Dual Role of Auxin in Regulating Plant Defense and Bacterial Virulence Gene Expression During Pseudomonas syringae PtoDC3000 Pathogenesis

Rooted in Communication: Exploring Auxin-Salicylic Acid Nexus in Root Growth and Development

Plant hormones are pivotal in orchestrating diverse aspects of growth and developmental processes. Among various phytohormones, auxin and salicylic acid (SA) stand out as important regulators, often exerting opposing effects on overall plant growth. Essentially, research has indicated that auxin and SA-mediated pathways exhibit mutual antagonism during pathogen challenge. Additionally, in recent years, significant advancements have been made in uncovering the molecular intricacies that govern the action and interplay between these two phytohormones during various essential growth-related processes. In this discussion, we briefly delve into the genetic and molecular mechanisms involved in auxin and SA antagonism. We then analyse in detail how this dialogue impacts critical aspects of root development, with an emphasis on the transcriptional and protein regulatory networks. Finally, we propose the potential of exploring their interaction in various other aspects of below ground root growth processes. Understanding this relationship could provide valuable insights for optimizing and enhancing crop growth and yields.

Identification and characterization of auxin response factor (ARF) gene family in five Bambusoideae species reveals the role of PedARF 23 in regulating lignin synthesis through auxin signaling

Auxin plays a critical role in plant growth and development, mediating responses to environmental stimuli and directing organ growth by establishing and maintaining auxin concentration gradients. Auxin Response Factors (ARFs) are key transcription factors that regulate the expression of auxin-responsive genes. Different ARF genes can act as transcriptional activators or repressors, functioning through various auxin signaling pathways at different developmental stages and under various environmental conditions. Studying the ARF gene family in bamboo is essential for uncovering the molecular mechanisms underlying rapid growth, maintenance of apical dominance, and root development in bamboo. However, research on the function of ARF genes in bamboo is still limited. In this study, we utilized the latest bamboo genome data to comprehensively analyze the genomes of five representative bamboo species, identifying a total of 216 ARF genes and classifying them into 12 subfamilies based on phylogenetic relationships. Through a combination of transcriptome date and RT-qPCR analysis, we observed that the PedARF23 gene in Moso bamboo was significantly upregulated following NAA treatment. To further explore the mechanisms by which PedARF23 modulates plant growth and development via auxin signaling, we developed PedARF23 overexpression lines. By measuring auxin and lignin levels, the expression of key genes (4CL3, 4CL7, and CCoAOMT1) in the lignin biosynthesis pathway in both Moso bamboo and transgenic Arabidopsis, as well as the Yeast one-hybrid assay and LUC activation assay, study discovered that PedARF23 could enhances auxin synthesis and activates the auxin signaling pathway, thereby regulating lignin biosynthesis. Overall, this study provides a foundation for understanding the classification, characteristics, and functions of the ARF gene family in the Bambusoideae. It elucidates the role of ARFs in regulating lignin synthesis, offering valuable insights into the regulation of lignin content in bamboo.

In vivo dynamics of indole- and phenol-derived plant hormones: Long-term, continuous, and minimally invasive phytohormone sensor

Specific phytohormone combinations regulate plant growth and responses to environmental stimuli. Monitoring their distribution is key for understanding signaling cross-talk and detecting plant stress early. However, typical means of monitoring these chemicals are often laborious, destructive, or limited to model plants. In this study, we present an amperometric and minimally invasive sensing platform that can be attached to plant leaves for the simultaneous detection of two key phytohormones, auxin [indole-3-acetic acid (IAA)] and salicylic acid (SA). The platform incorporates magnetized microneedles coated with superparamagnetic Fe3O4 intercalated into a scaffold of multiwalled carbon nanotubes (MWCNTs). It achieves detection limits of 1.41 μM (IAA) and 1.15 μM (SA) with a strong correlation (R2 ≥ 0.7) to ultrahigh-performance liquid chromatography-tandem mass spectrometry measurements. Furthermore, implementing cyclical amperometric cleaning extends the sensor lifespan by preventing electrode passivation. Last, the sensor's capability to monitor the real-time plant responses to several stressors is validated, showcasing its potential for phytodiagnostics and precision farming.

Integrated Transcriptomic and Metabolomic Analyses Reveal the Importance of the Terpenoid, Fatty Acid, and Flavonoid Pathways in Rice Cell Death and Defense

Lesion mimic mutants provide unique tools to investigate plant-pathogen interactions, often exhibiting hypersensitive responses in the absence of biotic or abiotic stresses. The overexpression of the S-domain receptor-like kinase gene, SPL11 cell-death suppressor 2 (SDS2), in rice leads to constitutive programmed cell death and enhanced resistance to fungal and bacterial pathogens. However, the mechanisms underlying this broad-spectrum resistance remain unclear. This study integrates transcriptomic and metabolomic analyses of the SDS2-ACT mutant to uncover gene expression and metabolic shifts associated with disease resistance. To identify SDS2-specific physiological changes related to pathogen resistance, leaf tissues from the SDS2-ACT mutant and the Kitkaake WT line were subjected to both transcriptomic and non-targeted metabolic profiling. Transcriptomic analyses identified 1497 differentially expressed genes (DEGs), including up-regulated genes involved in terpenoid and flavonoid biosynthesis, phytohormone signaling, and defense-related pathways (including pathogenesis-related [PR] genes). Metabolomic profiling revealed significant alterations in the accumulation of several compound classes, including putative: terpenoids, phenylpropanoids, phytohormones, fatty acids, and sugars. These changes are likely correlated with the observed cell death and resistance phenotypes in the SDS2-ACT mutant. This study provides an overall landscape of the transcriptomic and metabolomic alterations in a lesion mimic mutant, identifying candidate defense-related genes and metabolites for functional analysis in rice.

Regulation of Bacterial Growth and Behavior by Host Plant

Plants are associated with diverse bacteria in nature. Some bacteria are pathogens that decrease plant fitness, and others are beneficial bacteria that promote plant growth and stress resistance. Emerging evidence also suggests that plant-associated commensal bacteria collectively contribute to plant health and are essential for plant survival in nature. Bacteria with different characteristics simultaneously colonize plant tissues. Thus, plants need to accommodate bacteria that provide service to the host plants, but they need to defend against pathogens at the same time. How do plants achieve this? In this review, we summarize how plants use physical barriers, control common goods such as water and nutrients, and produce antibacterial molecules to regulate bacterial growth and behavior. Furthermore, we highlight that plants use specialized metabolites that support or inhibit specific bacteria, thereby selectively recruiting plant-associated bacterial communities and regulating their function. We also raise important questions that need to be addressed to improve our understanding of plant-bacteria interactions.

