The biochemical properties of the two Arabidopsis thaliana isochorismate synthases

Defense strategies and associated phytohormonal regulation in Brassica plants in response to chewing and sap-sucking insects

Plants have evolved distinct defense strategies in response to a diverse range of chewing and sucking insect herbivory. While chewing insect herbivores, exemplified by caterpillars and beetles, cause visible tissue damage and induce jasmonic acid (JA)-mediated defense responses, sucking insects, such as aphids and whiteflies, delicately tap into the phloem sap and elicit salicylic acid (SA)-mediated defense responses. This review aims to highlight the specificity of defense strategies in Brassica plants and associated underlying molecular mechanisms when challenged by herbivorous insects from different feeding guilds (i.e., chewing and sucking insects). To establish such an understanding in Brassica plants, the typical defense responses were categorized into physical, chemical, and metabolic adjustments. Further, the impact of contrasting feeding patterns on Brassica is discussed in context to unique biochemical and molecular modus operandi that governs the resistance against chewing and sucking insect pests. Grasping these interactions is crucial to developing innovative and targeted pest management approaches to ensure ecosystem sustainability and Brassica productivity.

Deficiency of the Arabidopsis mismatch repair MSH6 attenuates Pseudomonas syringae invasion

The MutS homolog 6 (MSH6) is a nuclear DNA mismatch repair (MMR) gene that encodes the MSH6 protein. MSH6 interacts with MSH2 to form the MutSα heterodimer. MutSα corrects DNA mismatches and unpaired nucleotides arising during DNA replication, deamination of 5-methylcytosine, and recombination between non-identical DNA sequences. In addition to correcting DNA biosynthetic errors, MutSα also recognizes chemically damaged DNA bases. Here, we show that inactivation of MSH6 affects the basal susceptibility of Arabidopsis thaliana to Pseudomonas syringae pv tomato DC3000. The msh6 T-DNA insertional mutant exhibited a reduced susceptibility to the bacterial invasion. This heightened basal resistance of msh6 mutants appears to be dependent on an increased stomatal closure, an accumulation of H₂O₂ and double-strand breaks (DSBs) and a constitutive expression of pathogenesis-related (NPR1 and PR1) and DNA damage response (RAD51D and SOG1) genes. Complementation of this mutant with the MSH6 wild type allele under the control of its own promoter resulted in reversal of the basal bacterial resistance phenotype and the stomatal closure back to wild type levels. Taken together, these results demonstrate that inactivation of MSH6 increases Arabidopsis basal susceptibility to the bacterial pathogen and suggests a link between DNA repair and stress signaling in plants.

Plant environmental sensing relies on specialized plastids

In plants, plastids are thought to interconvert to various forms that are specialized for photosynthesis, starch and oil storage, and diverse pigment accumulation. Post-endosymbiotic evolution has led to adaptations and specializations within plastid populations that align organellar functions with different cellular properties in primary and secondary metabolism, plant growth, organ development, and environmental sensing. Here, we review the plastid biology literature in light of recent reports supporting a class of 'sensory plastids' that are specialized for stress sensing and signaling. Abundant literature indicates that epidermal and vascular parenchyma plastids display shared features of dynamic morphology, proteome composition, and plastid-nuclear interaction that facilitate environmental sensing and signaling. These findings have the potential to reshape our understanding of plastid functional diversification.

The Cassava NBS-LRR Genes Confer Resistance to Cassava Bacterial Blight

Cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis (Xam) seriously affects cassava yield. Genes encoding nucleotide-binding site (NBS) and leucine-rich repeat (LRR) domains are among the most important disease resistance genes in plants that are specifically involved in the response to diverse pathogens. However, the in vivo roles of NBS-LRR remain unclear in cassava (Manihot esculenta). In this study, we isolated four MeLRR genes and assessed their expression under salicylic acid (SA) treatment and Xam inoculation. Four MeLRR genes positively regulate cassava disease general resistance against Xam via virus-induced gene silencing (VIGS) and transient overexpression. During cassava-Xam interaction, MeLRRs positively regulated endogenous SA and reactive oxygen species (ROS) accumulation and pathogenesis-related gene 1 (PR1) transcripts. Additionally, we revealed that MeLRRs positively regulated disease resistance in Arabidopsis. These pathogenic microorganisms include Pseudomonas syringae pv. tomato, Alternaria brassicicola, and Botrytis cinerea. Our findings shed light on the molecular mechanism underlying the regulation of cassava resistance against Xam inoculation.

