Balancing trade-offs between biotic and abiotic stress responses through leaf age-dependent variation in stress hormone cross-talk

Are miRNAs applicable for balancing crop growth and defense trade-off?

Securing agricultural supplies for the increasing population without negative impacts on environment demands new crop varieties with higher yields, better quality, and stronger stress resilience. But breeding such super crop varieties is restrained by growth-defense (G-D) trade-off. MicroRNAs (miRNAs) are versatile regulators of plant growth and immune responses, with several being demonstrated to simultaneously regulate crop growth and defense against biotic stresses and to balance G-D trade-off. Increasing evidence also links miRNAs to the metabolism and signaling of phytohormones, another type of master regulator of plant growth and defense. Here, we synthesize the reported functions of miRNAs in crop growth, development, and responses to bio-stressors, summarize the regulatory scenarios of miRNAs based on their relationship with target(s), and discuss how miRNAs, particularly those involved in crosstalk with phytohormones, can be applied in balancing G-D trade-off in crops. We also propose several open questions to be addressed for adopting miRNAs in balancing crop G-D trade-off.

Roots of synthetic ecology: microbes that foster plant resilience in the changing climate

Microbes orchestrate nearly all major biogeochemical processes. The ability to program their influence on plant growth and development is attractive for sustainable agriculture. However, the complexity of microbial ecosystems and our limited understanding of the mechanisms by which plants and microbes interact with each other and the environment make it challenging to use microbiomes to influence plant growth. Novel technologies at the intersection of microbial ecology, systems biology, and bioengineering provide new tools to probe the role of plant microbiomes across environments. Here, we summarize recent studies on plant and microbe responses to abiotic stresses, showcasing key molecules and micro-organisms that are important for plant health. We highlight opportunities to use synthetic microbial communities to understand the complexity of plant-microbial interactions and discuss future avenues of programming ecology to improve plant and ecosystem health.

Unique regulatory network of dragon fruit simultaneously mitigates the effect of vanadium pollutant and environmental factors

Under changing climatic conditions, plants are simultaneously facing conflicting stresses in nature. Plants can sense different stresses, induce systematic ROS signals, and regulate transcriptomic, hormonal, and stomatal responses. We performed transcriptome analysis to reveal the integrative stress response regulatory mechanism underlying heavy metal stress alone or in combination with heat and drought conditions in pitaya (dragon fruit). A total of 70 genes were identified from 31,130 transcripts with conserved differential expression. Furthermore, weighted gene co-expression network analysis (WGCNA) identified trait-associated modules. By integrating information from three modules and protein-protein interaction (PPI) networks, we identified 10 interconnected genes associated with the multifaceted defense mechanism employed by pitaya against co-occurring stresses. To further confirm the reliability of the results, we performed a comparative analysis of 350 genes identified by three trait modules and 70 conserved genes exhibiting their dynamic expression under all treatments. Differential expression pattern of genes and comparative analysis, have proven instrumental in identifying ten putative structural genes. These ten genes were annotated as PLAT/LH2, CAT, MLP, HSP, PB1, PLA, NAC, HMA, and CER1 transcription factors involved in antioxidant activity, defense response, MAPK signaling, detoxification of metals and regulating the crosstalk between the complex pathways. Predictive analysis of putative candidate genes, potentially governing single, double, and multifactorial stress response, by several signaling systems and molecular patterns. These findings represent a valuable resource for pitaya breeding programs, offering the potential to develop resilient "super pitaya" plants.

Sweet Cherry Plants Prioritize Their Response to Cope with Summer Drought, Overshadowing the Defense Response to Pseudomonas syringae pv. syringae

Disease severity and drought due to climate change present significant challenges to orchard productivity. This study examines the effects of spring inoculation with Pseudomonas syringae pv. syringae (Pss) on sweet cherry plants, cvs. Bing and Santina with varying defense responses, assessing plant growth, physiological variables (water potential, gas exchange, and plant hydraulic conductance), and the levels of abscisic acid (ABA) and salicylic acid (SA) under two summer irrigation levels. Pss inoculation elicited a more pronounced response in 'Santina' compared to 'Bing' at 14 days post-inoculation (dpi), and those plants inoculated with Pss exhibited a slower leaf growth and reduced transpiration compared to control plants during 60 dpi. During differential irrigations, leaf area was reduced 14% and 44% in Pss inoculated plants of 'Bing' and 'Santina' respectively, under well-watered (WW) conditions, without changes in plant water status or gas exchange. Conversely, water-deficit (WD) conditions led to gas exchange limitations and a 43% decrease in plant biomass compared to that under WW conditions, with no differences between inoculation treatments. ABA levels were lower under WW than under WD at 90 dpi, while SA levels were significantly higher in Pss-inoculated plants under WW conditions. These findings underscore the influence on plant growth during summer in sweet cherry cultivars that showed a differential response to Pss inoculations and how the relationship between ABA and SA changes in plant drought level responses.

Endophytic Microorganisms in Tomato Roots, Changes in the Structure and Function of the Community at Different Growing Stages

This study analyzed flower bud differentiation and fruiting stages to investigate how the structure of the plant endophytic microbial community in the roots of tomatoes changes with plant senescence. Based on high-throughput sequencing technology, the diversity and relative abundance of endophytic microorganisms (bacteria and fungi) in tomato stems at different growth stages were analyzed. At the same time, based on LEfSe analysis, the differences in endophytic microorganisms in tomato stems at different growth stages were studied. Based on PICRUSt2 function prediction and FUNGuild, we predicted the functions of endophytic bacterial and fungal communities in tomato stems at different growth stages to explore potential microbial functional traits. The results demonstrated that not only different unique bacterial genera but also unique fungal genera could be found colonizing tomato roots at different growth stages. In tomato seedlings, flower bud differentiation, and fruiting stages, the functions of colonizing endophytes in tomato roots could primarily contribute to the promotion of plant growth, stress resistance, and improvement in nutrient cycling, respectively. These results also suggest that different functional endophytes colonize tomato roots at different growth stages.

Cadmium Alters the Metabolism and Perception of Abscisic Acid in Pisum sativum Leaves in a Developmentally Specific Manner

Abscisic acid (ABA) plays a crucial role in plant defense mechanisms under adverse environmental conditions, but its metabolism and perception in response to heavy metals are largely unknown. In Pisum sativum exposed to CdCl2, an accumulation of free ABA was detected in leaves at different developmental stages (A, youngest, unexpanded; B1, youngest, fully expanded; B2, mature; C, old), with the highest content found in A and B1 leaves. In turn, the content of ABA conjugates, which was highest in B2 and C leaves under control conditions, increased only in A leaves and decreased in leaves of later developmental stages after Cd treatment. Based on the expression of PsNCED2, PsNCED3 (9-cis-epoxycarotenoid dioxygenase), PsAO3 (aldehyde oxidase) and PsABAUGT1 (ABA-UDP-glucosyltransferase), and the activity of PsAOγ, B2 and C leaves were found to be the main sites of Cd-induced de novo synthesis of ABA from carotenoids and ABA conjugation with glucose. In turn, β-glucosidase activity and the expression of genes encoding ABA receptors (PsPYL2, PsPYL4, PsPYL8, PsPYL9) suggest that in A and B1 leaves, Cd-induced release of ABA from inactive ABA-glucosyl esters and enhanced ABA perception comes to the forefront when dealing with Cd toxicity. The distinct role of leaves at different developmental stages in defense against the harmful effects of Cd is discussed.

Drought: A context-dependent damper and aggravator of plant diseases

Drought dynamically influences the interactions between plants and pathogens, thereby affecting disease outbreaks. Understanding the intricate mechanistic aspects of the multiscale interactions among plants, pathogens, and the environment-known as the disease triangle-is paramount for enhancing the climate resilience of crop plants. In this review, we systematically compile and comprehensively analyse current knowledge on the influence of drought on the severity of plant diseases. We emphasise that studying these stresses in isolation is not sufficient to predict how plants respond to combined stress from both drought and pathogens. The impact of drought and pathogens on plants is complex and multifaceted, encompassing the activation of antagonistic signalling cascades in response to stress factors. The nature, intensity, and temporality of drought and pathogen stress occurrence significantly influence the outcome of diseases. We delineate the drought-sensitive nodes of plant immunity and highlight the emerging points of crosstalk between drought and defence signalling under combined stress. The limited mechanistic understanding of these interactions is acknowledged as a key research gap in this area. The information synthesised herein will be crucial for crafting strategies for the accurate prediction and mitigation of future crop disease risks, particularly in the context of a changing climate.