Priming of Exogenous Salicylic Acid under Field Conditions Enhances Crop Yield through Resistance to Magnaporthe oryzae by Modulating Phytohormones and Antioxidant Enzymes

This study explores the impact of exogenous salicylic acid (SA) alongside conventional treatment by farmers providing positive (Mancozeb 80 % WP) and negative (water) controls on rice plants (Oryza sativa L.), focusing on antioxidant enzyme activities, phytohormone levels, disease resistance, and yield components under greenhouse and field conditions. In greenhouse assays, SA application significantly enhanced the activities of peroxidase (POX), polyphenol oxidase (PPO), catalase (CAT), and superoxide dismutase (SOD) within 12-24 h post-inoculation (hpi) with Magnaporthe oryzae. Additionally, SA-treated plants showed higher levels of endogenous SA and indole-3-acetic acid (IAA) within 24 hpi compared to the controls. In terms of disease resistance, SA-treated plants exhibited a reduced severity of rice blast under greenhouse conditions, with a significant decrease in disease symptoms compared to negative control treatment. The field study was extended over three consecutive crop seasons during 2021-2023, further examining the efficacy of SA in regular agricultural practice settings. The SA treatment consistently led to a reduction in rice blast disease severity across all three seasons. Yield-related parameters such as plant height, the number of tillers and panicles per hill, grains per panicle, and 1000-grain weight all showed improvements under SA treatment compared to both positive and negative control treatments. Specifically, SA-treated plants yielded higher grain outputs in all three crop seasons, underscoring the potential of SA as a growth enhancer and as a protective agent against rice blast disease under both controlled and field conditions. These findings state the broad-spectrum benefits of SA application in rice cultivation, highlighting its role not only in bolstering plant defense mechanisms and growth under greenhouse conditions but also in enhancing yield and disease resistance in field settings across multiple crop cycles. This research presents valuable insights into the practical applications of SA in improving rice plant resilience and productivity, offering a promising approach for sustainable agriculture practices.

Isolation of bacteria with plant growth-promoting properties from microalgae-bacterial flocs produced in high-rate oxidation ponds

Exploring plant growth-promoting (PGP) bacterial activity of microbial components aggregated by wastewater treatment can reduce dependence on fossil fuel-derived fertilisers. This study describes the isolation and identification of bacteria from microalgae-bacteria flocs (MaB-flocs) generated in high-rate algal oxidation ponds (HRAOP) of an integrated algal pond system (IAPS) remediating municipal wastewater. Amplified 16S rRNA gene sequence analysis determined the molecular identity of the individual strains. Genetic relatedness to known PGP rhizobacteria in the NCBI GenBank database was by metagenomics. Isolated strains were screened for the production of indoles (measured as indole-3-acetic acid; IAA) and an ability to mineralise NH 4 + , PO 4 3 - , and K + . Of the twelve bacterial strains isolated from HRAOP MaB-flocs, four produced indoles, nine mineralised NH 4 + , seven solubilised P, and one K. Potential of isolated strains for PGP activity according to one-way ANOVA on ranks was: ECCN 7b > ECCN 4b > ECCN 6b > ECCN 3b = ECCN 10b > ECCN 1b = ECCN 5b > ECCN 8b > ECCN 2b > ECCN 12b > ECCN 9b = ECCN 11b. Further study revealed that cell-free filtrate from indole-producing cultures of Aeromonas strain ECCN 4b, Enterobacter strain ECCN 7b, and Arthrobacter strain ECCN 6b promoted mung bean adventitious root formation suggestive of the presence of auxin-like biological activity.

Investigating the biosynthesis and roles of the auxin phenylacetic acid during Pseudomonas syringae-Arabidopsis thaliana pathogenesis

Several plant-associated microbes synthesize the auxinic plant growth regulator phenylacetic acid (PAA) in culture; however, the role of PAA in plant-pathogen interactions is not well understood. In this study, we investigated the role of PAA during interactions between the phytopathogenic bacterium Pseudomonas syringae strain PtoDC3000 (PtoDC3000) and the model plant host, Arabidopsis thaliana. Previous work demonstrated that indole-3-acetaldehyde dehydrogenase A (AldA) of PtoDC3000 converts indole-3-acetaldehyde (IAAld) to the auxin indole-3-acetic acid (IAA). Here, we further demonstrated the biochemical versatility of AldA by conducting substrate screening and steady-state kinetic analyses, and showed that AldA can use both IAAld and phenylacetaldehyde as substrates to produce IAA and PAA, respectively. Quantification of auxin in infected plant tissue showed that AldA-dependent synthesis of either IAA or PAA by PtoDC3000 does not contribute significantly to the increase in auxin levels in infected A. thaliana leaves. Using available arogenate dehydratase (adt) mutant lines of A. thaliana compromised for PAA synthesis, we observed that a reduction in PAA-Asp and PAA-Glu is correlated with elevated levels of IAA and increased susceptibility. These results provide evidence that PAA/IAA homeostasis in A. thaliana influences the outcome of plant-microbial interactions.

Fighting for Survival at the Stomatal Gate

Stomata serve as the battleground between plants and plant pathogens. Plants can perceive pathogens, inducing closure of the stomatal pore, while pathogens can overcome this immune response with their phytotoxins and elicitors. In this review, we summarize new discoveries in stomata-pathogen interactions. Recent studies have shown that stomatal movement continues to occur in a close-open-close-open pattern during bacterium infection, bringing a new understanding of stomatal immunity. Furthermore, the canonical pattern-triggered immunity pathway and ion channel activities seem to be common to plant-pathogen interactions outside of the well-studied Arabidopsis-Pseudomonas pathosystem. These developments can be useful to aid in the goal of crop improvement. New technologies to study intact leaves and advances in available omics data sets provide new methods for understanding the fight at the stomatal gate. Future studies should aim to further investigate the defense-growth trade-off in relation to stomatal immunity, as little is known at this time.

Abolishing ARF8A activity promotes disease resistance in tomato

Auxin response factors (ARFs) are a family of transcription factors that regulate auxin-dependent developmental processes. Class A ARFs function as activators of auxin-responsive gene expression in the presence of auxin, while acting as transcriptional repressors in its absence. Despite extensive research on the functions of ARF transcription factors in plant growth and development, the extent, and mechanisms of their involvement in plant resistance, remain unknown. We have previously reported that mutations in the tomato AUXIN RESPONSE FACTOR8 (ARF8) genes SlARF8A and SlARF8B result in the decoupling of fruit development from pollination and fertilization, leading to partial or full parthenocarpy and increased yield under extreme temperatures. Here, we report that fine-tuning of SlARF8 activity results in increased resistance to fungal and bacterial pathogens. This resistance is mostly preserved under fluctuating temperatures. Thus, fine-tuning SlARF8 activity may be a potent strategy for increasing overall growth and yield.

Indole Alkaloid Production by the Halo Blight Bacterium Treated with the Phytoalexin Genistein

When Pseudomonas savastanoi pv. phaseolicola, the bacterium that causes halo blight, induces hypersensitive immunity in common bean leaves, salicylic acid and phytoalexins accumulate at the site of infection. Both salicylic acid and the phytoalexin resveratrol exert antibiotic activities and toxicities in vitro, adversely disrupting the P. savastanoi pv. phaseolicola proteome and metabolism and stalling replication and motility. These efficacious properties likely contribute to the cessation of bacterial spread in beans. Genistein is an isoflavonoid phytoalexin that also accumulates during bean immunity, so we tested its antibiotic potential in vitro. Quantitative proteomics revealed that genistein did not induce proteomic changes in P. savastanoi pv. phaseolicola in the same way that salicylic acid or resveratrol did. Rather, a dioxygenase that could function to metabolize genistein was among the most highly induced enzymes. Indeed, high-throughput metabolomics provided direct evidence for genistein catabolism. Metabolomics also revealed that genistein induced the bacterium to produce indole compounds, several of which had structural similarity to auxin. Additional mass spectrometry analyses proved that the bacterium produced an isomer of the auxin indole-3-acetic acid but not indole-3-acetic acid proper. These results reveal that P. savastanoi pv. phaseolicola can tolerate bean genistein and that the bacterium likely responds to bean-produced genistein during infection, using it as a signal to increase pathogenicity, possibly by altering host cell physiology or metabolism through the production of potential auxin mimics.