A Secreted Chorismate Mutase from Xanthomonas arboricola pv. juglandis Attenuates Virulence and Walnut Blight Symptoms

Walnut blight is a significant above-ground disease of walnuts caused by Xanthomonas arboricola pv. juglandis (Xaj). The secreted form of chorismate mutase (CM), a key enzyme of the shikimate pathway regulating plant immunity, is highly conserved between plant-associated beta and gamma proteobacteria including phytopathogens belonging to the Xanthomonadaceae family. To define its role in walnut blight disease, a dysfunctional mutant of chorismate mutase was created in a copper resistant strain Xaj417 (XajCM). Infections of immature walnut Juglans regia (Jr) fruit with XajCM were hypervirulent compared with infections with the wildtype Xaj417 strain. The in vitro growth rate, size and cellular morphology were similar between the wild-type and XajCM mutant strains, however the quantification of bacterial cells by dPCR within walnut hull tissues showed a 27% increase in XajCM seven days post-infection. To define the mechanism of hypervirulence, proteome analysis was conducted to compare walnut hull tissues inoculated with the wild type to those inoculated with the XajCM mutant strain. Proteome analysis revealed 3296 Jr proteins (five decreased and ten increased with FDR ≤ 0.05) and 676 Xaj417 proteins (235 increased in XajCM with FDR ≤ 0.05). Interestingly, the most abundant protein in Xaj was a polygalacturonase, while in Jr it was a polygalacturonase inhibitor. These results suggest that this secreted chorismate mutase may be an important virulence suppressor gene that regulates Xaj417 virulence response, allowing for improved bacterial survival in the plant tissues.

Metabolic regulation of systemic acquired resistance

Plants achieve an optimal balance between growth and defense by a fine-tuned biosynthesis and metabolic inactivation of immune-stimulating small molecules. Recent research illustrates that three common hubs are involved in the cooperative regulation of systemic acquired resistance (SAR) by the defense hormones N-hydroxypipecolic acid (NHP) and salicylic acid (SA). First, a common set of regulatory proteins is involved in their biosynthesis. Second, NHP and SA are glucosylated by the same glycosyltransferase, UGT76B1, and thereby inactivated in concert. And third, NHP confers immunity via the SA receptor NPR1 to reprogram plants at the level of transcription and primes plants for an enhanced defense capacity. An overview of SA and NHP metabolism is provided, and their contribution to long-distance signaling in SAR is discussed.

A new take on organelle-mediated stress sensing in plants

Plants are able to adjust phenotype in response to changes in the environment. This system depends on an internal capacity to sense environmental conditions and to process this information to plant response. Recent studies have pointed to mitochondria and plastids as important environmental sensors, capable of perceiving stressful conditions and triggering gene expression, epigenomic, metabolic and phytohormone changes in the plant. These processes involve integrated gene networks that ultimately modulate the energy balance between growth and plant defense. This review attempts to link several unusual recent findings into a comprehensive hypothesis for the regulation of plant phenotypic plasticity.

An update on salicylic acid biosynthesis, its induction and potential exploitation by plant viruses

Salicylic acid (SA) is a plant hormone essential for effective resistance to viral and non-viral pathogens. SA biosynthesis increases rapidly in resistant hosts when a dominant host resistance gene product recognizes a pathogen. SA stimulates resistance to viral replication, intercellular spread and systemic movement. However, certain viruses stimulate SA biosynthesis in susceptible hosts. This paradoxical effect limits virus titer and prevents excessive host damage, suggesting that these viruses exploit SA-induced resistance to optimize their accumulation. Recent work showed that SA production in plants does not simply recapitulate bacterial SA biosynthetic mechanisms, and that the relative contributions of the shikimate and phenylpropanoid pathways to the SA pool differ markedly between plant species.

