High CO2 - and pathogen-driven expression of the carbonic anhydrase βCA3 confers basal immunity in tomato

The plant disease triangle facing climate change: a molecular perspective

Variations in climate conditions can dramatically affect plant health and the generation of climate-resilient crops is imperative to food security. In addition to directly affecting plants, it is predicted that more severe climate conditions will also result in greater biotic stresses. Recent studies have identified climate-sensitive molecular pathways that can result in plants being more susceptible to infection under unfavorable conditions. Here, we review how expected changes in climate will impact plant-pathogen interactions, with a focus on mechanisms regulating plant immunity and microbial virulence strategies. We highlight the complex interactions between abiotic and biotic stresses with the goal of identifying components and/or pathways that are promising targets for genetic engineering to enhance adaptation and strengthen resilience in dynamically changing environments.

Phosphorylation of sugar transporter TST2 by protein kinase CPK27 enhances drought tolerance in tomato

Drought is a major environmental stress threatening plant growth and productivity. Calcium-dependent protein kinases (CPKs) are plant-specific Ca2+ sensors with multifaceted roles in signaling drought responses. Nonetheless, the mechanisms underpinning how CPKs transmit downstream drought signaling remain unresolved. Through genetic investigations, our study unveiled that knocking out CPK27 reduced drought tolerance in tomato (Solanum lycopersicum) plants and impaired abscisic acid (ABA)-orchestrated plant response to drought stress. Proteomics and phosphoproteomics revealed that CPK27-dependent drought-induced proteins were highly associated with the sugar metabolism pathway, which was further verified by reduced soluble sugar content in the cpk27 mutant under drought conditions. Using protein-protein interaction assays and phosphorylation assessments, we demonstrated that CPK27 directly interacted with and phosphorylated tonoplast sugar transporter 2 (TST2), promoting intercellular soluble sugar accumulation during drought stress. Furthermore, Ca2+ and ABA enhanced CPK27-mediated interaction and phosphorylation of TST2, thus revealing a role of TST2 in tomato plant drought tolerance. These findings extend the toolbox of potential interventions for enhancing plant drought stress tolerance and provide a target to improve drought tolerance by manipulating CPK27-mediated soluble sugar accumulation for rendering drought tolerance in a changing climate.

Transcriptional reprogramming in sound-treated Micro-Tom plants inoculated with Pseudomonas syringae pv. tomato DC3000

Sound vibrations (SV) are known to influence molecular and physiological processes that can improve crop performance and yield. In this study, the effects of three audible frequencies (100, 500 and 1000 Hz) at constant amplitude (90 dB) on tomato Micro-Tom physiological responses were evaluated 1 and 3 days post-treatment. Moreover, the potential use of SV treatment as priming agent for improved Micro-Tom resistance to Pseudomonas syringae pv. tomato DC3000 was tested by microarray. Results showed that the SV-induced physiological changes were frequency- and time-dependent, with the largest changes registered at 1000 Hz at day 3. SV treatments tended to alter the foliar content of photosynthetic pigments, soluble proteins, sugars, phenolic composition, and the enzymatic activity of polyphenol oxidase, peroxidase, superoxide dismutase and catalase. Microarray data revealed that 1000 Hz treatment is effective in eliciting transcriptional reprogramming in tomato plants grown under normal conditions, but particularly after the infection with Pst DC3000. Broadly, in plants challenged with Pst DC3000, the 1000 Hz pretreatment provoked the up-regulation of unique differentially expressed genes (DEGs) involved in cell wall reinforcement, phenylpropanoid pathway and defensive proteins. In addition, in those plants, DEGs associated with enhancing plant basal immunity, such as proteinase inhibitors, pathogenesis-related proteins, and carbonic anhydrase 3, were notably up-regulated in comparison with non-SV pretreated, infected plants. These findings provide new insights into the modulation of Pst DC3000-tomato interaction by sound and open up prospects for further development of strategies for plant disease management through the reinforcement of defense mechanisms in Micro-Tom plants.