Harnessing root-soil-microbiota interactions for drought-resilient cereals

Cereal plants form complex networks with their associated microbiome in the soil environment. A complex system including variations of numerous parameters of soil properties and host traits shapes the dynamics of cereal microbiota under drought. These multifaceted interactions can greatly affect carbon and nutrient cycling in soil and offer the potential to increase plant growth and fitness under drought conditions. Despite growing recognition of the importance of plant microbiota to agroecosystem functioning, harnessing the cereal root microbiota remains a significant challenge due to interacting and synergistic effects between root traits, soil properties, agricultural practices, and drought-related features. A better mechanistic understanding of root-soil-microbiota associations could lead to the development of novel strategies to improve cereal production under drought. In this review, we discuss the root-soil-microbiota interactions for improving the soil environment and host fitness under drought and suggest a roadmap for harnessing the benefits of these interactions for drought-resilient cereals. These methods include conservative trait-based approaches for the selection and breeding of plant genetic resources and manipulation of the soil environments.

Screening of core microorganisms in healthy and diseased peaches and effect evaluation of biocontrol bacteria (Burkholderia sp.)

Biological antagonists serve as the most important green alternatives to chemical fungicides, a class of microorganism that inhibits the growth of pathogenic fungi to reduce fruit incidence. In this paper, healthy and diseased peach fruit was selected for amplicon sequencing of the epiphytic microbiota on their surface to obtain a comprehensive understanding. Community structure, diversity and LefSe analysis were performed to screen Acetobacter, Muribaculaceae and Burkholderia as the core bacteria, Mycosphaerella, Penicillium and Alternaria as the core fungi, they showed significant differences and were highly enriched. Two strains fungi (Penicillium K3 and N1) and one strain antagonistic bacteria (Burkholderia J2) were isolated. The in intro test results indicated the bacterial suspension, fermentation broth and volatile organic compounds of antagonistic bacteria J2 were able to significantly inhibit pathogen growth. In vivo experiments, peach was stored at 28 °C for 6 days after different treatments, and samples were taken every day. It was found that Burkholderia J2 enhanced peach resistance by increasing the activities of antioxidant-related enzymes such as SOD, POD, PAL, PPO, GR, MDHAR, and DHAR. The results improved that Burkholderia J2 has great biocontrol potential and could be used as a candidate strain for green control of blue mold.

Physiochemical interaction between osmotic stress and a bacterial exometabolite promotes plant disease

Various microbes isolated from healthy plants are detrimental under laboratory conditions, indicating the existence of molecular mechanisms preventing disease in nature. Here, we demonstrated that application of sodium chloride (NaCl) in natural and gnotobiotic soil systems is sufficient to induce plant disease caused by an otherwise non-pathogenic root-derived Pseudomonas brassicacearum isolate (R401). Disease caused by combinatorial treatment of NaCl and R401 triggered extensive, root-specific transcriptional reprogramming that did not involve down-regulation of host innate immune genes, nor dampening of ROS-mediated immunity. Instead, we identified and structurally characterized the R401 lipopeptide brassicapeptin A as necessary and sufficient to promote disease on salt-treated plants. Brassicapeptin A production is salt-inducible, promotes root colonization and transitions R401 from being beneficial to being detrimental on salt-treated plants by disturbing host ion homeostasis, thereby bolstering susceptibility to osmolytes. We conclude that the interaction between a global change stressor and a single exometabolite from a member of the root microbiome promotes plant disease in complex soil systems.

Nitric Oxide Mitigates the Deleterious Effects Caused by Infection of Pseudomonas syringae pv. syringae and Modulates the Carbon Assimilation Process in Sweet Cherry under Water Stress

Bacterial canker is an important disease of sweet cherry plants mainly caused by Pseudomonas syringae pv. syringae (Pss). Water deficit profoundly impairs the yield of this crop. Nitric oxide (NO) is a molecule that plays an important role in the plant defense mechanisms. To evaluate the protection exerted by NO against Pss infection under normal or water-restricted conditions, sodium nitroprusside (SNP), a NO donor, was applied to sweet cherry plants cv. Lapins, before they were exposed to Pss infection under normal or water-restricted conditions throughout two seasons. Well-watered plants treated with exogenous NO presented a lower susceptibility to Pss. A lower susceptibility to Pss was also induced in plants by water stress and this effect was increased when water stress was accompanied by exogenous NO. The lower susceptibility to Pss induced either by exogenous NO or water stress was accompanied by a decrease in the internal bacterial population. In well-watered plants, exogenous NO increased the stomatal conductance and the net CO2 assimilation. In water-stressed plants, NO induced an increase in the leaf membranes stability and proline content, but not an increase in the CO2 assimilation or the stomatal conductance.

Complexity of responses to ionizing radiation in plants, and the impact on interacting biotic factors

In nature, plants are simultaneously exposed to different abiotic (e.g., heat, drought, and salinity) and biotic (e.g., bacteria, fungi, and insects) stresses. Climate change and anthropogenic pressure are expected to intensify the frequency of stress factors. Although plants are well equipped with unique and common defense systems protecting against stressors, they may compromise their growth and development for survival in such challenging environments. Ionizing radiation is a peculiar stress factor capable of causing clustered damage. Radionuclides are both naturally present on the planet and produced by human activities. Natural and artificial radioactivity affects plants on molecular, biochemical, cellular, physiological, populational, and transgenerational levels. Moreover, the fitness of pests, pathogens, and symbionts is concomitantly challenged in radiologically contaminated areas. Plant responses to artificial acute ionizing radiation exposure and laboratory-simulated or field chronic exposure are often discordant. Acute or chronic ionizing radiation exposure may occasionally prime the defense system of plants to better tolerate the biotic stress or could often exhaust their metabolic reserves, making plants more susceptible to pests and pathogens. Currently, these alternatives are only marginally explored. Our review summarizes the available literature on the responses of host plants, biotic factors, and their interaction to ionizing radiation exposure. Such systematic analysis contributes to improved risk assessment in radiologically contaminated areas.

Bacillus cereus NJ01 induces SA- and ABA-mediated immunity against bacterial pathogens through the EDS1-WRKY18 module

Emerging evidence suggests a beneficial role of rhizobacteria in ameliorating plant disease resistance in an environment-friendly way. In this study, we characterize a rhizobacterium, Bacillus cereus NJ01, that enhances bacterial pathogen resistance in rice and Arabidopsis. Transcriptome analyses show that root inoculation of NJ01 induces the expression of salicylic acid (SA)- and abscisic acid (ABA)-related genes in Arabidopsis leaves. Genetic evidence showed that EDS1, PAD4, and WRKY18 are required for B. cereus NJ01-induced bacterial resistance. An EDS1-PAD4 complex interacts with WRKY18 and enhances its DNA binding activity. WRKY18 directly binds to the W box in the promoter region of the SA biosynthesis gene ICS1 and ABA biosynthesis genes NCED3 and NCED5 and contributes to the NJ01-induced bacterial resistance. Taken together, our findings indicate a role of the EDS1/PAD4-WRKY18 complex in rhizobacteria-induced disease resistance.

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.

Soil Application of Bacillus subtilis Regulates Flavonoid and Alkaloids Biosynthesis in Mulberry Leaves

Flavonoids and alkaloids are the major active ingredients in mulberry leaves that have outstanding medicinal value. Bacillus subtilis can effectively activate the plants defense response and regulate the plant secondary metabolism. In this study, we explored the effects of soil application of B. subtilis on the content of flavonoids and the most important alkaloids (1-deoxynojirimycin, DNJ) in mulberry leaves. Significant decreases in flavonoid content were observed in tender leaves and mature leaves after treatment with B. subtilis; at the same time, significant increases in DNJ content were observed in tender leaves. Based on widely targeted LC-MS/MS and high-throughput approaches, we screened out 904 differentially synthesized metabolites (DSMs) and 9715 differentially expressed genes (DEGs). KEGG analyses showed that these DSMs and DEGs were both significantly enriched in the biosynthesis of secondary metabolites, flavonoid synthesis and plant hormone signal transduction. Further correlation analysis of DEMs and DEGs showed that 40 key genes were involved in flavonoid biosynthesis, with 6 key genes involved in DNJ biosynthesis. The expression of CHS, CHI, F3H, F3'H, FLS, UGT and AOC significantly responded to B. subtilis soil application. This study broadens our understanding of the molecular mechanisms underlying the accumulation of flavonoids and alkaloids in mulberry leaves.