Inositol hexakisphosphate biosynthesis underpins PAMP-triggered immunity to Pseudomonas syringae pv. tomato in Arabidopsis thaliana but is dispensable for establishment of systemic acquired resistance

Phytic acid (inositol hexakisphosphate, InsP₆ ) is an important phosphate store and signal molecule necessary for maintenance of basal resistance to plant pathogens. Arabidopsis thaliana ('arabidopsis') has three genes encoding myo-inositol phosphate synthases (IPS1-3), the enzymes that catalyse conversion of glucose-6-phosphate to InsP, the first step in InsP₆ biosynthesis. There is one gene for inositol-(1,3,4,5,6)-pentakisphosphate 2-kinase (IPK1), which catalyses the final step. Previously, we showed that mutation of IPS2 and IPK1 but not IPS1 increased susceptibility to pathogens. Our aim was to better understand the InsP₆ biosynthesis pathway in plant defence. Here we found that the susceptibility of arabidopsis (Col-0) to virulent and avirulent Pseudomonas syringae pv. tomato was also increased in ips3 and ips2/3 double mutants. Also, ipk1 plants had compromised expression of local acquired resistance induced by treatment with the pathogen-derived molecular pattern (PAMP) molecule flg22, but were unaffected in other responses to flg22, including Ca²⁺ influx and the oxidative burst, seedling root growth inhibition, and transcriptional up-regulation of the PAMP-triggered genes MITOGEN-ACTIVATED PROTEIN KINASE (MPK) 3, MPK11, CINNAMYL ALCOHOL DEHYDROGENASE 5, and FLG22-INDUCED RECEPTOR-LIKE KINASE 1. IPK1 mutation did not prevent the induction of systemic acquired resistance by avirulent P. syringae. Also, ips2 and ips2/3 double mutant plants, like ipk1, were hypersusceptible to P. syringae but were not compromised in flg22-induced local acquired resistance. The results support the role of InsP₆ biosynthesis enzymes in effective basal resistance and indicate that there is more than one basal resistance mechanism dependent upon InsP₆ biosynthesis.

N-hydroxypipecolic acid and salicylic acid: a metabolic duo for systemic acquired resistance

Recent research has established that the pipecolate pathway, a three-step biochemical sequence from l-lysine to N-hydroxypipecolic acid (NHP), is central for plant systemic acquired resistance (SAR). NHP orchestrates SAR establishment in concert with the immune signal salicylic acid (SA). Here, we outline the biochemistry of NHP formation from l-Lys and address novel progress on SA biosynthesis in Arabidopsis and other plant species. In Arabidopsis, the pathogen-inducible pipecolate and salicylate pathways are activated by common and distinct regulatory elements and mutual interactions between both metabolic branches exist. The mode of action of NHP in SAR involves direct induction of SAR gene expression, signal amplification, priming for enhanced defense activation and positive interplay with SA signaling to ensure elevated plant immunity.

Plant defense signals: Players and pawns in plant-virus-vector interactions

Plant viruses face an array of host defenses. Well-studied responses that protect against viruses include effector-triggered immunity, induced resistance (such as systemic acquired resistance mediated by salicylic acid), and RNA silencing. Recent work shows that viruses are also affected by non-host resistance mechanisms; previously thought to affect only bacteria, oomycetes and fungi. However, an enduring puzzle is how viruses are inhibited by several inducible host resistance mechanisms. Many viruses have been shown to encode factors that inhibit antiviral silencing. A number of these, including the cucumoviral 2b protein, the poytviral P1/HC-Pro and, respectively, geminivirus or satellite DNA-encoded proteins such as the C2 or βC1, also inhibit defensive signaling mediated by salicylic acid and jasmonic acid. This helps to explain how viruses can, in some cases, overcome host resistance. Additionally, interference with defensive signaling provides a means for viruses to manipulate plant-insect interactions. This is important because insects, particularly aphids and whiteflies, transmit many viruses. Indeed, there is now substantial evidence that viruses can enhance their own transmission through their effects on hosts. Even more surprisingly, it appears that viruses may be able to manipulate plant interactions with beneficial insects by, for example, 'paying back' their hosts by attracting pollinators.