H2 supplied via ammonia borane stimulates lateral root branching via phytomelatonin signaling

A reliable and stable hydrogen gas (H2) supply will benefit agricultural laboratory and field trials. Here, we assessed ammonia borane (AB), an efficient hydrogen storage material used in the energy industry, and determined its effect on plant physiology and the corresponding mechanism. Through hydroponics and pot experiments, we discovered that AB increases tomato (Solanum lycopersicum) lateral root (LR) branching and this function depended on the increased endogenous H2 level caused by the sustainable H2 supply. In particular, AB might trigger LR primordia initiation. Transgenic tomato and Arabidopsis (Arabidopsis thaliana) expressing hydrogenase1 (CrHYD1) from Chlamydomonas reinhardtii not only accumulated higher endogenous H2 and phytomelatonin levels but also displayed pronounced LR branching. These endogenous H2 responses achieved by AB or genetic manipulation were sensitive to the pharmacological removal of phytomelatonin, indicating the downstream role of phytomelatonin in endogenous H2 control of LR formation. Consistently, extra H2 supply failed to influence the LR defective phenotypes in phytomelatonin synthetic mutants. Molecular evidence showed that the phytomelatonin-regulated auxin signaling network and cell-cycle regulation were associated with the AB/H2 control of LR branching. Also, AB and melatonin had little effect on LR branching in the presence of auxin synthetic inhibitors. Collectively, our integrated approaches show that supplying H2 via AB increases LR branching via phytomelatonin signaling. This finding might open the way for applying hydrogen storage materials to horticultural production.

Climate change reshaping plant-fungal interaction

Plant diseases pose a severe threat to modern agriculture, necessitating effective and lasting control solutions. Environmental factors significantly shape plant ecology. Human-induced greenhouse gas emissions have led to global temperature rise, impacting various aspects, including carbon dioxide (CO2) concentration, temperature, ozone (O3), and ultraviolet-B, all of which influence plant diseases. Altered pathogen ranges can accelerate disease transmission. This review explores environmental effects on plant diseases, with climate change affecting fungal biogeography, disease incidence, and severity, as well as agricultural production. Moreover, we have discussed how climate change influences pathogen development, host-fungal interactions, the emergence of new races of fungi, and the dissemination of emerging fungal diseases across the globe. The discussion about environment-mediated impact on pattern-triggered immunity (PTI), effector-triggered immunity (ETI), and RNA interference (RNAi) is also part of this review. In conclusion, the review underscores the critical importance of understanding how climate change is reshaping plant-fungal interactions. It highlights the need for continuous research efforts to elucidate the mechanisms driving these changes and their ecological consequences. As the global climate continues to evolve, it is imperative to develop innovative strategies for mitigating the adverse effects of fungal pathogens on plant health and food security.

The pivotal ripening gene SlDML2 participates in regulating disease resistance in tomato

Fruit ripening and disease resistance are two essential biological processes for quality formation and maintenance. DNA methylation, in the form of 5-methylcytosine (5mC), has been elucidated to modulate fruit ripening, but its role in regulating fruit disease resistance remains poorly understood. In this study, we show that mutation of SlDML2, the DNA demethylase gene essential for fruit ripening, affects multiple developmental processes of tomato besides fruit ripening, including seed germination, leaf length and width and flower branching. Intriguingly, loss of SlDML2 function decreased the resistance of tomato fruits against the necrotrophic fungal pathogen Botrytis cinerea. Comparative transcriptomic analysis revealed an obvious transcriptome reprogramming caused by SlDML2 mutation during B. cinerea invasion. Among the thousands of differentially expressed genes, SlβCA3 encoding a β-carbonic anhydrase and SlFAD3 encoding a ω-3 fatty acid desaturase were demonstrated to be transcriptionally activated by SlDML2-mediated DNA demethylation and positively regulate tomato resistance to B. cinerea probably in the same genetic pathway with SlDML2. We further show that the pericarp tissue surrounding B. cinerea infection exhibited a delay in ripening with singnificant decrease in expression of ripening genes that are targeted by SlDML2 and increase in expression of SlβCA3 and SlFAD3. Taken together, our results uncover an essential layer of gene regulation mediated by DNA methylation upon B. cinerea infection and raise the possible that the DNA demethylase gene SlDML2, as a multifunctional gene, participates in modulating the trade-off between fruit ripening and disease resistance.