Transcription factor WRKY75 maintains auxin homeostasis to promote tomato defense against Pseudomonas syringae

The hemibiotrophic bacterial pathogen Pseudomonas syringae infects a range of plant species and causes enormous economic losses. Auxin and WRKY transcription factors play crucial roles in plant responses to P. syringae, but their functional relationship in plant immunity remains unclear. Here, we characterized tomato (Solanum lycopersicum) SlWRKY75, which promotes defenses against P. syringae pv. tomato (Pst) DC3000 by regulating plant auxin homeostasis. Overexpressing SlWRKY75 resulted in low free indole-3-acetic acid (IAA) levels, leading to attenuated auxin signaling, decreased expansin transcript levels, upregulated expression of PATHOGENESIS-RELATED GENES (PRs) and NONEXPRESSOR OF PATHOGENESIS-RELATED GENE 1 (NPR1), and enhanced tomato defenses against Pst DC3000. RNA interference-mediated repression of SlWRKY75 increased tomato susceptibility to Pst DC3000. Yeast one-hybrid, electrophoretic mobility shift assays, and luciferase activity assays suggested that SlWRKY75 directly activates the expression of GRETCHEN HAGEN 3.3 (SlGH3.3), which encodes an IAA-amido synthetase. SlGH3.3 enhanced tomato defense against Pst DC3000 by converting free IAA to the aspartic acid (Asp)-conjugated form IAA-Asp. In addition, SlWRKY75 interacted with a tomato valine-glutamine (VQ) motif-containing protein 16 (SlVQ16) in vivo and in vitro. SlVQ16 enhanced SlWRKY75-mediated transcriptional activation of SlGH3.3 and promoted tomato defense responses to Pst DC3000. Our findings illuminate a mechanism in which the SlVQ16-SlWRKY75 complex participates in tomato pathogen defense by positively regulating SlGH3.3-mediated auxin homeostasis.

Bacterial indole-3-acetic acid: A key regulator for plant growth, plant-microbe interactions, and agricultural adaptive resilience

Indole-3-acetic acid (IAA), a fundamental phytohormone categorized under auxins, not only influences plant growth and development but also plays a critical role in plant-microbe interactions. This study reviews the role of IAA in bacteria-plant communication, with a focus on its biosynthesis, regulation, and the subsequent effects on host plants. Bacteria synthesize IAA through multiple pathways, which include the indole-3-acetamide (IAM), indole-3-pyruvic acid (IPyA), and several other routes, whose full mechanisms remain to be fully elucidated. The production of bacterial IAA affects root architecture, nutrient uptake, and resistance to various abiotic stresses such as drought, salinity, and heavy metal toxicity, enhancing plant resilience and thus offering promising routes to sustainable agriculture. Bacterial IAA synthesis is regulated through complex gene networks responsive to environmental cues, impacting plant hormonal balances and symbiotic relationships. Pathogenic bacteria have adapted mechanisms to manipulate the host's IAA dynamics, influencing disease outcomes. On the other hand, beneficial bacteria utilize IAA to promote plant growth and mitigate abiotic stresses, thereby enhancing nutrient use efficiency and reducing dependency on chemical fertilizers. Advancements in analytical methods, such as liquid chromatography-tandem mass spectrometry, have improved the quantification of bacterial IAA, enabling accurate measurement and analysis. Future research focusing on molecular interactions between IAA-producing bacteria and host plants could facilitate the development of biotechnological applications that integrate beneficial bacteria to improve crop performance, which is essential for addressing the challenges posed by climate change and ensuring global food security. This integration of bacterial IAA producers into agricultural practice promises to revolutionize crop management strategies by enhancing growth, fostering resilience, and reducing environmental impact.

Genome-Wide Identification of the Paulownia fortunei Aux/IAA Gene Family and Its Response to Witches' Broom Caused by Phytoplasma

The typical symptom of Paulownia witches' broom (PaWB), caused by phytoplasma infection, is excessive branching, which is mainly triggered by auxin metabolism disorder. Aux/IAA is the early auxin-responsive gene that participates in regulating plant morphogenesis such as apical dominance, stem elongation, lateral branch development, and lateral root formation. However, no studies have investigated the response of the Aux/IAA gene family to phytoplasma infection in Paulownia fortunei. In this study, a total of 62 Aux/IAA genes were found in the genome. Phylogenetic analysis showed that PfAux/IAA genes could be divided into eight subgroups, which were formed by tandem duplication and fragment replication. Most of them had a simple gene structure, and several members lacked one or two conserved domains. By combining the expression of PfAux/IAA genes under phytoplasma stress and SA-treated phytoplasma-infected seedlings, we found that PfAux/IAA13/33/45 may play a vital role in the occurrence of PaWB. Functional analysis based on homologous relationships showed a strong correlation between PfAux/IAA45 and branching. Protein-protein interaction prediction showed that PfARF might be the binding partner of PfAux/IAA, and the yeast two-hybrid assay and bimolecular fluorescent complementary assay confirmed the interaction of PfAux/IAA45 and PfARF13. This study provides a theoretical basis for further understanding the function of the PfAux/IAA gene family and exploring the regulatory mechanism of branching symptoms caused by PaWB.

AefR, a TetR Family Transcriptional Repressor, Regulates Several Auxin Responses in Pseudomonas syringae Strain PtoDC3000

The plant hormone indole-3-acetic acid (IAA), also known as auxin, plays important roles in plant growth and development, as well as in several plant-microbe interactions. IAA also acts as a microbial signal and in many bacteria regulates metabolism, stress responses, and virulence. In the bacterial plant pathogen Pseudomonas syringae pv. tomato strain DC3000 (PtoDC3000), exposure to IAA results in large-scale transcriptional reprogramming, including the differential expression of several known virulence genes. However, how PtoDC3000 senses and responds to IAA and what aspects of its biology are regulated by IAA is not understood. To investigate the mechanisms involved in perceiving and responding to IAA, we carried out a genetic screen for mutants with altered responses to IAA. One group of mutants of particular interest carried disruptions in the aefR gene encoding a TetR family transcriptional regulator. Gene expression analysis confirmed that the aefR mutants have altered responses to IAA. Thus, AefR is the first demonstrated auxin response regulator in PtoDC3000. We also investigated several aspects of PtoDC3000 biology that are regulated by both AefR and IAA, including antibiotic resistance, motility, and virulence. The observation that the aefR mutant has altered virulence on Arabidopsis, suggests that the sector of the IAA response regulated by aefR is important during pathogenesis. Our findings also provide evidence that AefR plays a role in coordinating changes in gene expression during the transition from early to late stages of infection. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Salicylic and Jasmonic Acid Synergism during Black Knot Disease Progression in Plums

Black knot (BK) is a deadly disease of European (Prunus domestica) and Japanese (Prunus salicina) plums caused by the hemibiotrophic fungus Apiosporina morbosa. Generally, phytopathogens hamper the balance of primary defense phytohormones, such as salicylic acid (SA)-jasmonic acid (JA) balance, for disease progression. Thus, we quantified the important phytohormone titers in tissues of susceptible and resistant genotypes belonging to European and Japanese plums at five different time points. Our previous results suggested that auxin-cytokinins interplay driven by A. morbosa appeared to be vital in disease progression by hampering the plant defense system. Here, we further show that such hampering of disease progression is likely mediated by perturbance in SA, JA, and, to some extent, gibberellic acid. The results further indicate that SA and JA in plant defense are not always necessarily antagonistic as most of the studies suggest but can be different, especially in woody perennials. Together, our results suggest that the changes in phytohormone levels, especially in terms of SA and JA content due to BK infection and progression in plums, could be used as phytohormonal markers in the identification of BK-resistant cultivars.