Seasonality and longer-term development generate temporal dynamics in the Populus microbiome

Temporal variation in community composition is central to our understanding of the assembly and functioning of microbial communities, yet the controls over temporal dynamics for microbiomes of long-lived plants, such as trees, remain unclear. Temporal variation in tree microbiomes could arise primarily from seasonal (i.e., intra-annual) fluctuations in community composition or from longer-term changes across years as host plants age. To test these alternatives, we experimentally isolated temporal variation in plant microbiome composition using a common garden and clonally propagated plants, and we used amplicon sequencing to characterize bacterial/archaeal and fungal communities in the leaf endosphere, root endosphere, and rhizosphere of two Populus spp. over four seasons across two consecutive years. Microbial community composition differed among seasons and years (which accounted for up to 21% of the variation in microbial community composition) and was correlated with seasonal dissimilarity in climatic conditions. However, microbial community dissimilarity was also positively correlated with time, reflecting longer-term compositional shifts as host trees aged. Together, our findings demonstrate that temporal patterns in tree microbiomes arise from both seasonal fluctuations and longer-term changes, which interact to generate unique seasonal patterns each year. In addition to shedding light on two important controls over the assembly of plant microbiomes, our results also suggest future studies of tree microbiomes should account for background temporal dynamics when testing the drivers of spatial patterns in microbial community composition and temporal responses of plant microbiomes to environmental change.IMPORTANCEMicrobiomes are integral to the health of host plants, but we have a limited understanding of the factors that control how the composition of plant microbiomes changes over time. Especially little is known about the microbiome of long-lived trees, relative to annual and non-woody plants. We tested how tree microbiomes changed between seasons and years in poplar (genus Populus), which are widespread and ecologically important tree species that also serve as important biofuel feedstocks. We found the composition of bacterial, archaeal, and fungal communities differed among seasons, but these seasonal differences depended on year. This dependence was driven by longer-term changes in microbial composition as host trees developed across consecutive years. Our findings suggest that temporal variation in tree microbiomes is driven by both seasonal fluctuations and longer-term (i.e., multiyear) development.

Maize multi-omics reveal leaf water status controlling of differential transcriptomes, proteomes and hormones as mechanisms of age-dependent osmotic stress response in leaves

Drought-induced osmotic stress severely affects the growth and yield of maize. However, the mechanisms underlying the different responses of young and old maize leaves to osmotic stress remain unclear. To gain a systematic understanding of age-related stress responses, we compared osmotic-stress-induced changes in maize leaves of different ages using multi-omics approaches. After short-term osmotic stress, old leaves suffered more severe water deficits than young leaves. The adjustments of transcriptomes, proteomes, and hormones in response to osmotic stress were more dynamic in old leaves. Metabolic activities, stress signaling pathways, and hormones (especially abscisic acid) responded to osmotic stress in an age-dependent manner. We identified multiple functional clusters of genes and proteins with potential roles in stress adaptation. Old leaves significantly accumulated stress proteins such as dehydrin, aquaporin, and chaperones to cope with osmotic stress, accompanied by senescence-like cellular events, whereas young leaves exhibited an effective water conservation strategy mainly by hydrolyzing transitory starch and increasing proline production. The stress responses of individual leaves are primarily determined by their intracellular water status, resulting in differential transcriptomes, proteomes, and hormones. This study extends our understanding of the mechanisms underlying plant responses to osmotic stress.

The impact of multifactorial stress combination on plants, crops, and ecosystems: how should we prepare for what comes next?

The complexity of environmental conditions encountered by plants in the field, or in nature, is gradually increasing due to anthropogenic activities that promote global warming, climate change, and increased levels of pollutants. While in the past it seemed sufficient to study how plants acclimate to one or even two different stresses affecting them simultaneously, the complex conditions developing on our planet necessitate a new approach of studying stress in plants: Acclimation to multiple stress conditions occurring concurrently or consecutively (termed, multifactorial stress combination [MFSC]). In an initial study of the plant response to MFSC, conducted with Arabidopsis thaliana seedlings subjected to an MFSC of six different abiotic stresses, it was found that with the increase in the number and complexity of different stresses simultaneously impacting a plant, plant growth and survival declined, even if the effects of each stress involved in such MFSC on the plant was minimal or insignificant. In three recent studies, conducted with different crop plants, MFSC was found to have similar effects on a commercial rice cultivar, a maize hybrid, tomato, and soybean, causing significant reductions in growth, biomass, physiological parameters, and/or yield traits. As the environmental conditions on our planet are gradually worsening, as well as becoming more complex, addressing MFSC and its effects on agriculture and ecosystems worldwide becomes a high priority. In this review, we address the effects of MFSC on plants, crops, agriculture, and different ecosystems worldwide, and highlight potential avenues to enhance the resilience of crops to MFSC.

Genome-wide association analysis reveals genes controlling an antagonistic effect of biotic and osmotic stress on Arabidopsis thaliana growth

While the response of Arabidopsis thaliana to drought, herbivory or fungal infection has been well-examined, the consequences of exposure to a series of such (a)biotic stresses are not well studied. This work reports on the genetic mechanisms underlying the Arabidopsis response to single osmotic stress, and to combinatorial stress, either fungal infection using Botrytis cinerea or herbivory using Pieris rapae caterpillars followed by an osmotic stress treatment. Several small-effect genetic loci associated with rosette dry weight (DW), rosette water content (WC), and the projected rosette leaf area in response to combinatorial stress were mapped using univariate and multi-environment genome-wide association approaches. A single-nucleotide polymorphism (SNP) associated with DROUGHT-INDUCED 19 (DI19) was identified by both approaches, supporting its potential involvement in the response to combinatorial stress. Several SNPs were found to be in linkage disequilibrium with known stress-responsive genes such as PEROXIDASE 34 (PRX34), BASIC LEUCINE ZIPPER 25 (bZIP25), RESISTANCE METHYLATED GENE 1 (RMG1) and WHITE RUST RESISTANCE 4 (WRR4). An antagonistic effect between biotic and osmotic stress was found for prx34 and arf4 mutants, which suggests PRX34 and ARF4 play an important role in the response to the combinatorial stress.

Application of Chlorophyll Fluorescence Analysis Technique in Studying the Response of Lettuce (Lactuca sativa L.) to Cadmium Stress

To reveal the impact of cadmium stress on the physiological mechanism of lettuce, simultaneous determination and correlation analyses of chlorophyll content and photosynthetic function were conducted using lettuce seedlings as the research subject. The changes in relative chlorophyll content, rapid chlorophyll fluorescence induction kinetics curve, and related chlorophyll fluorescence parameters of lettuce seedling leaves under cadmium stress were detected and analyzed. Furthermore, a model for estimating relative chlorophyll content was established. The results showed that cadmium stress at 1 mg/kg and 5 mg/kg had a promoting effect on the relative chlorophyll content, while cadmium stress at 10 mg/kg and 20 mg/kg had an inhibitory effect on the relative chlorophyll content. Moreover, with the extension of time, the inhibitory effect became more pronounced. Cadmium stress affects both the donor and acceptor sides of photosystem II in lettuce seedling leaves, damaging the electron transfer chain and reducing energy transfer in the photosynthetic system. It also inhibits water photolysis and decreases electron transfer efficiency, leading to a decline in photosynthesis. However, lettuce seedling leaves can mitigate photosystem II damage caused by cadmium stress through increased thermal dissipation. The model established based on the energy captured by a reaction center for electron transfer can effectively estimate the relative chlorophyll content of leaves. This study demonstrates that chlorophyll fluorescence techniques have great potential in elucidating the physiological mechanism of cadmium stress in lettuce, as well as in achieving synchronized determination and correlation analyses of chlorophyll content and photosynthetic function.