Autophagy promotes jasmonate-mediated defense against nematodes

Autophagy, as an intracellular degradation system, plays a critical role in plant immunity. However, the involvement of autophagy in the plant immune system and its function in plant nematode resistance are largely unknown. Here, we show that root-knot nematode (RKN; Meloidogyne incognita) infection induces autophagy in tomato (Solanum lycopersicum) and different atg mutants exhibit high sensitivity to RKNs. The jasmonate (JA) signaling negative regulators JASMONATE-ASSOCIATED MYC2-LIKE 1 (JAM1), JAM2 and JAM3 interact with ATG8s via an ATG8-interacting motif (AIM), and JAM1 is degraded by autophagy during RKN infection. JAM1 impairs the formation of a transcriptional activation complex between ETHYLENE RESPONSE FACTOR 1 (ERF1) and MEDIATOR 25 (MED25) and interferes with transcriptional regulation of JA-mediated defense-related genes by ERF1. Furthermore, ERF1 acts in a positive feedback loop and regulates autophagy activity by transcriptionally activating ATG expression in response to RKN infection. Therefore, autophagy promotes JA-mediated defense against RKNs via forming a positive feedback circuit in the degradation of JAMs and transcriptional activation by ERF1.

Ubiquitylation of PHYTOSULFOKINE RECEPTOR 1 modulates the defense response in tomato

Phytosulfokine (PSK) is a danger-associated molecular pattern recognized by PHYTOSULFOKINE RECEPTOR 1 (PSKR1) and initiates intercellular signaling to coordinate different physiological processes, especially in the defense response to the necrotrophic fungus Botrytis cinerea. The activity of peptide receptors is largely influenced by different posttranslational modifications, which determine intercellular peptide signal outputs. To date, the posttranslational modification to PHYTOSULFOKINE RECEPTOR 1 (PSKR1) remains largely unknown. Here, we show that tomato (Solanum lycopersicum) PSKR1 is regulated by the ubiquitin/proteasome degradation pathway. Using multiple protein-protein interactions and ubiquitylation analyses, we identified that plant U-box E3 ligases PUB12 and PUB13 interacted with PSKR1, among which PUB13 caused PSKR1 ubiquitylation at Lys-748 and Lys-905 sites to control PSKR1 abundance. However, this posttranslational modification was attenuated upon addition of PSK. Moreover, the disease symptoms observed in PUB13 knock-down and overexpression lines demonstrated that PUB13 significantly suppressed the PSK-initiated defense response. This highlights an important regulatory function for the turnover of a peptide receptor by E3 ligase-mediated ubiquitylation in the plant defense response.

The Carbonic Anhydrase βCA1 Functions in PopW-Mediated Plant Defense Responses in Tomato

β-Carbonic anhydrase (βCA) is very important for plant growth and development, but its function in immunity has also been examined. In this study, we found that the expression level of Solanum lycopersicum βCA1 (SlβCA1) was significantly upregulated in plants treated with Xanthomonas euvesicatoria 85-10. The protein was localized in the nucleus, cell membrane and chloroplast. Using tomato plants silenced with SlβCA1, we demonstrated that SlβCA1 plays an active role in plant disease resistance. Moreover, we found that the elicitor PopW upregulated the expression of SlβCA1, while the microbe-associated molecular pattern response induced by PopW was inhibited in TRV-SlβCA1. The interaction between PopW and SlβCA1 was confirmed. Here, we found that SlβCA1 was positively regulated during PopW-induced resistance to Xanthomonas euvesicatoria 85-10. These data indicate the importance of SlβCA1 in plant basic immunity and its recognition by the Harpin protein PopW as a new target for elicitor recognition.

Salicylic acid and jasmonic acid in elevated CO2-induced plant defense response to pathogens

Plants respond to elevated CO₂ (eCO₂) via a variety of signaling pathways that often rely on plant hormones. In particular, phytohormone salicylic acid (SA) and jasmonic acid (JA) play a key role in plant defense against diverse pathogens at eCO₂. eCO₂ affects the synthesis and signaling of SA and/or JA and variations in SA and JA signaling lead to variations in plant defense responses to pathogens. In general, eCO₂ promotes SA signaling and represses the JA pathway, and thus diseases caused by biotrophic and hemibiotrophic pathogens are typically suppressed, while the incidence and severity of diseases caused by necrotrophic fungal pathogens are enhanced under eCO₂ conditions. Moreover, eCO₂-induced modulation of antagonism between SA and JA leads to altered plant immunity to different pathogens. Notably, research in this area has often yielded contradictory findings and these responses vary depending on plant species, growth conditions, photoperiod, and fertilizer management. In this review, we focus on the recent advances in SA, and JA signaling pathways in plant defense and their involvement in plant immune responses to pathogens under eCO₂. Since atmospheric CO₂ will continue to increase, it is crucial to further explore how eCO₂ may alter plant defense and host-pathogen interactions in the context of climate change in both natural as well as agricultural ecosystems.