Indole-3-acetic acid as a cross-talking molecule in algal-bacterial interactions and a potential driving force in algal bloom formation

Most signaling molecules are involved in inter-or intra-species communication, and signaling involving cross-kingdom cell-to-cell communication is limited. Howerver, algae and bacteria exchange nutrients and information in a range of interactions in marine environments. Multiple signaling molecules exist between algae and bacteria, including quorum-sensing molecules, nitric oxide, and volatile organic compounds. Recently, indole-3-acetic acid (IAA), an auxin hormone that is a well-studied signaling molecule in terrestrial ecosystems, was found to act as a cue in cross-kingdom communication between algae and bacteria in aquatic environments. To increase understanding of the roles of IAA in the phycosphere, the latest evidence regarding the ecological functions of IAA in cross-kingdom communication between algae and bacteria has been compiled in this review. The pathways of IAA biosynthesis, effects of IAA on algal growth & reproduction, and potential mechanisms at phenotypic and molecular levels are summarized. It is proposed that IAA is an important molecule regulating algal-bacterial interactions and acts as an invisible driving force in the formation of algal blooms.

Ill Communication: Host Metabolites as Virulence-Regulating Signals for Plant-Pathogenic Bacteria

Plant bacterial pathogens rely on host-derived signals to coordinate the deployment of virulence factors required for infection. In this review, I describe how diverse plant-pathogenic bacteria detect and respond to plant-derived metabolic signals for the purpose of virulence gene regulation. I highlight examples of how pathogens perceive host metabolites through membrane-localized receptors as well as intracellular response mechanisms. Furthermore, I describe how individual strains may coordinate their virulence using multiple distinct host metabolic signals, and how plant signals may positively or negatively regulate virulence responses. I also describe how plant defenses may interfere with the perception of host metabolites as a means to dampen pathogen virulence. The emerging picture is that recognition of host metabolic signals for the purpose of virulence gene regulation represents an important primary layer of interaction between pathogenic bacteria and host plants that shapes infection outcomes.

Catalases mediate tobacco resistance to virus infection through crosstalk between salicylic acid and auxin signaling pathways

Catalases (CATs) play important roles in plant growth, development and defense responses. Previous studies have shown that CATs exhibit different or even opposite effects on plant immunity in different plant-pathogen interactions, but little is known about the mechanisms. In this study, Nicotiana tabacum plants with overexpression or knockout of CAT genes, tobacco mosaic virus (TMV) and cucumber mosaic virus (CMV) were employed to investigate the role of CAT in compatible plant-virus interactions. The results showed that there were dynamic changes in the effect of CAT on N. tabacum defense responses. Overexpression of catalase 1 (CAT1) and catalase 3 (CAT3) improved N. tabacum resistance in the early stage of virus infection but depressed it during the late stages of pathogenesis, especially in CAT3 overexpressing plants. The lower level of electrolyte leakage, lower contents of malonaldehyde and hydrogen peroxide (H2 O2 ), higher activities of antioxidant enzymes and improved functions of photosystem II corresponded to the milder symptoms and higher resistance of infected tobacco plants. In addition, the infection of TMV and CMV resulted in expression changes of CATs in tobacco plants, and pretreatment with H2 O2 facilitated TMV and CMV infection. Further experiments showed that the content of salicylic acid (SA) and the expression of genes related to SA signaling pathway were positively correlated with plant resistance, whereas auxin and its related signaling pathway were related to the viral susceptibility of plants. Taken together, our results demonstrated that CAT1 and CAT3 mediated tobacco resistance to virus infection through crosstalk between SA and auxin signaling pathways.

Entomopathogenic Fungus-Related Priming Defense Mechanisms in Cucurbits Impact Spodoptera littoralis (Boisduval) Fitness

Entomopathogenic fungi (EPF) exhibit direct and indirect mechanisms to increase plant resistance against biotic and abiotic stresses. Plant responses to these stresses are interconnected by common regulators such as ethylene (ET), which is involved in both iron (Fe) deficiency and induced systemic resistance responses. In this work, the roots of cucurbit seedlings were primed with Metarhizium brunneum (EAMa 01/58-Su strain), and relative expression levels of 18 genes related to ethylene (ET), jasmonic acid (JA), and salicylic acid (SA) synthesis, as well as pathogen-related (PR) protein genes, were studied by reverse transcription-quantitative PCR (qRT-PCR). Effects of priming on Spodoptera littoralis were studied by feeding larvae for 15 days with primed and control plants. Genes showed upregulation in studied species; however, the highest relative expression was observed in roots and shoots of plants with Fe deficiency, demonstrating the complexity and the overlapping degree of the regulatory network. EIN2 and EIN3 should be highlighted; both are key genes of the ET transduction pathway that enhanced their expression levels up to eight and four times, respectively, in shoots of primed cucumber. Also, JA and SA synthesis and PR genes showed significant upregulation during the observation period (e.g., the JA gene LOX1 increased 506 times). Survival and fitness of S. littoralis were affected with significant effects on mortality of larvae fed on primed plants versus controls, length of the larval stage, pupal weight, and the percentage of abnormal pupae. These results highlight the role of the EAMa 01/58-Su strain in the induction of resistance, which could be translated into direct benefits for plant development. IMPORTANCE Entomopathogenic fungi are multipurpose microorganisms with direct and indirect effects on insect pests. Also, EPF provide multiple benefits to plants by solubilizing minerals and facilitating nutrient acquisition. A very interesting and novel effect of these fungi is the enhancement of plant defense systems by inducing systematic and acquired resistance. However, little is known about this function. This study sheds light on the molecular mechanisms involved in cucurbits plants' defense activation after being primed by the EPF M. brunneum. Furthermore, the subsequent effects on the fitness of the lepidopteran pest S. littoralis are shown. In this regard, a significant upregulation was recorded for the genes that regulate JA, SA, and ET pathways. This increased expression of defense genes caused lethal and sublethal effects on S. littoralis. This could be considered an added value for the implementation of EPF in integrated pest management programs.

Biosynthetic Pathways and Functions of Indole-3-Acetic Acid in Microorganisms

Indole-3-acetic acid (IAA) belongs to the family of auxin indole derivatives. IAA regulates almost all aspects of plant growth and development, and is one of the most important plant hormones. In microorganisms too, IAA plays an important role in growth, development, and even plant interaction. Therefore, mechanism studies on the biosynthesis and functions of IAA in microorganisms can promote the production and utilization of IAA in agriculture. This mini-review mainly summarizes the biosynthesis pathways that have been reported in microorganisms, including the indole-3-acetamide pathway, indole-3-pyruvate pathway, tryptamine pathway, indole-3-acetonitrile pathway, tryptophan side chain oxidase pathway, and non-tryptophan dependent pathway. Some pathways interact with each other through common key genes to constitute a network of IAA biosynthesis. In addition, functional studies of IAA in microorganisms, divided into three categories, have also been summarized: the effects on microorganisms, the virulence on plants, and the beneficial impacts on plants.