TOR coordinates cytokinin and gibberellin signals mediating development and defense

Plants constantly perceive and process environmental signals and balance between the energetic demands of growth and defense. Growth arrest upon pathogen attack was previously suggested to result from a redirection of the plants' metabolic resources towards the activation of plant defense. The energy sensor Target of Rapamycin (TOR) kinase is a conserved master coordinator of growth and development in all eukaryotes. Although TOR is positioned at the interface between development and defense, little is known about the mechanisms by which TOR may potentially regulate the relationship between these two modalities. The plant hormones cytokinin (CK) and gibberellin (GA) execute various aspects of plant development and defense. The ratio between CK and GA was reported to determine the outcome of developmental programmes. Here, investigating the interplay between TOR-mediated development and TOR-mediated defense in tomato, we found that TOR silencing resulted in rescue of several different aberrant developmental phenotypes, demonstrating that TOR is required for the execution of developmental cues. In parallel, TOR inhibition enhanced immunity in genotypes with a low CK/GA ratio but not in genotypes with a high CK/GA ratio. TOR-inhibition mediated disease resistance was found to depend on developmental status, and was abolished in strongly morphogenetic leaves, while being strongest in mature, differentiated leaves. CK repressed TOR activity, suggesting that CK-mediated immunity may rely on TOR downregulation. At the same time, TOR activity was promoted by GA, and TOR silencing reduced GA sensitivity, indicating that GA signalling requires normal TOR activity. Our results demonstrate that TOR likely acts in concert with CK and GA signalling, executing signalling cues in both defense and development. Thus, differential regulation of TOR or TOR-mediated processes could regulate the required outcome of development-defense prioritisation.

Understanding the mechanobiology of phytoacoustics through molecular Lens: Mechanisms and future perspectives

BACKGROUND

How plants emit, perceive, and respond to sound vibrations (SVs) is a long-standing question in the field of plant sensory biology. In recent years, there have been numerous studies on how SVs affect plant morphological, physiological, and biochemical traits related to growth and adaptive responses. For instance, under drought SVs navigate plant roots towards water, activate their defence responses against stressors, and increase nectar sugar in response to pollinator SVs. Also, plants emit SVs during stresses which are informative in terms of ecological and adaptive perspective. However, the molecular mechanisms underlying the SV perception and emission in plants remain largely unknown. Therefore, deciphering the complexity of plant-SV interactions and identifying bonafide receptors and signaling players will be game changers overcoming the roadblocks in phytoacoustics.

AIM OF REVIEW

The aim of this review is to provide an overview of recent developments in phytoacoustics. We primarily focuss on SV signal perception and transduction with current challenges and future perspectives.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Timeline breakthroughs in phytoacoustics have constantly shaped our understanding and belief that plants may emit and respond to SVs like other species. However, unlike other plant mechanostimuli, little is known about SV perception and signal transduction. Here, we provide an update on phytoacoustics and its ecological importance. Next, we discuss the role of cell wall receptor-like kinases, mechanosensitive channels, intracellular organelle signaling, and other key players involved in plant-SV receptive pathways that connect them. We also highlight the role of calcium (Ca2+), reactive oxygen species (ROS), hormones, and other emerging signaling molecules in SV signal transduction. Further, we discuss the importance of molecular, biophysical, computational, and live cell imaging tools for decoding the molecular complexity of acoustic signaling in plants. Finally, we summarised the role of SV priming in plants and discuss how SVs could modulate plant defense and growth trade-offs during other stresses.

Shoot Maturation Strengthens FLS2-Mediated Resistance to Pseudomonas syringae

Temporospatial regulation of immunity components is essential for properly activating plant defense response. Flagellin-sensing 2 (FLS2) is a surface-localized receptor that recognizes bacterial flagellin. The immune function of FLS2 is compromised in early stages of shoot development. However, the underlying mechanism for the age-dependent FLS2 signaling is not clear. Here, we show that the reduced basal immunity of juvenile leaves against Pseudomonas syringae pv. tomato DC3000 is independent of FLS2. The flg22-induced marker gene expression and reactive oxygen species activation were comparable in juvenile and adult stages, but callose deposition was more evident in the adult stage than the juvenile stage. We further demonstrated that microRNA156, a master regulator of plant aging, does not influence the expression of FLS2 and FRK1 (Flg22-induced receptor-like kinase 1) but mildly suppresses callose deposition in juvenile leaves. Our experiments revealed an intrinsic mechanism that regulates the amplitude of FLS2-mediated resistance during aging. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Functional characterization of two NAC transcription factors HfNAP1 and HfNAC090 associated with flower programmed cell death in daylily (Hemerocallis fulva)

Daylily (Hemerocallis fulva) is one of the most widely used perennial flowers, but its ornamental and economic value is greatly limited due to its ephemeral flowering period. In general, the flower senescence is regulated by the developmental signals and considered as an irreversible process of programmed cell death (PCD). However, the molecular mechanism of flower PCD in daylily still remains unclear. In this study, two NAC transcription factors, namely HfNAP1 and HfNAC090, are first identified and found to be upregulated significantly in both the age-induced and the ABA-induced flower PCD processes in daylily. Then, the functions of HfNAP1 and HfNAC090 in regulating the flower PCD are investigated through transgenic phenotypes analysis. The results demonstrate that the ectopic and transient overexpression of these two genes can effectively regulate the flower PCD in tobacco and daylily. While the overexpression of HfNAP1 accelerates the flower PCD process, the overexpression of HfNAC090 significantly delays that. Furthermore, the yeast two-hybrid assay is performed to discover potential interactions related to these two genes, and the results demonstrate that HfNAP1 and HfNAC090 can interact with each other, or interact with other flower aging-related genes. Additionally, the yeast one-hybrid assay suggests that HfNAP1 and HfNAC090 can bind directly to the promoters of downstream senescence-associated genes HfSAG39 and HfSAG15. Taken overall, this study provides sufficient evidences to confirm that HfNAP1 and HfNAC090 play dominant roles in regulating the flower PCD in daylily, supporting the development of new strategies to prolong the longevity of daylily flowers.

Genetic architecture of source-sink-regulated senescence in maize

Source and sink interactions play a critical but mechanistically poorly understood role in the regulation of senescence. To disentangle the genetic and molecular mechanisms underlying source-sink-regulated senescence (SSRS), we performed a phenotypic, transcriptomic, and systems genetics analysis of senescence induced by the lack of a strong sink in maize (Zea mays). Comparative analysis of genotypes with contrasting SSRS phenotypes revealed that feedback inhibition of photosynthesis, a surge in reactive oxygen species, and the resulting endoplasmic reticulum (ER) stress were the earliest outcomes of weakened sink demand. Multienvironmental evaluation of a biparental population and a diversity panel identified 12 quantitative trait loci and 24 candidate genes, respectively, underlying SSRS. Combining the natural diversity and coexpression networks analyses identified 7 high-confidence candidate genes involved in proteolysis, photosynthesis, stress response, and protein folding. The role of a cathepsin B like protease 4 (ccp4), a candidate gene supported by systems genetic analysis, was validated by analysis of natural alleles in maize and heterologous analyses in Arabidopsis (Arabidopsis thaliana). Analysis of natural alleles suggested that a 700-bp polymorphic promoter region harboring multiple ABA-responsive elements is responsible for differential transcriptional regulation of ccp4 by ABA and the resulting variation in SSRS phenotype. We propose a model for SSRS wherein feedback inhibition of photosynthesis, ABA signaling, and oxidative stress converge to induce ER stress manifested as programed cell death and senescence. These findings provide a deeper understanding of signals emerging from loss of sink strength and offer opportunities to modify these signals to alter senescence program and enhance crop productivity.

The Homeodomain-Leucine Zipper Subfamily I Contributes to Leaf Age- and Time-Dependent Resistance to Pathogens in Arabidopsis thaliana

In Arabidopsis thaliana (Arabidopsis), nonhost resistance (NHR) is influenced by both leaf age and the moment of inoculation. While the circadian clock and photoperiod have been linked to the time-dependent regulation of NHR in Arabidopsis, the mechanism underlying leaf age-dependent NHR remains unclear. In this study, we investigated leaf age-dependent NHR to Pyricularia oryzae in Arabidopsis. Our findings revealed that this NHR type is regulated by both miR156-dependent and miR156-independent pathways. To identify the key players, we utilized rice-FOX Arabidopsis lines and identified the rice HD-Zip I OsHOX6 gene. Notably, OsHOX6 expression confers robust NHR to P. oryzae and Colletotrichum nymphaeae in Arabidopsis, with its effect being contingent upon leaf age. Moreover, we explored the role of AtHB7 and AtHB12, the Arabidopsis closest homologues of OsHOX6, by studying mutants and overexpressors in Arabidopsis-C. higginsianum interaction. AtHB7 and AtHB12 were found to contribute to both penetration resistance and post-penetration resistance to C. higginsianum in a leaf age- and time-dependent manner. These findings highlight the involvement of HD-Zip I AtHB7 and AtHB12, well-known regulators of development and abiotic stress responses, in biotic stress responses in Arabidopsis.