Melatonin mediates elevated carbon dioxide-induced photosynthesis and thermotolerance in tomato

Increasing carbon dioxide (CO₂ ) promotes photosynthesis and mitigates heat stress-induced deleterious effects on plants, but the regulatory mechanisms remain largely unknown. Here, we found that tomato (Solanum lycopersicum L.) plants treated with high atmospheric CO₂ concentrations (600, 800, and 1000 µmol mol^-1 ) accumulated increased levels of melatonin (N-acetyl-5-methoxy tryptamine) in their leaves and this response is conserved across many plant species, including Arabidopsis, rice, wheat, mustard, cucumber, watermelon, melon, and hot pepper. Elevated CO₂ (eCO₂ ; 800 µmol mol^-1 ) caused a 6.8-fold increase in leaf melatonin content, and eCO₂ -induced melatonin biosynthesis preferentially occurred through chloroplast biosynthetic pathways in tomato plants. Crucially, manipulation of endogenous melatonin levels by genetic means affected the eCO₂ -induced accumulation of sugar and starch in tomato leaves. Furthermore, net photosynthetic rate, maximum photochemical efficiency of photosystem II, and transcript levels of chloroplast- and nuclear-encoded photosynthetic genes, such as rbcL, rbcS, rbcA, psaD, petB, and atpA, significantly increased in COMT1 overexpressing (COMT1-OE) tomato plants, but not in melatonin-deficient comt1 mutants at eCO₂ conditions. While eCO₂ enhanced plant tolerance to heat stress (42°C) in wild-type and COMT1-OE, melatonin deficiency compromised eCO₂ -induced thermotolerance in comt1 plants. The expression of heat shock proteins genes increased in COMT1-OE but not in comt1 plants in response to eCO₂ under heat stress. Further analysis revealed that eCO₂ -induced thermotolerance was closely linked to the melatonin-dependent regulation of reactive oxygen species, redox homeostasis, cellular protein protection, and phytohormone metabolism. This study unveiled a crucial mechanism of elevated CO₂ -induced thermotolerance in which melatonin acts as an essential endogenous signaling molecule in tomato plants.

Phytosulfokine peptide optimizes plant growth and defense via glutamine synthetase GS2 phosphorylation in tomato

Phytosulfokine (PSK) is a plant pentapeptide hormone that fulfills a wide range of functions. Although PSK has frequently been reported to function in the inverse regulation of growth and defense in response to (hemi)biotrophic pathogens, the mechanisms involved remain largely unknown. Using the tomato (Solanum lycopersicum) and Pseudomonas syringae pv. tomato (Pst) DC3000 pathogen system, we present compelling evidence that the PSK receptor PSKR1 interacts with the calcium-dependent protein kinase CPK28, which in turn phosphorylates the key enzyme of nitrogen assimilation glutamine synthetase GS2 at two sites (Serine-334 and Serine-360). GS2 phosphorylation at S334 specifically regulates plant defense, whereas S360 regulates growth, uncoupling the PSK-induced effects on defense responses and growth regulation. The discovery of these sites will inform breeding strategies designed to optimize the growth-defense balance in a compatible manner.