The changes in the maize root cell walls after exogenous application of auxin in the presence of cadmium

Cadmium (Cd) is a transition metal and hazardous pollutant that has many toxic effects on plants. This heavy metal poses a health risk for both humans and animals. The cell wall is the first structure of a plant cell that is in contact with Cd; therefore, it can change its composition and/or ratio of wall components accordingly. This paper investigates the changes in the anatomy and cell wall architecture of maize (Zea mays L.) roots grown for 10 days in the presence of auxin indole-3-butyric acid (IBA) and Cd. The application of IBA in the concentration 10-9 M delayed the development of apoplastic barriers, decreased the content of lignin in the cell wall, increased the content of Ca2+ and phenols, and influenced the composition of monosaccharides in polysaccharide fractions when compared to the Cd treatment. Application of IBA improved the Cd2+ fixation to the cell wall and increased the endogenous concentration of auxin depleted by Cd treatment. The proposed scheme from obtained results may explain the possible mechanisms of the exogenously applied IBA and its effects on the changes in the binding of Cd2+ within the cell wall, and on the stimulation of growth that resulted in the amelioration of Cd stress.

HexR Transcription Factor Contributes to Pseudomonas cannabina pv. alisalensis Virulence by Coordinating Type Three Secretion System Genes

Pseudomonas cannabina pv. alisalensis (Pcal) causes bacterial blight on cabbage. We previously conducted a screening for reduced virulence using Tn5 transposon mutants and identified one of the transcriptional factors, HexR, as a potential Pcal virulence factor. However, the role of HexR in plant pathogenic Pseudomonas virulence has not been investigated well. Here, we demonstrated that the Pcal hexR mutant showed reduced disease symptoms and bacterial populations on cabbage, indicating that HexR contributes to Pcal virulence. We used RNA-seq analysis to characterize the genes regulated by HexR. We found that several type three secretion system (T3SS)-related genes had lower expression of the Pcal hexR mutant. Five genes were related to T3SS machinery, two genes were related to type three helper proteins, and three genes encoded type three effectors (T3Es). We also confirmed that T3SS-related genes, including hrpL, avrPto, hopM1, and avrE1, were also down-regulated in the Pcal hexR mutant both in culture and in vivo by using RT-qPCR. T3SS functions to suppress plant defense in host plants and induce hypersensitive response (HR) cell death in non-host plants. Therefore, we investigated the expression profiles of cabbage defense-related genes, including PR1 and PR5, and found that the expressions of these genes were greater in the Pcal hexR mutant. We also demonstrated that the hexR mutant did not induce HR cell death in non-host plants, indicating that HexR contributes in causing HR in nonhost plants. Together, these results indicate that the mutation in hexR leads to a reduction in the T3SS-related gene expression and thus an impairment in plant defense suppression, reducing Pcal virulence.

The birth of a giant: evolutionary insights into the origin of auxin responses in plants

The plant signaling molecule auxin is present in multiple kingdoms of life. Since its discovery, a century of research has been focused on its action as a phytohormone. In land plants, auxin regulates growth and development through transcriptional and non-transcriptional programs. Some of the molecular mechanisms underlying these responses are well understood, mainly in Arabidopsis. Recently, the availability of genomic and transcriptomic data of green lineages, together with phylogenetic inference, has provided the basis to reconstruct the evolutionary history of some components involved in auxin biology. In this review, we follow the evolutionary trajectory that allowed auxin to become the "giant" of plant biology by focusing on bryophytes and streptophyte algae. We consider auxin biosynthesis, transport, physiological, and molecular responses, as well as evidence supporting the role of auxin as a chemical messenger for communication within ecosystems. Finally, we emphasize that functional validation of predicted orthologs will shed light on the conserved properties of auxin biology among streptophytes.

Genome Analysis of Two Pseudomonas syringae pv. aptata Strains with Different Virulence Capacity Isolated from Sugar Beet: Features of Successful Pathogenicity in the Phyllosphere Microbiome

Members of the Pseudomonas syringae species complex are heterogeneous bacteria that are the most abundant bacterial plant pathogens in the plant phyllosphere, with strong abilities to exist on and infect different plant hosts and survive in/outside agroecosystems. In this study, the draft genome sequences of two pathogenic P. syringae pv. aptata strains with different in planta virulence capacities isolated from the phyllosphere of infected sugar beet were analyzed to evaluate putative features of survival strategies and to determine the pathogenic potential of the strains. The draft genomes of P. syringae pv. aptata strains P16 and P21 are 5,974,057 bp and 6,353,752 bp in size, have GC contents of 59.03% and 58.77%, respectively, and contain 3,439 and 3,536 protein-coding sequences, respectively. For both average nucleotide identity and pangenome analysis, P16 and P21 largely clustered with other pv. aptata strains from the same isolation source. We found differences in the repertoire of effectors of the type III secretion system among all 102 selected strains, suggesting that the type III secretion system is a critical factor in the different virulent phenotypes of P. syringae pv. aptata. During genome analysis of the highly virulent strain P21, we discovered genes for T3SS effectors (AvrRpm1, HopAW1, and HopAU1) that were not previously found in genomes of P. syringae pv. aptata. We also identified coding sequences for pantothenate kinase, VapC endonuclease, phospholipase, and pectate lyase in both genomes, which may represent novel effectors of the type III secretion system. IMPORTANCE Genome analysis has an enormous effect on understanding the life strategies of plant pathogens. Comparing similarities with pathogens involved in other epidemics could elucidate the pathogen life cycle when a new outbreak happens. This study represents the first in-depth genome analysis of Pseudomonas syringae pv. aptata, the causative agent of leaf spot disease of sugar beet. Despite the increasing number of disease reports in recent years worldwide, there is still a lack of information about the genomic features, epidemiology, and pathogenic life strategies of this particular pathogen. Our findings provide advances in disease etiology (especially T3SS effector repertoire) and elucidate the role of environmental adaptations required for prevalence in the pathobiome of the sugar beet. From the perspective of the very heterogeneous P. syringae species complex, this type of analysis has specific importance in reporting the characteristics of individual strains.

TIR1/AFB proteins: Active players in abiotic and biotic stress signaling

The TIR1/AFB family of proteins is a group of functionally diverse auxin receptors that are only found in plants. TIR1/AFB family members are characterized by a conserved N-terminal F-box domain followed by 18 leucine-rich repeats. In the past few decades, extensive research has been conducted on the role of these proteins in regulating plant development, metabolism, and responses to abiotic and biotic stress. In this review, we focus on TIR1/AFB proteins that play crucial roles in plant responses to diverse abiotic and biotic stress. We highlight studies that have shed light on the mechanisms by which TIR1/AFB proteins are regulated at the transcriptional and post-transcriptional as well as the downstream in abiotic or biotic stress pathways regulated by the TIR1/AFB family.

A novel function of the key nitrogen-fixation activator NifA in beta-rhizobia: Repression of bacterial auxin synthesis during symbiosis

Rhizobia fix nitrogen within root nodules of host plants where nitrogenase expression is strictly controlled by its key regulator NifA. We recently discovered that in nodules infected by the beta-rhizobial strain Paraburkholderia phymatum STM815, NifA controls expression of two bacterial auxin synthesis genes. Both the iaaM and iaaH transcripts, as well as the metabolites indole-acetamide (IAM) and indole-3-acetic acid (IAA) showed increased abundance in nodules occupied by a nifA mutant compared to wild-type nodules. Here, we document the structural changes that a P. phymatum nifA mutant induces in common bean (Phaseolus vulgaris) nodules, eventually leading to hypernodulation. To investigate the role of the P. phymatum iaaMH genes during symbiosis, we monitored their expression in presence and absence of NifA over different stages of the symbiosis. The iaaMH genes were found to be under negative control of NifA in all symbiotic stages. While a P. phymatum iaaMH mutant produced the same number of nodules and nitrogenase activity as the wild-type strain, the nifA mutant produced more nodules than the wild-type that clustered into regularly-patterned root zones. Mutation of the iaaMH genes in a nifA mutant background reduced the presence of these nodule clusters on the root. We further show that the P. phymatum iaaMH genes are located in a region of the symbiotic plasmid with a significantly lower GC content and exhibit high similarity to two genes of the IAM pathway often used by bacterial phytopathogens to deploy IAA as a virulence factor. Overall, our data suggest that the increased abundance of rhizobial auxin in the non-fixing nifA mutant strain enables greater root infection rates and a role for bacterial auxin production in the control of early stage symbiotic interactions.