Microbe-associated molecular pattern recognition receptors have little effect on endophytic Arabidopsis thaliana microbiome assembly in the field

Plant microbiome structure affects plant health and productivity. A limited subset of environmental microbes successfully establishes within plant tissues, but the forces underlying this selectivity remain poorly characterized. Transmembrane pattern recognition receptors (PRRs), used by plants to detect microbe-associated molecular patterns (MAMPs), are strong candidates for achieving this selectivity because PRRs can potentially interact with many members of the microbiome. Indeed, MAMPs found in many microbial taxa, including beneficials and commensals, can instigate a robust immune response that affects microbial growth. Surprisingly, we found that MAMP-detecting PRRs have little effect on endophytic bacterial and fungal microbiome structure in the field. We compared the microbiomes of four PRR knockout lines of Arabidopsis thaliana to wild-type plants in multiple tissue types over several developmental stages and detected only subtle shifts in fungal, but not bacterial, β-diversity in one of the four PRR mutants. In one developmental stage, lore mutants had slightly altered fungal β-diversity, indicating that LORE may be involved in plant-fungal interactions in addition to its known role in detecting certain bacterial lipids. No other effects of PRRs on α-diversity, microbiome variability, within-individual homogeneity, or microbial load were found. The general lack of effect suggests that individual MAMP-detecting PRRs are not critical in shaping the endophytic plant microbiome. Rather, we suggest that MAMP-detecting PRRs must either act in concert and/or are individually maintained through pleiotropic effects or interactions with coevolved mutualists or pathogens. Although unexpected, these results offer insights into the role of MAMP-detecting PRRs in plant-microbe interactions and help direct future efforts to uncover host genetic elements that control plant microbiome assembly.

Differential response of phenylpropanoid pathway as linked to hormonal change in two Brassica napus cultivars contrasting drought tolerance

Oilseed rape (Brassica napus L.) is a significant agro-economic crop with a wide range of uses. Drought is the most frequent unfavourable environmental stressor restraining its growth and development worldwide. This study was conducted to characterize the drought-responsive phenylpropanoid pathway and its link to hormonal changes in two cultivars, drought-resistant "Saturnin" and drought-susceptible "Mosa." Drought susceptibility in cv. Mosa was confirmed by its lower water use efficiency and higher lipid peroxidation levels with reactive oxygen species (ROS) accumulation. In cv. Saturnin, higher salicylic acid (SA) levels and expression of dehydration-responsive element binding 2 (DREB2) and non-expressor of pathogenesis-related gene 1 (NPR1) led to an upregulation of production of anthocyanin pigment 1 (PAP1) and phenylpropanoid pathway-related gene (CHS, F5H and COMT1) expression, increasing hydroxycinnamic acid and flavonoid compound concentrations. However, in cv. Mosa, higher increases in the activity of lignifying enzymes (polyphenol oxidase, coniferyl alcohol peroxidase, syringaldazine peroxidase, guaiacol peroxidase) and expression of the lignin synthesis-related gene cinnamyl alcohol dehydrogenase 2 (CAD2) were found along with greater increases in abscisic acid (ABA) levels and upregulation of ABA-responsive element binding 2 (AREB2) and basic helix-loop-helix transcription factor MYC2. These results indicate that drought-induced SA-mediated activation of the hydroxycinnamic acid and flavonoid pathways contributes to drought resistance, whereas ABA-mediated lignification contributes to drought susceptibility.

Deep discovery informs difficult deployment in plant microbiome science

Plant-associated microbiota can extend plant immune system function, improve nutrient acquisition and availability, and alleviate abiotic stresses. Thus, naturally beneficial microbial therapeutics are enticing tools to improve plant productivity. The basic definition of plant microbiota across species and ecosystems, combined with the development of reductionist experimental models and the manipulation of plant phenotypes with microbes, has fueled interest in its translation to agriculture. However, the great majority of microbes exhibiting plant-productivity traits in the lab and greenhouse fail in the field. Therapeutic microbes must reach détente, the establishment of uneasy homeostasis, with the plant immune system, invade heterogeneous pre-established plant-associated communities, and persist in a new and potentially remodeled community. Environmental conditions can alter community structure and thus impact the engraftment of therapeutic microbes. We survey recent breakthroughs, challenges, and opportunities in translating beneficial microbes from the lab to the field.

Phyllosphere Microbiome in Plant Health and Disease

The phyllosphere refers to the aboveground surface of plants colonized by diverse microorganisms. Microbes inhabiting this environment play an important role in enhancing the host's genomic and metabolic capabilities, including defense against pathogens. Compared to the large volume of studies on rhizosphere microbiome for plant health and defense, our understanding of phyllosphere microbiome remains in its infancy. In this review, we aim to explore the mechanisms that govern the phyllosphere assembly and their function in host defence, as well as highlight the knowledge gaps. These efforts will help develop strategies to harness the phyllosphere microbiome toward sustainable crop production.

Genetic control underlying the flowering-drought tolerance trade-off in the Antarctic plant Colobanthus quitensis

Plants inhabiting environments with stressful conditions often exhibit a low number of flowers, which can be attributed to the energetic cost associated with reproduction. One of the most stressful environments for plants is the Antarctic continent, characterized by limited soil water availability and low temperatures. Induction of dehydrins like those from the COR gene family and auxin transcriptional response repressor genes (IAAs), which are involved in floral repression, has been described in response to water stress. Here, we investigated the relationship between the water deficit-induced stress response and the number of flowers in Colobanthus quitensis plants collected from populations along a latitudinal gradient. The expression levels of COR47 and IAA12 genes in response to water deficit were found to be associated with the number of flowers. The relationship was observed both in the field and growth chambers. Watering the plants in the growth chambers alleviated the stress and stimualted flowering, thereby eliminating the trade-off observed in the field. Our study provides a mechanistic understanding of the ecological constraints on plant reproduction along a water availability gradient. However, further experiments are needed to elucidate the primary role of water availability in regulating resource allocation to reproduction in plants inhibiting extreme environments.

Identification of genes associated with abiotic stress tolerance in sweetpotato using weighted gene co-expression network analysis

Sweetpotato, Ipomoea batatas (L.), a key food security crop, is negatively impacted by heat, drought, and salinity stress. The orange-fleshed sweetpotato cultivar "Beauregard" was exposed to heat, salt, and drought treatments for 24 and 48 h to identify genes responding to each stress condition in leaves. Analysis revealed both common (35 up regulated, 259 down regulated genes in the three stress conditions) and unique sets of up regulated (1337 genes by drought, 516 genes by heat, and 97 genes by salt stress) and down regulated (2445 genes by drought, 678 genes by heat, and 204 genes by salt stress) differentially expressed genes (DEGs) suggesting common, yet stress-specific transcriptional responses to these three abiotic stressors. Gene Ontology analysis of down regulated DEGs common to both heat and salt stress revealed enrichment of terms associated with "cell population proliferation" suggestive of an impact on the cell cycle by the two stress conditions. To identify shared and unique gene co-expression networks under multiple abiotic stress conditions, weighted gene co-expression network analysis was performed using gene expression profiles from heat, salt, and drought stress treated 'Beauregard' leaves yielding 18 co-expression modules. One module was enriched for "response to water deprivation," "response to abscisic acid," and "nitrate transport" indicating synergetic crosstalk between nitrogen, water, and phytohormones with genes encoding osmotin, cell expansion, and cell wall modification proteins present as key hub genes in this drought-associated module. This research lays the groundwork for exploring to a further degree, mechanisms for abiotic stress tolerance in sweetpotato.