Herbivore-induced Ca2+ signals trigger a jasmonate burst by activating ERF16-mediated expression in tomato

Herbivory severely affects plant growth, posing a threat to crop production. Calcium ion (Ca²⁺ ) signaling and accumulation of jasmonates (JAs) are activated in plant response to herbivore attack, leading to the expression of defense pathways. However, little is known about how the Ca²⁺ signal modulates JA biosynthesis. We used diverse techniques, including CRISPR/Cas9, UPLC-MS/MS and molecular biology methods to explore the role of ETHYLENE RESPONSE FACTOR 16 in Ca²⁺ signal-triggered JA burst during herbivore defense in tomato. Here we show that simulated herbivory induces GLUTAMATE RECEPTOR LIKE3.3/3.5 (GLR3.3/3.5)-dependent increases in electrical activity, Ca²⁺ influx and increases the abundance of CALMODULIN2 (CaM2) and ERF16 transcripts in tomato. The interaction between CaM2 and ERF16 promotes JA biosynthesis by enhancing the transcriptional activity of ERF16, which increases the activation of ERF16 expression and causes expression of LIPOXYGENASE D (LOXD), AOC and 12-OXO-PHYTODIENOIC ACID REDUCTASE 3 (OPR3), the key genes in JA biosynthesis. Mutation of CaM2 results in decreased JA accumulation, together with the expression of JA biosynthesis-related genes, leading to reduced resistance to the cotton bollworm Helicoverpa armigera. These findings reveal a molecular mechanism underpinning the Ca²⁺ signal-initiated systemic JA burst and emphasize the pivotal role of Ca²⁺ signal/ERF16 crosstalk in herbivore defense.

Climate change impedes plant immunity mechanisms

Rapid climate change caused by human activity is threatening global crop production and food security worldwide. In particular, the emergence of new infectious plant pathogens and the geographical expansion of plant disease incidence result in serious yield losses of major crops annually. Since climate change has accelerated recently and is expected to worsen in the future, we have reached an inflection point where comprehensive preparations to cope with the upcoming crisis can no longer be delayed. Development of new plant breeding technologies including site-directed nucleases offers the opportunity to mitigate the effects of the changing climate. Therefore, understanding the effects of climate change on plant innate immunity and identification of elite genes conferring disease resistance are crucial for the engineering of new crop cultivars and plant improvement strategies. Here, we summarize and discuss the effects of major environmental factors such as temperature, humidity, and carbon dioxide concentration on plant immunity systems. This review provides a strategy for securing crop-based nutrition against severe pathogen attacks in the era of climate change.

CO2 enrichment in greenhouse production: Towards a sustainable approach

As the unique source of carbon in the atmosphere, carbon dioxide (CO2) exerts a strong impact on crop yield and quality. However, CO2 deficiency in greenhouses during the daytime often limits crop productivity. Crucially, climate warming, caused by increased atmospheric CO2, urges global efforts to implement carbon reduction and neutrality, which also bring challenges to current CO2 enrichment systems applied in greenhouses. Thus, there is a timely need to develop cost-effective and environmentally friendly CO2 enrichment technologies as a sustainable approach to promoting agricultural production and alleviating environmental burdens simultaneously. Here we review several common technologies of CO2 enrichment in greenhouse production, and their characteristics and limitations. Some control strategies of CO2 enrichment in distribution, period, and concentration are also discussed. We further introduce promising directions for future CO2 enrichment including 1) agro-industrial symbiosis system (AIS); 2) interdisciplinary application of carbon capture and utilization (CCU); and 3) optimization of CO2 assimilation in C3 crops via biotechnologies. This review aims to provide perspectives on efficient CO2 utilization in greenhouse production.

Glucose sensing by regulator of G protein signaling 1 (RGS1) plays a crucial role in coordinating defense in response to environmental variation in tomato

Low light intensities affect the outbreak of plant diseases. However, the underlying molecular mechanisms remain poorly understood. High-performance liquid chromatography analysis of tomato (Solanum lycopersicum) revealed that apoplastic glucose (Glc) levels decreased in response to low light. Conversely, low-light-induced susceptibility to Pseudomonas syringae pv tomato (Pst) DC3000 was significantly alleviated by exogenous Glc treatment. Using cell-based biolayer interferometry assays, we found that Glc specifically binds to the tomato regulator of G protein signaling 1 (RGS1). Laser scanning confocal microscopy imaging revealed that Glc triggers RGS1 endocytosis, which influences the uncoupling of the RGS1-Gα (GPA1) and GPA1-Gβ (SlGB1) proteins, in a dose- and duration-dependent manner. Analysis of G protein single and double mutants revealed that RGS1 negatively regulates disease resistance under low light and is required for Glc-enhanced defense. Downstream of RGS1-Glc binding, GPA1 negatively mediates the light-intensity-regulated defense, whereas SlGB1 positively regulates this process. These results reveal a novel light-intensity-responsive defense system that is mediated by a Glc-RGS1-G protein signaling pathway. This information will be critical for future investigations of how plant cells sense extracellular sugars and adjust defense under different environments, as well as for genetic engineering approaches to improve stress resilience.

