properties of water-soluble alkali lignin solutions, before and after UV-B irradiation, as well as to clarify their effect on colon cancer cell viability (Colon 26), applied at low (tolerable) concentrations. The results showed a high antioxidant capacity of lignin solutions, compared to a water-soluble control antioxidant standard (Trolox) and remarkable radical scavenging activity was observed after their UV-B irradiation. Diminishment of cell viability as well as inhibition of the proliferative activity of the colon cancer cell line with an increase in alkali lignin concentrations were observed. Our results confirmed that, due to its biodegradable and biocompatible nature, lignin could be a potential agent for cancer therapy, especially in nanomedicine as a drug delivery system.

Plant Exosome-like Nanovesicles and Their Role in the Innovative Delivery of RNA Therapeutics

Exosomes are single membrane-bound spheres released from cells carrying complex cargoes, including lipids, proteins, and nucleic acids. Exosomes transfer specific cargoes from donor to acceptor cells, playing important roles in cell-to-cell communication. Current studies have reported that plant exosomes are prominent in transferring small RNA between host and pathogens in a cross-kingdom manner. Plant exosomes are excellent RNA interference (RNAi) delivery agents with similar physical and chemical properties to mammalian exosomes and have potential applications in therapeutic delivery systems. Recent data have suggested that plant exosome-like nanovesicles (PENVs) and artificial PENV-derived nano-vectors (APNVs) are beneficial for delivering therapeutic small RNA in mammalian systems and exhibit excellent competitiveness in future clinical applications. This review features their preparation methods, composition, roles in small RNA delivery for health functionalities, and their potency as functional nanomedicine.

Extracellular vesicles: cross-organismal RNA trafficking in plants, microbes, and mammalian cells

Extracellular vesicles (EVs) are membrane-enclosed nanometer-scale particles that transport biological materials such as RNAs, proteins, and metabolites. EVs have been discovered in nearly all kingdoms of life as a form of cellular communication across different cells and between interacting organisms. EV research has primarily focused on EV-mediated intra-organismal transport in mammals, which has led to the characterization of a plethora of EV contents from diverse cell types with distinct and impactful physiological effects. In contrast, research into EV-mediated transport in plants has focused on inter-organismal interactions between plants and interacting microbes. However, the overall molecular content and functions of plant and microbial EVs remain largely unknown. Recent studies into the plant-pathogen interface have demonstrated that plants produce and secrete EVs that transport small RNAs into pathogen cells to silence virulence-related genes. Plant-interacting microbes such as bacteria and fungi also secrete EVs which transport proteins, metabolites, and potentially RNAs into plant cells to enhance their virulence. This review will focus on recent advances in EV-mediated communications in plant-pathogen interactions compared to the current state of knowledge of mammalian EV capabilities and highlight the role of EVs in cross-kingdom RNA interference.

Silencing of the calcium-dependent protein kinase TaCDPK27 improves wheat resistance to powdery mildew

BACKGROUND

Calcium ions (Ca²⁺), secondary messengers, are crucial for the signal transduction process of the interaction between plants and pathogens. Ca²⁺ signaling also regulates autophagy. As plant calcium signal-decoding proteins, calcium-dependent protein kinases (CDPKs) have been found to be involved in biotic and abiotic stress responses. However, information on their functions in response to powdery mildew attack in wheat crops is limited.

RESULT

In the present study, the expression levels of TaCDPK27, four essential autophagy-related genes (ATGs) (TaATG5, TaATG7, TaATG8, and TaATG10), and two major metacaspase genes, namely, TaMCA1 and TaMCA9, were increased by powdery mildew (Blumeria graminis f. sp. tritici, Bgt) infection in wheat seedling leaves. Silencing TaCDPK27 improves wheat seedling resistance to powdery mildew, with fewer Bgt hyphae occurring on TaCDPK27-silenced wheat seedling leaves than on normal seedlings. In wheat seedling leaves under powdery mildew infection, silencing TaCDPK27 induced excess contents of reactive oxygen species (ROS); decreased the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT); and led to an increase in programmed cell death (PCD). Silencing TaCDPK27 also inhibited autophagy in wheat seedling leaves, and silencing TaATG7 also enhanced wheat seedling resistance to powdery mildew infection. TaCDPK27-mCherry and GFP-TaATG8h colocalized in wheat protoplasts. Overexpressed TaCDPK27-mCherry fusions required enhanced autophagy activity in wheat protoplast under carbon starvation.

CONCLUSION

These results suggested that TaCDPK27 negatively regulates wheat resistance to PW infection, and functionally links with autophagy in wheat.

Comprehensive expression analyses of the ABCG subfamily reveal SvABCG17 as a potential transporter of lignin monomers in the model C4 grass Setaria viridis

Although several aspects of lignin metabolism have been extensively characterized, the mechanism(s) by which lignin monomers are transported across the plasma membrane remains largely unknown. Biochemical, proteomic, expression and co-expression analyses from several plant species support the involvement of active transporters, mainly those belonging to the ABC superfamily. Here, we report on the genome-wide characterization of the ABCG gene subfamily in the model C4 grass Setaria viridis and further identification of the members potentially involved in monolignol transport. A total of 48 genes encoding SvABCGs were found in the S. viridis genome, from which 21 SvABCGs were classified as full-size transporters and 27 as half-size transporters. Comprehensive analysis of the ABCG subfamily in S. viridis based on expression and co-expression analyses support a role for SvABCG17 in monolignol transport: (i) SvABCG17 is orthologous to AtABCG29, a monolignol transporter in Arabidopsis thaliana; (ii) SvABCG17 displays a similar expression profile to that of lignin biosynthetic genes in a set of different S. viridis tissues and along the elongating internode; (iii) SvABCG17 is highly co-expressed with lignin-related genes in a public transcriptomic database; (iv) SvABCG17displays particularly high expression in the top of the S. viridis elongating internode, a tissue undergoing active lignification; (v) SvABCG17 mRNA localization coincides with the histochemical pattern of lignin deposition; and (vi) the promoter of SvABCG17 is activated by secondary cell wall-associated transcription factors, especially by lignin-specific activators of the MYB family. Further studies might reveal further aspects of this potential monolignol transporter, including its real substrate specificity and whether it works redundantly with other ABC members during S. viridis lignification.