Reduction in PLANT DEFENSIN 1 expression in Arabidopsis thaliana results in increased resistance to pathogens and zinc toxicity

Ectopic expression of defensins in plants correlates with their increased capacity to withstand abiotic and biotic stresses. This applies to Arabidopsis thaliana, where some of the seven members of the PLANT DEFENSIN 1 family (AtPDF1) are recognised to improve plant responses to necrotrophic pathogens and increase seedling tolerance to excess zinc (Zn). However, few studies have explored the effects of decreased endogenous defensin expression on these stress responses. Here, we carried out an extensive physiological and biochemical comparative characterization of (i) novel artificial microRNA (amiRNA) lines silenced for the five most similar AtPDF1s, and (ii) a double null mutant for the two most distant AtPDF1s. Silencing of five AtPDF1 genes was specifically associated with increased aboveground dry mass production in mature plants under excess Zn conditions, and with increased plant tolerance to different pathogens - a fungus, an oomycete and a bacterium, while the double mutant behaved similarly to the wild type. These unexpected results challenge the current paradigm describing the role of PDFs in plant stress responses. Additional roles of endogenous plant defensins are discussed, opening new perspectives for their functions.

In planta dynamics, transport biases, and endogenous functions of mobile siRNAs in Arabidopsis

In RNA interference (RNAi), small interfering RNAs (siRNAs) produced from double-stranded RNA guide ARGONAUTE (AGO) proteins to silence sequence-complementary RNA/DNA. RNAi can propagate locally and systemically in plants, but despite recent advances in our understanding of the underlying mechanisms, basic questions remain unaddressed. For instance, RNAi is inferred to diffuse through plasmodesmata (PDs), yet how its dynamics in planta compares with that of established symplastic diffusion markers remains unknown. Also is why select siRNA species, or size classes thereof, are apparently recovered in RNAi recipient tissues, yet only under some experimental settings. Shootward movement of endogenous RNAi in micro-grafted Arabidopsis is also yet to be achieved, while potential endogenous functions of mobile RNAi remain scarcely documented. Here, we show (i) that temporal, localized PD occlusion in source leaves' companion cells (CCs) suffices to abrogate all systemic manifestations of CC-activated mobile transgene silencing, including in sink leaves; (ii) that the presence or absence of specific AGOs in incipient/traversed/recipient tissues likely explains the apparent siRNA length selectivity observed upon vascular movement; (iii) that stress enhancement allows endo-siRNAs of a single inverted repeat (IR) locus to translocate against the shoot-to-root phloem flow; and (iv) that mobile endo-siRNAs generated from this locus have the potential to regulate hundreds of transcripts. Our results close important knowledge gaps, rationalize previously noted inconsistencies between mobile RNAi settings, and provide a framework for mobile endo-siRNA research.

The Overexpression of Peanut (Arachis hypogaea L.) AhALDH2B6 in Soybean Enhances Cold Resistance

Soybeans are the main source of oils and protein for humans and animals; however, cold stress jeopardizes their growth and limits the soybean planting area. Aldehyde dehydrogenases (ALDH) are conserved enzymes that catalyze aldehyde oxidation for detoxification in response to stress. Additionally, transgenic breeding is an efficient method for producing stress-resistant germplasms. In this study, the peanut ALDH gene AhALDH2B6 was heterologously expressed in soybean, and its function was tested. We performed RNA-seq using transgenic and wild-type soybeans with and without cold treatment to investigate the potential mechanism. Transgenic soybeans developed stronger cold tolerance, with longer roots and taller stems than P3 soybeans. Biochemically, the transgenic soybeans exhibited a decrease in malondialdehyde activity and an increase in peroxidase and catalase content, both of which are indicative of stress alleviation. They also possessed higher levels of ALDH enzyme activity. Two phenylpropanoid-related pathways were specifically enriched in up-regulated differentially expressed genes (DEGs), including the phenylpropanoid metabolic process and phenylpropanoid biosynthetic process. Our findings suggest that AhALDH2B6 specifically up-regulates genes involved in oxidoreductase-related functions such as peroxidase, oxidoreductase, monooxygenase, and antioxidant activity, which is partially consistent with our biochemical data. These findings established the function of AhALDH2B6, especially its role in cold stress processes, and provided a foundation for molecular plant breeding, especially plant-stress-resistance breeding.

The OsSGS3-tasiRNA-OsARF3 module orchestrates abiotic-biotic stress response trade-off in rice

Recurrent heat stress and pathogen invasion seriously threaten crop production, and abiotic stress often antagonizes biotic stress response against pathogens. However, the molecular mechanisms of trade-offs between thermotolerance and defense remain obscure. Here, we identify a rice thermo-sensitive mutant that displays a defect in floret development under high temperature with a mutation in SUPPRESSOR OF GENE SILENCING 3a (OsSGS3a). OsSGS3a interacts with its homolog OsSGS3b and modulates the biogenesis of trans-acting small interfering RNA (tasiRNA) targeting AUXIN RESPONSE FACTORS (ARFs). We find that OsSGS3a/b positively, while OsARF3a/b and OsARF3la/lb negatively modulate thermotolerance. Moreover, OsSGS3a negatively, while OsARF3a/b and OsARF3la/lb positively regulate disease resistance to the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo) and the fungal pathogen Magnaporthe oryzae (M. oryzae). Taken together, our study uncovers a previously unknown trade-off mechanism that regulates distinct immunity and thermotolerance through the OsSGS3-tasiRNA-OsARF3 module, highlighting the regulation of abiotic-biotic stress response trade-off in plants.

WHIRLY1 Acts Upstream of ABA-Related Reprogramming of Drought-Induced Gene Expression in Barley and Affects Stress-Related Histone Modifications

WHIRLY1, a small plant-specific ssDNA-binding protein, dually located in chloroplasts and the nucleus, is discussed to act as a retrograde signal transmitting a stress signal from the chloroplast to the nucleus and triggering there a stress-related gene expression. In this work, we investigated the function of WHIRLY1 in the drought stress response of barley, employing two overexpression lines (oeW1-2 and oeW1-15). The overexpression of WHIRLY1 delayed the drought-stress-related onset of senescence in primary leaves. Two abscisic acid (ABA)-dependent marker genes of drought stress, HvNCED1 and HvS40, whose expression in the wild type was induced during drought treatment, were not induced in overexpression lines. In addition, a drought-related increase in ABA concentration in the leaves was suppressed in WHIRLY1 overexpression lines. To analyze the impact of the gain-of-function of WHIRLY1 on the drought-related reprogramming of nuclear gene expression, RNAseq was performed comparing the wild type and an overexpression line. Cluster analyses revealed a set of genes highly up-regulated in response to drought in the wild type but not in the WHIRLY1 overexpression lines. Among these genes were many stress- and abscisic acid (ABA)-related ones. Another cluster comprised genes up-regulated in the oeW1 lines compared to the wild type. These were related to primary metabolism, chloroplast function and growth. Our results indicate that WHIRLY1 acts as a hub, balancing trade-off between stress-related and developmental pathways. To test whether the gain-of-function of WHIRLY1 affects the epigenetic control of stress-related gene expression, we analyzed drought-related histone modifications in different regions of the promoter and at the transcriptional start sites of HvNCED1 and HvS40. Interestingly, the level of euchromatic marks (H3K4me3 and H3K9ac) was clearly decreased in both genes in a WHIRLY1 overexpression line. Our results indicate that WHIRLY1, which is discussed to act as a retrograde signal, affects the ABA-related reprogramming of nuclear gene expression during drought via differential histone modifications.

Understanding the plant-microbe interactions in environments exposed to abiotic stresses: An overview

Abiotic stress poses a severe danger to agriculture since it negatively impacts cellular homeostasis and eventually stunts plant growth and development. Abiotic stressors like drought and excessive heat are expected to occur more frequently in the future due to climate change, which would reduce the yields of important crops like maize, wheat, and rice which may jeopardize the food security of human populations. The plant microbiomes are a varied and taxonomically organized microbial community that is connected to plants. By supplying nutrients and water to plants, and regulating their physiology and metabolism, plant microbiota frequently helps plants develop and tolerate abiotic stresses, which can boost crop yield under abiotic stresses. In this present study, with emphasis on temperature, salt, and drought stress, we describe current findings on how abiotic stresses impact the plants, microbiomes, microbe-microbe interactions, and plant-microbe interactions as the way microorganisms affect the metabolism and physiology of the plant. We also explore crucial measures that must be taken in applying plant microbiomes in agriculture practices faced with abiotic stresses.