Countering elevated CO2 induced Fe and Zn reduction in Arabidopsis seeds

Growth at increased concentrations of CO₂ induces a reduction in seed zinc (Zn) and iron (Fe). Using Arabidopsis thaliana, we investigated whether this could be mitigated by reducing the elevated CO₂ -induced decrease in transpiration. We used an infrared imaging-based screen to isolate mutants in At1g08080 that encodes ALPHA CARBONIC ANHYDRASE 7 (ACA7). aca7 mutant alleles display wild-type (WT) responses to abscisic acid (ABA) and light but are compromised in their response to elevated CO₂ . ACA7 is expressed in guard cells. When aca7 mutants are grown at 1000 ppm CO₂ they exhibit higher transpiration and higher seed Fe and Zn content than WT grown under the same conditions. Our data show that by increasing transpiration it is possible to partially mitigate the reduction in seed Fe and Zn content when Arabidopsis is grown at elevated CO₂ .

ROS production and signalling in chloroplasts: cornerstones and evolving concepts

Reactive oxygen species (ROS) such as singlet oxygen, superoxide (O₂^●- ) and hydrogen peroxide (H₂ O₂ ) are the markers of living cells. Oxygenic photosynthesis produces ROS in abundance, which act as a readout of a functional electron transport system and metabolism. The concept that photosynthetic ROS production is a major driving force in chloroplast to nucleus retrograde signalling is embedded in the literature, as is the role of chloroplasts as environmental sensors. The different complexes and components of the photosynthetic electron transport chain (PETC) regulate O₂^●- production in relation to light energy availability and the redox state of the stromal Cys-based redox systems. All of the ROS generated in chloroplasts have the potential to act as signals and there are many sulphhydryl-containing proteins and peptides in chloroplasts that have the potential to act as H₂ O₂ sensors and function in signal transduction. While ROS may directly move out of the chloroplasts to other cellular compartments, ROS signalling pathways can only be triggered if appropriate ROS-sensing proteins are present at or near the site of ROS production. Chloroplast antioxidant systems serve either to propagate these signals or to remove excess ROS that cannot effectively be harnessed in signalling. The key challenge is to understand how regulated ROS delivery from the PETC to the Cys-based redox machinery is organised to transmit redox signals from the environment to the nucleus. Redox changes associated with stromal carbohydrate metabolism also play a key role in chloroplast signalling pathways.

BAG9 Confers Thermotolerance by Regulating Cellular Redox Homeostasis and the Stability of Heat Shock Proteins in Solanum lycopersicum

The Bcl-2-associated athanogene (BAG) family, a group of co-chaperones that share conservative domains in flora and fauna, is involved in plant growth, development, and stress tolerance. However, the function of tomato BAG genes on thermotolerance remains largely unknown. Herein, we found that the expression of BAG9 was induced during heat stress in tomato plants. Knockout of the BAG9 gene by CRISPR/Cas9 reduced, while its overexpression increased thermotolerance in tomato plants as reflected by the phenotype, photosynthesis rate, and membrane peroxidation. Heat-induced reactive oxygen species and oxidative/oxidized proteins were further increased in bag9 mutants and were normalized in BAG9 overexpressing plants. Furthermore, the activities of antioxidant enzymes, ascorbic acid (AsA)/dehydroascorbic acid (DHA), and reduced glutathione (GSH)/oxidized glutathione (GSSG) were reduced in bag9 mutants and were increased in BAG9 overexpressing plants under heat stress. Additionally, BAG9 interacted with Hsp20 proteins in vitro and in vivo. Accumulation of Hsp proteins induced by heat showed a reduction in bag9 mutants; meanwhile, it was increased in BAG9 overexpressing plants. Thus, BAG9 played a crucial role in response to heat stress by regulating cellular redox homeostasis and the stability of heat shock proteins.