Inter-organismal phytohormone networks in plant-microbe interactions

Phytohormones are produced by plants and play central roles in interactions with pathogenic and beneficial microbes as well as plant growth and development. Each phytohormone pathway consists of its biosynthesis, transport, perception, and signaling and is intertwined with each other at various levels to form phytohormone networks in plants. Different kinds of microbes also produce phytohormones that exert physiological roles within microbes and manipulate phytohormone networks in plants by using phytohormones, their mimics, and proteinaceous effectors. In turn, plant-derived phytohormones can directly or indirectly through plant signaling networks affect microbial metabolism and community assembly. Therefore, phytohormone networks in plants and microbes are connected through plant and microbial phytohormones and other molecules to form inter-organismal phytohormone networks. In this review, we summarize recent progress on molecular mechanisms of inter-organismal phytohormone networks and discuss future steps necessary for advancing our understanding of phytohormone networks.

Probing into the unique relationship between a soil bacterium, Pseudomonas putida AKMP7 and Arabidopsis thaliana: A case of "conditional pathogenesis"

Plant growth-promoting rhizobacteria (PGPR) are beneficial soil bacteria that colonise the rhizosphere and help plants in growth, development, and stress tolerance. While there is a significant body of research elucidating their benefits to plants, studies on the "abnormal" or "unexpected" behavior of these bacteria are almost non-existent. One such study from our laboratory has previously reported a unique situation in which a certain strain of drought and thermo-tolerant PGPR, namely, Pseudomonas putida AKMP7, becomes pathogenic towards Arabidopsis thaliana under drought conditions, but not under normal (well-watered) conditions. In this study, we have probed deeper into this phenomenon of "conditional pathogenesis". We found that, AKMP7 imparts an enhancement in plant growth under well-watered conditions, while, causing a deterioration in plant health under drought conditions. In an attempt to understand the underlying reasons for this phenomenon, we analysed the phytohormones released by Pseudomonas putida AKMP7 using LC-ESI-MS/MS technique. We identified that AKMP7 releases zeatin (a cytokinin), the auxin derivative -indole acetamide and amino acid-conjugates of auxin (indole-3-acetyl-L-alanine, indole-3-acetyl-L-phenylalanine and indole-3-acetyl-L-aspartate) in the growth medium. By treating the plants with commercially obtained forms of these phytohormones, individually or in combination with AKMP7, we identified that zeatin and auxin derivative indole acetamide can play a crucial role in the conditional pathogenesis exhibited by this bacterium on A. thaliana under drought conditions. Our work lays a foundation for further understanding the precise molecular mechanisms involved in this unique phenomenon of conditional/opportunistic pathogenesis.

Distinct phenotypic behaviours within a clonal population of Pseudomonas syringae pv. actinidiae

Bacterial canker of the kiwifruit caused by the etiological agent Pseudomonas syringae pv. actinidiae is the most severe disease in kiwifruit production. Since 2008 a hypervirulent Psa biovar 3 has spread rapidly worldwide. Different genomic and phenotypic approaches have been used to understand the origin of the dissemination and geographical evolution of populations associated with this pandemic. This study aimed to characterize the genetic and phenotypic diversity of 22 Psa isolates collected in different regions of Portugal between 2013 and 2017. Genotypic and phenotypic characterization was based on Multi-Locus Sequence Analysis (MLSA), motility, IAA production, Biolog GEN III, and copper sensitivity. No polymorphisms were detected for the concatenated sequence (1950 bp) of the housekeeping genes gltA, gapA, gyrB, and rpoD. Results support the analysed Portuguese Psa isolates (2013–2017) belonging to Psa3, and MLSA indicates high genetic clonality and stability of these populations. The phenotypic analysis through Biolog revealed a heterogeneous pattern in the Psa collection and its position in the Pseudomonas complex. This heterogeneity reflects a genomic diversity that may reflect distinct adaptive trends associated with the environmental conditions and widespread. The Portuguese Psa collection showed no resistance to copper. This information is relevant to kiwi producers that predominantly use Cu-treatments to control kiwifruit bacterial canker.

Identification and Characterization of Auxin/IAA Biosynthesis Pathway in the Rice Blast Fungus Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae has been known to produce the phytohormone auxin/IAA from its hyphae and conidia, but the detailed biological function and biosynthesis pathway is largely unknown. By sequence homology, we identified a complete indole-3-pyruvic acid (IPA)-based IAA biosynthesis pathway in M. oryzae, consisting of the tryptophan aminotransferase (MoTam1) and the indole-3-pyruvate decarboxylase (MoIpd1). In comparison to the wild type, IAA production was significantly reduced in the motam1Δ mutant, and further reduced in the moipd1Δ mutant. Correspondingly, mycelial growth, conidiation, and pathogenicity were defective in the motam1Δ and the moipd1Δ mutants to various degrees. Targeted metabolomics analysis further confirmed the presence of a functional IPA pathway, catalyzed by MoIpd1, which contributes to IAA/auxin production in M. oryzae. Furthermore, the well-established IAA biosynthesis inhibitor, yucasin, suppressed mycelial growth, conidiation, and pathogenicity in M. oryzae. Overall, this study identified an IPA-dependent IAA synthesis pathway crucial for M. oryzae mycelial growth and pathogenic development.

Regulation of plant biotic interactions and abiotic stress responses by inositol polyphosphates

Inositol pyrophosphates (PP-InsPs), derivatives of inositol hexakisphosphate (phytic acid, InsP₆) or lower inositol polyphosphates, are energy-rich signaling molecules that have critical regulatory functions in eukaryotes. In plants, the biosynthesis and the cellular targets of these messengers are not fully understood. This is because, in part, plants do not possess canonical InsP₆ kinases and are able to synthesize PP-InsP isomers that appear to be absent in yeast or mammalian cells. This review will shed light on recent discoveries in the biosynthesis of these enigmatic messengers and on how they regulate important physiological processes in response to abiotic and biotic stresses in plants.

Exposure of Brassica to Red Light Antagonizes Low Production of IAA in Leaf Through Root Signaling Under Stress Conditions

Plant leaf is highly sensitive to various growth promoting and restraining components. This sensitivity is normally caused by the alteration of different phyto-hormones (predominately by IAA), when the plants exposed to certain environmental conditions. We exposed the hydroponically grown Brassica campestris seedlings (7 days old) to red and green light in order to observe its effect on IAA secretion at leaf. The evaluated data showed that red light antagonized the low production of IAA in leaf by initiating the root signaling through flavonoids production and high redox activity. The study also explored the link between the differential phytohormonal response and biotic or abiotic stress elimination in leaf through root signaling under green or red light. The results exhibited that the biotic (P. syringae or F. alni) or abiotic stresses (100 mM AgNO₃ or 100 mM tert-butyl alcohol) inhibited flavonoids at the roots and resisted the restoration of IAA at the leaf. However, under green light where IAA was not inhibited, the stresses could not produce flavonoid at the root and further passing the signals to leaf. The results concluded that the growth and photosynthetic rates of the seedlings were improved under red light exposure through flavonoid inducing stresses.