Distinct function of SPL genes in age-related resistance in Arabidopsis

In plants, age-related resistance (ARR) refers to a gain of disease resistance during shoot or organ maturation. ARR associated with vegetative phase change, a transition from juvenile to adult stage, is a widespread agronomic trait affecting resistance against multiple pathogens. How innate immunity in a plant is differentially regulated during successive stages of shoot maturation is unclear. In this work, we found that Arabidopsis thaliana showed ARR against its bacterial pathogen Pseudomonas syringae pv. tomato DC3000 during vegetative phase change. The timing of the ARR activation was associated with a temporal drop of miR156 level. The microRNA miR156 maintains juvenile phase by inhibiting the accumulation and translation of SPL transcripts. A systematic inspection of the loss- and gain-of-function mutants of 11 SPL genes revealed that a subset of SPL genes, notably SPL2, SPL10, and SPL11, activated ARR in adult stage. The immune function of SPL10 was independent of its role in morphogenesis. Furthermore, the SPL10 mediated an age-dependent augmentation of the salicylic acid (SA) pathway partially by direct activation of PAD4. Disrupting SA biosynthesis or signaling abolished the ARR against Pto DC3000. Our work demonstrated that the miR156-SPL10 module in Arabidopsis is deployed to operate immune outputs over developmental timing.

The Arabidopsis holobiont: a (re)source of insights to understand the amazing world of plant-microbe interactions

As holobiont, a plant is intrinsically connected to its microbiomes. However, some characteristics of these microbiomes, such as their taxonomic composition, biological and evolutionary role, and especially the drivers that shape them, are not entirely elucidated. Reports on the microbiota of Arabidopsis thaliana first appeared more than ten years ago. However, there is still a lack of a comprehensive understanding of the vast amount of information that has been generated using this holobiont. The main goal of this review was to perform an in-depth, exhaustive, and systematic analysis of the literature regarding the Arabidopsis-microbiome interaction. A core microbiota was identified as composed of a few bacterial and non-bacterial taxa. The soil (and, to a lesser degree, air) were detected as primary microorganism sources. From the plant perspective, the species, ecotype, circadian cycle, developmental stage, environmental responses, and the exudation of metabolites were crucial factors shaping the plant-microbe interaction. From the microbial perspective, the microbe-microbe interactions, the type of microorganisms belonging to the microbiota (i.e., beneficial or detrimental), and the microbial metabolic responses were also key drivers. The underlying mechanisms are just beginning to be unveiled, but relevant future research needs were identified. Thus, this review provides valuable information and novel analyses that will shed light to deepen our understanding of this plant holobiont and its interaction with the environment.

In vivo surface-enhanced Raman scattering nanosensor for the real-time monitoring of multiple stress signalling molecules in plants

When under stress, plants release molecules to activate their defense system. Detecting these stress-related molecules offers the possibility to address stress conditions and prevent the development of diseases. However, detecting endogenous signalling molecules in living plants remains challenging due to low concentrations of these analytes and interference with other compounds; additionally, many methods currently used are invasive and labour-intensive. Here we show a non-destructive surface-enhanced Raman scattering (SERS)-based nanoprobe for the real-time detection of multiple stress-related endogenous molecules in living plants. The nanoprobe, which is placed in the intercellular space, is optically active in the near-infrared region (785 nm) to avoid interferences from plant autofluorescence. It consists of a Si nanosphere surrounded by a corrugated Ag shell modified by a water-soluble cationic polymer poly(diallyldimethylammonium chloride), which can interact with multiple plant signalling molecules. We measure a SERS enhancement factor of 2.9 × 10⁷ and a signal-to-noise ratio of up to 64 with an acquisition time of ~100 ms. To show quantitative multiplex detection, we adopted a binding model to interpret the SERS intensities of two different analytes bound to the SERS hot spot of the nanoprobe. Under either abiotic or biotic stress, our optical nanosensors can successfully monitor salicylic acid, extracellular adenosine triphosphate, cruciferous phytoalexin and glutathione in Nasturtium officinale, Triticum aestivum L. and Hordeum vulgare L.-all stress-related molecules indicating the possible onset of a plant disease. We believe that plasmonic nanosensor platforms can enable the early diagnosis of stress, contributing to a timely disease management of plants.

Crossroads in the evolution of plant specialized metabolism

The monophyletic group of embryophytes (land plants) stands out among photosynthetic eukaryotes: they are the sole constituents of the macroscopic flora on land. In their entirety, embryophytes account for the majority of the biomass on land and constitute an astounding biodiversity. What allowed for the massive radiation of this particular lineage? One of the defining features of all land plants is the production of an array of specialized metabolites. The compounds that the specialized metabolic pathways of embryophytes produce have diverse functions, ranging from superabundant structural polymers and compounds that ward off abiotic and biotic challenges, to signaling molecules whose abundance is measured at the nanomolar scale. These specialized metabolites govern the growth, development, and physiology of land plants-including their response to the environment. Hence, specialized metabolites define the biology of land plants as we know it. And they were likely a foundation for their success. It is thus intriguing to find that the closest algal relatives of land plants, freshwater organisms from the grade of streptophyte algae, possess homologs for key enzymes of specialized metabolic pathways known from land plants. Indeed, some studies suggest that signature metabolites emerging from these pathways can be found in streptophyte algae. Here we synthesize the current understanding of which routes of the specialized metabolism of embryophytes can be traced to a time before plants had conquered land.

Regulation of inositol 1,2,4,5,6-pentakisphosphate and inositol hexakisphosphate levels in Gossypium hirsutum by IPK1

The IPK1 genes, which code for 2-kinases that can synthesize Ins(1,2,4,5,6)P₅ from Ins(1,4,5,6)P₄, are expressed throughout cotton plants, resulting in the highest Ins(1,2,4,5,6)P₅ concentrations in young leaves and flower buds. Cotton leaves contain large amounts of Ins(1,2,4,5,6)P₅ and InsP₆ compared to plants not in the Malvaceae family. The inositol polyphosphate pathway has been linked to stress tolerance in numerous plant species. Accordingly, we sought to determine why cotton and other Malvaceae have such high levels of these inositol phosphates. We have quantified the levels of InsP₅ and InsP₆ in different tissues of cotton plants and determined the expression of IPK1 (inositol 1,3,4,5,6-pentakisphosphate 2-kinase gene) in vegetative and reproductive tissues. Gossypium hirsutum was found to contain four IPK1 genes that were grouped into two pair (AB, CD) where each pair consists of very similar sequences that were measured together. More IPK1AB is expressed in leaves than in roots, whereas more IPK1CD is expressed in roots than in leaves. Leaves and flower buds have more InsP₅ and InsP₆ than stems and roots. Leaves and roots contain more InsP₅ than InsP₆, whereas flower buds and stems contain more InsP₆ than InsP₅. Dark-grown seedlings contain more InsP₅ and InsP₆ than those grown under lights, and the ratio of InsP₅ to InsP₆ is greater in the light-grown seedlings. During 35 days of the life cycle of the third true leaf, InsP₅ and InsP₆ gradually decreased by more than 50%. Silencing IPK1AB and IPK1CD with Cotton Leaf Crumple Virus-induced gene silencing (VIGS) resulted in plants with an intense viral phenotype, reduced IPK1AB expression and lowered amounts of InsP₅. The results are consistent with Ins(1,2,4,5,6)P₅ synthesis from Ins(1,4,5,6)P₄ by IPK1. This study detailed the central role of IPK1 in cotton inositol polyphosphate metabolism, which has potential to be harnessed to improve the resistance of plants to different kinds of stress.

Distinct Phyllosphere Microbiome of Wild Tomato Species in Central Peru upon Dysbiosis

Plants are colonized by myriads of microbes across kingdoms, which affect host development, fitness, and reproduction. Hence, plant microbiomes have been explored across a broad range of host species, including model organisms, crops, and trees under controlled and natural conditions. Tomato is one of the world's most important vegetable crops; however, little is known about the microbiota of wild tomato species. To obtain insights into the tomato microbiota occurring in natural environments, we sampled epiphytic microbes from leaves of four tomato species, Solanum habrochaites, S. corneliomulleri, S. peruvianum, and S. pimpinellifolium, from two geographical locations within the Lima region of Peru over 2 consecutive years. Here, a high-throughput sequencing approach was applied to investigate microbial compositions including bacteria, fungi, and eukaryotes across tomato species and geographical locations. The phyllosphere microbiome composition varies between hosts and location. Yet, we identified persistent microbes across tomato species that form the tomato microbial core community. In addition, we phenotypically defined healthy and dysbiotic samples and performed a downstream analysis to reveal the impact on microbial community structures. To do so, we compared microbial diversities, unique OTUs, relative abundances of core taxa, and microbial hub taxa, as well as co-occurrence network characteristics in healthy and dysbiotic tomato leaves and found that dysbiosis affects the phyllosphere microbial composition in a host species-dependent manner. Yet, overall, the present data suggests an enrichment of plant-promoting microbial taxa in healthy leaves, whereas numerous microbial taxa containing plant pathogens occurred in dysbiotic leaves.Concluding, we identify the core phyllosphere microbiome of wild tomato species, and show that the overall phyllosphere microbiome can be impacted by sampling time point, geographical location, host genotype, and plant health. Future studies in these components will help understand the microbial contribution to plant health in natural systems and can be of use in cultivated tomatoes.