SlRBP1 promotes translational efficiency via SleIF4A2 to maintain chloroplast function in tomato

Many glycine-rich RNA-binding proteins (GR-RBPs) have critical functions in RNA processing and metabolism. Here, we describe a role for the tomato (Solanum lycopersicum) GR-RBP SlRBP1 in regulating mRNA translation. We found that SlRBP1 knockdown mutants (slrbp1) displayed reduced accumulation of total chlorophyll and impaired chloroplast ultrastructure. These phenotypes were accompanied by deregulation of the levels of numerous key transcripts associated with chloroplast functions in slrbp1. Furthermore, native RNA immunoprecipitation-sequencing (nRIP-seq) recovered 61 SlRBP1-associated RNAs, most of which are involved in photosynthesis. SlRBP1 binding to selected target RNAs was validated by nRIP-qPCR. Intriguingly, the accumulation of proteins encoded by SlRBP1-bound transcripts, but not the mRNAs themselves, was reduced in slrbp1 mutants. Polysome profiling followed by RT-qPCR assays indicated that the polysome occupancy of target RNAs was lower in slrbp1 plants than in wild-type. Furthermore, SlRBP1 interacted with the eukaryotic translation initiation factor SleIF4A2. Silencing of SlRBP1 significantly reduced SleIF4A2 binding to SlRBP1-target RNAs. Taking these observations together, we propose that SlRBP1 binds to and channels RNAs onto the SleIF4A2 translation initiation complex and promotes the translation of its target RNAs to regulate chloroplast functions.

Impact of Future Elevated Carbon Dioxide on C3 Plant Resistance to Biotic Stresses

Before the end of the century, atmospheric carbon dioxide levels are predicted to increase to approximately 900 ppm. This will dramatically affect plant physiology and influence environmental interactions and, in particular, plant resistance to biotic stresses. This review is a broad survey of the current research on the effects of elevated CO₂ (eCO₂) on phytohormone-mediated resistance of C₃ agricultural crops and related model species to pathogens and insect herbivores. In general, while plants grown in eCO₂ often have increased constitutive and induced salicylic acid levels and suppressed induced jasmonate levels, there are exceptions that implicate other environmental factors, such as light and nitrogen fertilization in modulating these responses. Therefore, this review sets the stage for future studies to delve into understanding the mechanistic basis behind how eCO₂ will affect plant defensive phytohormone signaling pathways under future predicted environmental conditions that could threaten global food security to inform the best agricultural management practices.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

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.

Mechanisms of elevated CO2-induced thermotolerance in plants: the role of phytohormones

Rising atmospheric CO₂ is a key driver of climate change, intensifying drastic changes in meteorological parameters. Plants can sense and respond to changes in environmental parameters including atmospheric CO₂ and temperatures. High temperatures beyond the physiological threshold can significantly affect plant growth and development and thus attenuate crop productivity. However, elevated atmospheric CO₂ can mitigate the deleterious effects of heat stress on plants. Despite a large body of literature supporting the positive impact of elevated CO₂ on thermotolerance, the underlying biological mechanisms and precise molecular pathways that lead to enhanced tolerance to heat stress remain largely unclear. Under heat stress, elevated CO₂-induced expression of respiratory burst oxidase homologs (RBOHs) and reactive oxygen species (ROS) signaling play a critical role in stomatal movement, which optimizes gas exchange to enhance photosynthesis and water use efficiency. Notably, elevated CO₂ also fortifies antioxidant defense and redox homeostasis to alleviate heat-induced oxidative damage. Both hormone-dependent and independent pathways have been shown to mediate high CO₂-induced thermotolerance. The activation of heat-shock factors and subsequent expression of heat-shock proteins are thought to be the essential mechanism downstream of hormone and ROS signaling. Here we review the role of phytohormones in plant response to high atmospheric CO₂ and temperatures. We also discuss the potential mechanisms of elevated CO₂-induced thermotolerance by focusing on several key phytohormones such as ethylene. Finally, we address some limitations of our current understanding and the need for further research to unveil the yet-unknown crosstalk between plant hormones in mediating high CO₂-induced thermotolerance in plants.