Manipulation of auxin signalling by plant viruses

Compatible plant-virus interactions result in dramatic changes of the plant transcriptome and morphogenesis, and are often associated with rapid alterations in plant hormone homeostasis and signalling. Auxin controls many aspects of plant organogenesis, development, and growth; therefore, plants can rapidly perceive and respond to changes in the cellular auxin levels. Auxin signalling is a tightly controlled process and, hence, is highly vulnerable to changes in the mRNA and protein levels of its components. There are several core nuclear components of auxin signalling. In the nucleus, the interaction of auxin response factors (ARFs) and auxin/indole acetic acid (Aux/IAA) proteins is essential for the control of auxin-regulated pathways. Aux/IAA proteins are negative regulators, whereas ARFs are positive regulators of the auxin response. The interplay between both is essential for the transcriptional regulation of auxin-responsive genes, which primarily regulate developmental processes but also modulate the plant immune system. Recent studies suggest that plant viruses belonging to different families have developed various strategies to disrupt auxin signalling, namely by (a) changing the subcellular localization of Aux/IAAs, (b) preventing degradation of Aux/IAAs by stabilization, or (c) inhibiting the transcriptional activity of ARFs. These interactions perturb auxin signalling and experimental evidence from various studies highlights their importance for virus replication, systemic movement, interaction with vectors for efficient transmission, and symptom development. In this microreview, we summarize and discuss the current knowledge on the interaction of plant viruses with auxin signalling components of their hosts.

Identification of IAA-regulated genes in Pseudomonas syringae pv. tomato strain DC3000

The auxin indole-3-acetic acid (IAA) is a plant hormone that not only regulates plant growth and development but also plays important roles in plant-microbe interactions. We previously reported that IAA alters expression of several virulence-related genes in the plant pathogen Pseudomonas syringae pv. tomato strain DC3000 (PtoDC3000). To learn more about the impact of IAA on regulation of PtoDC3000 gene expression we performed a global transcriptomic analysis of bacteria grown in culture, in the presence or absence of exogenous IAA. We observed that IAA repressed expression of genes involved in the Type III secretion (T3S) system and motility and promoted expression of several known and putative transcriptional regulators. Several of these regulators are orthologs of factors known to regulate stress responses and accordingly expression of several stress response-related genes was also upregulated by IAA. Similar trends in expression for several genes were also observed by RT-qPCR. Using an Arabidopsis thaliana auxin receptor mutant that accumulates elevated auxin, we found that many of the P. syringae genes regulated by IAA in vitro were also regulated by auxin in planta. Collectively the data indicate that IAA modulates many aspects of PtoDC3000 biology, presumably to promote both virulence and survival under stressful conditions, including those encountered in or on plant leaves. IMPORTANCE Indole-3-acetic acid (IAA), a form of the plant hormone auxin, is used by many plant-associated bacteria as a cue to sense the plant environment. Previously, we showed that IAA can promote disease in interactions between the plant pathogen Pseudomonas syringae strain PtoDC000 and one of its hosts, Arabidopsis thaliana. However, the mechanisms by which IAA impacts the biology of PtoDC3000 and promotes disease are not well understood. Here we demonstrate that IAA is a signal molecule that regulates gene expression in PtoDC3000. The presence of exogenous IAA affects expression of over 700 genes in the bacteria, including genes involved in Type III secretion and genes involved in stress response. This work offers insight into the roles of auxin promoting pathogenesis.

Plant immune system activation is necessary for efficient root colonization by auxin-secreting beneficial bacteria

Although plant roots encounter a plethora of microorganisms in the surrounding soil, at the rhizosphere, plants exert selective forces on their bacterial colonizers. Unlike immune recognition of pathogenic bacteria, the mechanisms by which beneficial bacteria are selected and how they interact with the plant immune system are not well understood. To better understand this process, we studied the interaction of auxin-producing Bacillus velezensis FZB42 with Arabidopsis roots and found that activation of the plant immune system is necessary for efficient bacterial colonization and auxin secretion. A feedback loop is established in which bacterial colonization triggers an immune reaction and production of reactive oxygen species, which, in turn, stimulate auxin production by the bacteria. Auxin promotes bacterial survival and efficient root colonization, allowing the bacteria to inhibit fungal infection and promote plant health. Thus, a feedback loop between bacteria and the plant immune system promotes the fitness of both partners.

Development of potent promoters that drive the efficient expression of genes in apple protoplasts

Protoplast transient expression is a powerful strategy for gene functional characterization, especially in biochemical mechanism studies. We herein developed a highly efficient transient expression system for apple protoplasts. The abilities of the Arabidopsis thaliana and Malus domestica ubiquitin-10 (AtUBQ10 and MdUBQ10) promoters to drive the expression of multiple genes were compared with that of the CaMV 35S promoter, and the results revealed that the AtUBQ10 and MdUBQ10 promoters were more efficient in apple protoplasts. With this system, we demonstrated that active AtMKK7ac could activate MAPK6/3/4 signaling cascades, which further regulated MdWRKY33 phosphorylation and stability in apple. Furthermore, the ligand-induced interaction between the immune receptor AtFLS2 and the coreceptor AtBAK1 was reconstituted in apple protoplasts. We also found that the stability of the bacterial effector AvrRpt2 was regulated by feedback involving auxin and the immune regulator RIN4. The system established herein will serve as a useful tool for the molecular and biochemical analyses of apple genes.

Auxin Plays Multiple Roles during Plant-Pathogen Interactions

The plant hormone auxin governs many aspects of normal plant growth and development. Auxin also plays an important role in plant-microbe interactions, including interactions between plant hosts and pathogenic microorganisms that cause disease. It is now well established that indole-3-acetic acid (IAA), the most well-studied form of auxin, promotes disease in many plant-pathogen interactions. Recent studies have shown that IAA can act both as a plant hormone that modulates host signaling and physiology to increase host susceptibility and as a microbial signal that directly impacts the pathogen to promote virulence, but large gaps in our understanding remain. In this article, we review recent studies on the roles that auxin plays during plant-pathogen interactions and discuss the virulence mechanisms that many plant pathogens have evolved to manipulate host auxin signaling and promote pathogenesis.

Plant susceptible responses: the underestimated side of plant-pathogen interactions

Plant susceptibility to pathogens is usually considered from the perspective of the loss of resistance. However, susceptibility cannot be equated with plant passivity since active host cooperation may be required for the pathogen to propagate and cause disease. This cooperation consists of the induction of reactions called susceptible responses that transform a plant from an autonomous biological unit into a component of a pathosystem. Induced susceptibility is scarcely discussed in the literature (at least compared to induced resistance) although this phenomenon has a fundamental impact on plant-pathogen interactions and disease progression. This review aims to summarize current knowledge on plant susceptible responses and their regulation. We highlight two main categories of susceptible responses according to their consequences and indicate the relevance of susceptible response-related studies to agricultural practice. We hope that this review will generate interest in this underestimated aspect of plant-pathogen interactions.