PBS3: a versatile player in and beyond salicylic acid biosynthesis in Arabidopsis

AVRPPHB SUSCEPTIBLE 3 (PBS3) belongs to the GH3 family of acyl acid amido synthetases, which conjugates amino acids to diverse acyl acid substrates. Recent studies demonstrate that PBS3 in Arabidopsis plays a key role in the biosynthesis of plant defense hormone salicylic acid (SA) by catalyzing the conjugation of glutamate to isochorismate to form isochorismate-9-glutamate, which is then used to produce SA through spontaneous decay or ENHANCED PSEUDOMONAS SUSCEPTIBILITY (EPS1) catalysis. Consistent with its function as an essential enzyme for SA biosynthesis, PBS3 is well known to be a positive regulator of plant immunity in Arabidopsis. Additionally, PBS3 is also involved in the trade-off between abiotic and biotic stress responses in Arabidopsis by suppressing the inhibitory effect of abscisic acid on SA-mediated plant immunity. Besides stress responses, PBS3 also plays a role in plant development. Under long-day conditions, PBS3 influences Arabidopsis flowering time by regulating the expression of flowering regulators FLOWERING LOCUS C and FLOWERING LOCUS T. Taken together, PBS3 functions in the signaling network of plant development and responses to biotic and/or abiotic stresses, but the molecular mechanisms underlying its diverse roles remain obscure.

Arabidopsis Cys2/His2 Zinc Finger Transcription Factor ZAT18 Modulates the Plant Growth-Defense Tradeoff

Plant defense responses under unfavorable conditions are often associated with reduced growth. However, the mechanisms underlying the growth-defense tradeoff remain to be fully elucidated, especially at the transcriptional level. Here, we revealed a Cys2/His2-type zinc finger transcription factor, namely, ZAT18, which played dual roles in plant immunity and growth by oppositely regulating the signaling of defense- and growth-related hormones. ZAT18 was first identified as a salicylic acid (SA)-inducible gene and was required for plant responses to SA in this study. In addition, we observed that ZAT18 enhanced the plant immunity with growth penalties that may have been achieved by activating SA signaling and repressing auxin signaling. Further transcriptome analysis of the zat18 mutant showed that the biological pathways of defense-related hormones, including SA, ethylene and abscisic acid, were repressed and that the biological pathways of auxin and cytokinin, which are growth-related hormones, were activated by abolishing the function of ZAT18. The ZAT18-mediated regulation of hormone signaling was further confirmed using qRT-PCR. Our results explored a mechanism by which plants handle defense and growth at the transcriptional level under stress conditions.

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.

Flooding and herbivory: the effect of concurrent stress factors on plant volatile emissions and gene expression in two heirloom tomato varieties

BACKGROUND

In nature and in cultivated fields, plants encounter multiple stress factors. Nonetheless, our understanding of how plants actively respond to combinatorial stress remains limited. Among the least studied stress combination is that of flooding and herbivory, despite the growing importance of these stressors in the context of climate change. We investigated plant chemistry and gene expression changes in two heirloom tomato varieties: Cherokee Purple (CP) and Striped German (SG) in response to flooding, herbivory by Spodoptera exigua, and their combination.

RESULTS

Volatile organic compounds (VOCs) identified in tomato plants subjected to flooding and/or herbivory included several mono- and sesquiterpenes. Flooding was the main factor altering VOCs emission rates, and impacting plant biomass accumulation, while different varieties had quantitative differences in their VOC emissions. At the gene expression levels, there were 335 differentially expressed genes between the two tomato plant varieties, these included genes encoding for phenylalanine ammonia-lyase (PAL), cinnamoyl-CoA-reductase-like, and phytoene synthase (Psy1). Flooding and variety effects together influenced abscisic acid (ABA) signaling genes with the SG variety showing higher levels of ABA production and ABA-dependent signaling upon flooding. Flooding downregulated genes associated with cytokinin catabolism and general defense response and upregulated genes associated with ethylene biosynthesis, anthocyanin biosynthesis, and gibberellin biosynthesis. Combining flooding and herbivory induced the upregulation of genes including chalcone synthase (CHS), PAL, and genes encoding BAHD acyltransferase and UDP-glucose iridoid glucosyltransferase-like genes in one of the tomato varieties (CP) and a disproportionate number of heat-shock proteins in SG. Only the SG variety had measurable changes in gene expression due to herbivory alone, upregulating zeatin, and O-glucosyltransferase and thioredoxin among others.

CONCLUSION

Our results suggest that both heirloom tomato plant varieties differ in their production of secondary metabolites including phenylpropanoids and terpenoids and their regulation and activation of ABA signaling upon stress associated with flooding. Herbivory and flooding together had interacting effects that were evident at the level of plant chemistry (VOCs production), gene expression and biomass markers. Results from our study highlight the complex nature of plant responses to combinatorial stresses and point at specific genes and pathways that are affected by flooding and herbivory combined.

Abiotic and Herbivory Combined Stress in Tomato: Additive, Synergic and Antagonistic Effects and Within-Plant Phenotypic Plasticity

Background: Drought, N deficiency and herbivory are considered the most important stressors caused by climate change in the agro- and eco-systems and varied in space and time shaping highly dynamic and heterogeneous stressful environments. This study aims to evaluate the tomato morpho-physiological and metabolic responses to combined abiotic and herbivory at different within-plant spatial levels and temporal scales. Methods: Leaf-level morphological, gas exchange traits and volatile organic compounds (VOCs) profiles were measured in tomato plants exposed to N deficiency and drought, Tuta absoluta larvae and their combination. Additive, synergistic or antagonistic effects of the single stress when combined were also evaluated. Morpho-physiological traits and VOCs profile were also measured on leaves located at three different positions along the shoot axes. Results: The combination of the abiotic and biotic stress has been more harmful than single stress with antagonistic and synergistic but non-additive effects for the morpho-physiological and VOCs tomato responses, respectively. Combined stress also determined a high within-plant phenotypic plasticity of the morpho-physiological responses. Conclusions: These results suggested that the combined stress in tomato determined a “new stress state” and a higher within-plant phenotypic plasticity which could permit an efficient use of the growth and defense resources in the heterogeneous and multiple stressful environmental conditions.

The Barley Heavy Metal Associated Isoprenylated Plant Protein HvFP1 Is Involved in a Crosstalk between the Leaf Development and Abscisic Acid-Related Drought Stress Responses

The heavy metal associated isoprenylated plant proteins (HIPPs) are characterized by at least one heavy metal associated (HMA) domain and a C-terminal isoprenylation motif. Hordeum vulgare farnesylated protein 1 (HvFP1), a barley HIPP, is upregulated during drought stress, in response to abscisic acid (ABA) and during leaf senescence. To investigate the role of HvFP1, two independent gain-of-function lines were generated. In a physiological level, the overexpression of HvFP1 results in the delay of normal leaf senescence, but not in the delay of rapid, drought-induced leaf senescence. In addition, the overexpression of HvFP1 suppresses the induction of the ABA-related genes during drought and senescence, e.g., HvNCED, HvS40, HvDhn1. Even though HvFP1 is induced during drought, senescence and the ABA treatment, its overexpression suppresses the ABA regulated genes. This indicates that HvFP1 is acting in a negative feedback loop connected to the ABA signaling. The genome-wide transcriptomic analysis via RNA sequencing revealed that the gain-of-function of HvFP1 positively alters the expression of the genes related to leaf development, photomorphogenesis, photosynthesis and chlorophyll biosynthesis. Interestingly, many of those genes encode proteins with zinc binding domains, implying that HvFP1 may act as zinc supplier via its HMA domain. The results show that HvFP1 is involved in a crosstalk between stress responses and growth control pathways.