A Novel Role of Pipecolic Acid Biosynthetic Pathway in Drought Tolerance through the Antioxidant System in Tomato

With global warming and water shortage, drought stress is provoking an increasing impact on plant growth, development, and crop productivity worldwide. Pipecolic acid (Pip) is an emerging lysine catabolite in plants, acting as a critical element in disease resistance with a related signal pathway of phytohormone salicylic acid (SA). While SA plays a vital role in various abiotic stresses, the role of Pip in plant response to abiotic stresses, especially drought, remains largely unknown. To address this issue, Pip biosynthetic gene Slald1 mutants and hydroxylated modification gene Slfmo1 mutants were generated using CRISPR-Cas9 gene-editing approaches. Drought resistance dramatically increased in Slald1 mutants compared with wild-type, which was associated with increased CO₂ assimilation, photosystems activities, antioxidant enzymes activities, ascorbate and glutathione content, and reduced reactive oxygen species accumulation, lipid peroxidation and protein oxidation. On the contrary, Slfmo1 mutants were more sensitive to drought, showing damaged photosystems and impaired antioxidant systems, which were significantly alleviated by exogenous ascorbate. Our results demonstrate that Pip biosynthesis and hydroxylated modification pathways play a critical role in drought tolerance through the antioxidant system in tomato. This knowledge can be helpful to breed improved crop cultivars that are better equipped with drought resistance.

Molecular Characterization of Carbonic Anhydrase Genes in Lotus japonicus and Their Potential Roles in Symbiotic Nitrogen Fixation

Carbonic anhydrase (CA) plays a vital role in photosynthetic tissues of higher plants, whereas its non-photosynthetic role in the symbiotic root nodule was rarely characterized. In this study, 13 CA genes were identified in the model legume Lotus japonicus by comparison with Arabidopsis CA genes. Using qPCR and promoter-reporter fusion methods, three previously identified nodule-enhanced CA genes (LjαCA2, LjαCA6, and LjβCA1) have been further characterized, which exhibit different spatiotemporal expression patterns during nodule development. LjαCA2 was expressed in the central infection zone of the mature nodule, including both infected and uninfected cells. LjαCA6 was restricted to the vascular bundle of the root and nodule. As for LjβCA1, it was expressed in most cell types of nodule primordia but only in peripheral cortical cells and uninfected cells of the mature nodule. Using CRISPR/Cas9 technology, the knockout of LjβCA1 or both LjαCA2 and its homolog, LjαCA1, did not result in abnormal symbiotic phenotype compared with the wild-type plants, suggesting that LjβCA1 or LjαCA1/2 are not essential for the nitrogen fixation under normal symbiotic conditions. Nevertheless, the nodule-enhanced expression patterns and the diverse distributions in different types of cells imply their potential functions during root nodule symbiosis, such as CO₂ fixation, N assimilation, and pH regulation, which await further investigations.

The protein kinase CPK28 phosphorylates ascorbate peroxidase and enhances thermotolerance in tomato

High temperatures are a major threat to plant growth and development, leading to yield losses in crops. Calcium-dependent protein kinases (CPKs) act as critical components of Ca2+ sensing in plants that transduce rapid stress-induced responses to multiple environmental stimuli. However, the role of CPKs in plant thermotolerance and their mechanisms of action remain poorly understood. To address this issue, tomato (Solanum lycopersicum) cpk28 mutants were generated using a CRISPR-Cas9 gene-editing approach. The responses of mutant and wild-type plants to normal (25°C) and high temperatures (45°C) were documented. Thermotolerance was significantly decreased in the cpk28 mutants, which showed increased heat stress-induced accumulation of reactive oxygen species (ROS) and levels of protein oxidation, together with decreased activities of ascorbate peroxidase (APX) and other antioxidant enzymes. The redox status of ascorbate and glutathione were also modified. Using a yeast two-hybrid library screen and protein interaction assays, we provide evidence that CPK28 directly interacts with cytosolic APX2. Mutations in APX2 rendered plants more sensitive to high temperatures, whereas the addition of exogenous reduced ascorbate (AsA) rescued the thermotolerance phenotype of the cpk28 mutants. Moreover, protein phosphorylation analysis demonstrated that CPK28 phosphorylates the APX2 protein at Thr-59 and Thr-164. This process is suggested to be responsive to Ca2+ stimuli and may be required for CPK28-mediated thermotolerance. Taken together, these results demonstrate that CPK28 targets APX2, thus improving thermotolerance. This study suggests that CPK28 is an attractive target for the development of improved crop cultivars that are better adapted to heat stress in a changing climate.