Double agent indole-3-acetic acid (IAA): Mechanistic analysis of indole-3-acetaldehyde dehydrogenase AldA that synthesizes IAA, an auxin that aids bacterial virulence

The large diversity of organisms inhabiting various environmental niches on our planet are engaged in a lively exchange of biomolecules, including nutrients, hormones, and vitamins. In a quest to survive, organisms that we define as pathogens employ innovative methods to extract valuable resources from their host leading to an infection. One such instance is where plant-associated bacterial pathogens synthesize and deploy hormones or their molecular mimics to manipulate the physiology of the host plant. This commentary describes one such specific example—the mechanism of the enzyme AldA, an aldehyde dehydrogenase (ALDH) from the bacterial plant pathogen Pseudomonas syringae which produces the plant auxin hormone indole-3-acetic acid (IAA) by oxidizing the substrate indole-3-acetaldehyde (IAAld) using the cofactor nicotinamide adenine dinucleotide (NAD⁺) (Bioscience Reports (2020) 40(12), https://doi.org/10.1042/BSR20202959). Using mutagenesis, enzyme kinetics, and structural analysis, Zhang et al. established that the progress of the reaction hinges on the formation of two distinct conformations of NAD(H) during the reaction course. Additionally, a key mutation in the AldA active site ‘aromatic box’ changes the enzyme’s preference for an aromatic substrate to an aliphatic one. Our commentary concludes that such molecular level investigations help to establish the nature of the dynamics of NAD(H) in ALDH-catalyzed reactions, and further show that the key active site residues control substrate specificity. We also contemplate that insights from the present study can be used to engineer novel ALDH enzymes for environmental, health, and industrial applications.

Regulation of the Pseudomonas syringae Type III Secretion System by Host Environment Signals

Pseudomonas syringae are Gram-negative, plant pathogenic bacteria that use a type III secretion system (T3SS) to disarm host immune responses and promote bacterial growth within plant tissues. Despite the critical role for type III secretion in promoting virulence, T3SS-encoding genes are not constitutively expressed by P. syringae and must instead be induced during infection. While it has been known for many years that culturing P. syringae in synthetic minimal media can induce the T3SS, relatively little is known about host signals that regulate the deployment of the T3SS during infection. The recent identification of specific plant-derived amino acids and organic acids that induce T3SS-inducing genes in P. syringae has provided new insights into host sensing mechanisms. This review summarizes current knowledge of the regulatory machinery governing T3SS deployment in P. syringae, including master regulators HrpRS and HrpL encoded within the T3SS pathogenicity island, and the environmental factors that modulate the abundance and/or activity of these key regulators. We highlight putative receptors and regulatory networks involved in linking the perception of host signals to the regulation of the core HrpRS–HrpL pathway. Positive and negative regulation of T3SS deployment is also discussed within the context of P. syringae infection, where contributions from distinct host signals and regulatory networks likely enable the fine-tuning of T3SS deployment within host tissues. Last, we propose future research directions necessary to construct a comprehensive model that (a) links the perception of host metabolite signals to T3SS deployment and (b) places these host–pathogen signaling events in the overall context of P. syringae infection.

Investigating the reaction and substrate preference of indole-3-acetaldehyde dehydrogenase from the plant pathogen Pseudomonas syringae PtoDC3000

Aldehyde dehydrogenases (ALDHs) catalyze the conversion of various aliphatic and aromatic aldehydes into corresponding carboxylic acids. Traditionally considered as housekeeping enzymes, new biochemical roles are being identified for members of ALDH family. Recent work showed that AldA from the plant pathogen Pseudomonas syringae strain PtoDC3000 (PtoDC3000) functions as an indole-3-acetaldehyde dehydrogenase for the synthesis of indole-3-acetic acid (IAA). IAA produced by AldA allows the pathogen to suppress salicylic acid-mediated defenses in the model plant Arabidopsis thaliana. Here we present a biochemical and structural analysis of the AldA indole-3-acetaldehyde dehydrogenase from PtoDC3000. Site-directed mutants targeting the catalytic residues Cys302 and Glu267 resulted in a loss of enzymatic activity. The X-ray crystal structure of the catalytically inactive AldA C302A mutant in complex with IAA and NAD+ showed the cofactor adopting a conformation that differs from the previously reported structure of AldA. These structures suggest that NAD+ undergoes a conformational change during the AldA reaction mechanism similar to that reported for human ALDH. Site-directed mutagenesis of the IAA binding site indicates that changes in the active site surface reduces AldA activity; however, substitution of Phe169 with a tryptophan altered the substrate selectivity of the mutant to prefer octanal. The present study highlights the inherent biochemical versatility of members of the ALDH enzyme superfamily in P. syringae.

Water Dipping of Auxin Coated Chrysanthemum Cuttings Confers Protection against Insect Herbivores

Simple Summary

Cultivated chrysanthemums are one of the most economically important ornamental greenhouse crops worldwide. Classical breeding programs have mainly focused on improving aesthetic characteristics to meet the continuous increasing customer demands for new flower varieties. Consequently, commercial cultivars often lack insect resistance traits. Among the most important production constraints are biotic foes, in particular thrips and leaf miner infestations form a prominent hazard during its vegetative state. To maintain the desired aesthetic characteristics, clonal commercial propagation is aided by the use of auxin hormones for root promotion. This study aims to evaluate the potential of root promoting auxins in antiherbivore defenses. We demonstrate that water dipping of unrooted basal cut ends, coated with the commercial rooting hormone indole-3-butyric acid (IBA), conferred protection in chrysanthemum against thrips and leaf miner. Our findings add an interesting twist to the traditional role of auxins. We advocate a new twist of auxins beyond its traditional role in rooting in order to maximize plant yield by reducing herbivory through feasible, cost-effective water dipping treatments.

Abstract

Auxins are commonly used for commercial propagation of chrysanthemums by stem cuttings. Recent studies imply that these root-promoting hormones also affect plant defense responses. The underlying motive of this study stems from the serendipitous observation that water dipping of auxin-coated cuttings beneficially affected thrips herbivory. Therefore, the primary objective of this investigation was to explore the role of indole-3-butyric acid (IBA) in relation to herbivore susceptibility in chrysanthemum. We observed contrasting findings concerning the physical presence of IBA and it’s role in promoting susceptibility of cuttings to thrips, which may in part be explained by the phenotypical variations of cuttings generated from mother plants. Nonetheless, we repeatedly demonstrated considerable protection, in some experiments up to 37%, against thrips and leaf miner upon water dipping of IBA-coated cuttings. Assessment of polyphenol oxidase activity (PPO), 14 days after dipping treatment, suggests that neither direct induction nor priming of plant defenses are involved. Future experiments aimed at understanding the early signaling events may help to explain the underlying mechanisms involved in conferring herbivore protection. We propose a dual role for auxins in early integrated pest management strategies to maximize plant development and minimize herbivory through feasible, cost-effective water dipping treatments.

The plant pathogen enzyme AldC is a long-chain aliphatic aldehyde dehydrogenase

Aldehyde dehydrogenases are versatile enzymes that serve a range of biochemical functions. Although traditionally considered metabolic housekeeping enzymes because of their ability to detoxify reactive aldehydes, like those generated from lipid peroxidation damage, the contributions of these enzymes to other biological processes are widespread. For example, the plant pathogen Pseudomonas syringae strain PtoDC3000 uses an indole-3-acetaldehyde dehydrogenase to synthesize the phytohormone indole-3-acetic acid to elude host responses. Here we investigate the biochemical function of AldC from PtoDC3000. Analysis of the substrate profile of AldC suggests that this enzyme functions as a long-chain aliphatic aldehyde dehydrogenase. The 2.5 Å resolution X-ray crystal of the AldC C291A mutant in a dead-end complex with octanal and NAD⁺ reveals an apolar binding site primed for aliphatic aldehyde substrate recognition. Functional characterization of site-directed mutants targeting the substrate- and NAD(H)-binding sites identifies key residues in the active site for ligand interactions, including those in the "aromatic box" that define the aldehyde-binding site. Overall, this study provides molecular insight for understanding the evolution of the prokaryotic aldehyde dehydrogenase superfamily and their diversity of function.