An E. coli biosensor for screening of cDNA libraries for isochorismate pyruvate lyase-encoding cDNAs

Salicylic acid (SA) is an essential hormone for development and induced defense against biotrophic pathogens in plants. The formation of SA mainly derives from chorismate via demonstrated isochorismate synthase (ICS) and presumed isochorismate pyruvate lyase (IPL)-mediated steps in Arabidopsis thaliana, but so far no plant enzyme displaying IPL activity has been identified. Here, we developed an E. coli SA biosensor to screen for IPL activity based on the SalR regulator/salA promoter combination from Acinetobacter sp ADP1, to control the expression of the reporter luxCDABE. The biosensor was responsive to micromolar concentrations of exogenous SA, and to endogenous SA produced after transformation with a plasmid permitting IPTG-inducible expression of bacterial IPL in this biosensor strain. After screening a cDNA library constructed from turnip crinkle virus (TCV)-infected Arabidopsis ecotype Di-17, we identified an enzyme, PRXR1, as a putative IPL that converts isochorismate into SA. Our results provide a new experimental approach to identify IPL and new insights into the SA biosynthesis pathway in Arabidopsis.

Systemic Acquired Resistance and Salicylic Acid: Past, Present, and Future

This article is part of the Distinguished Review Article Series in Conceptual and Methodological Breakthroughs in Molecular Plant-Microbe Interactions. Salicylic acid (SA) is a critical plant hormone that regulates numerous aspects of plant growth and development as well as the activation of defenses against biotic and abiotic stress. Here, we present a historical overview of the progress that has been made to date in elucidating the role of SA in signaling plant immune responses. The ability of plants to develop acquired immunity after pathogen infection was first proposed in 1933. However, most of our knowledge about plant immune signaling was generated over the last three decades, following the discovery that SA is an endogenous defense signal. During this timeframe, researchers have identified i) two pathways through which SA can be synthesized, ii) numerous proteins that regulate SA synthesis and metabolism, and iii) some of the signaling components that function downstream of SA, including a large number of SA targets or receptors. In addition, it has become increasingly evident that SA does not signal immune responses by itself but, rather, as part of an intricate network that involves many other plant hormones. Future efforts to develop a comprehensive understanding of SA-mediated immune signaling will therefore need to close knowledge gaps that exist within the SA pathway itself as well as clarify how crosstalk among the different hormone signaling pathways leads to an immune response that is both robust and optimized for maximal efficacy, depending on the identity of the attacking pathogen.

Genetic Analysis of Flooding Tolerance in an Andean Diversity Panel of Dry Bean (Phaseolus vulgaris L.)

Climate change models predict temporal and spatial shifts in precipitation resulting in more frequent incidents of flooding, particularly in regions with poor soil drainage. In these flooding conditions, crop losses are inevitable due to exposure of plants to hypoxia and the spread of root rot diseases. Improving the tolerance of bean cultivars to flooding is crucial to minimize crop losses. In this experiment, we evaluated the phenotypic responses of 277 genotypes from the Andean Diversity Panel to flooding at germination and seedling stages. A randomized complete block design, with a split plot arrangement, was employed for phenotyping germination rate, total weight, shoot weight, root weight, hypocotyl length, SPAD index, adventitious root rate, and survival score. A subset of genotypes (n = 20) were further evaluated under field conditions to assess correlations between field and greenhouse data and to identify the most tolerant genotypes. A genome-wide association study (GWAS) was performed using ~203 K SNP markers to understand the genetic architecture of flooding tolerance in this panel. Survival scores between field and greenhouse data were significantly correlated (r = 0.55, P = 0.01). Subsequently, a subset of the most tolerant and susceptible genotypes were evaluated under pathogenic Pythium spp. pressure. This experiment revealed a potential link between flooding tolerance and Pythium spp. resistance. Several tolerant genotypes were identified that could be used as donor parents in breeding pipelines, especially ADP-429 and ADP-604. Based on the population structure analysis, a subpopulation consisting of 20 genotypes from the Middle American gene pool was detected that also possessed the highest root weight, hypocotyl length, and adventitious root development under flooding conditions. Genomic regions associated with flooding tolerance were identified including a region on Pv08/3.2 Mb, which is associated with germination rate and resides in vicinity of SnRK1.1, a central gene involved in response of plants to hypoxia. Furthermore, a QTL at Pv07/4.7 Mb was detected that controls survival score of seedlings under flooding conditions. The association of these QTL with the survivability traits including germination rate and survival score, indicates that these loci can be used in marker-assisted selection breeding to improve flooding tolerance in the Andean germplasm.

Comparative analysis of plant isochorismate synthases reveals structural mechanisms underlying their distinct biochemical properties

Isochorismate synthase (ICS) converts chorismate into isochorismate, a precursor of primary and secondary metabolites including salicylic acid (SA). SA plays important roles in responses to stress conditions in plants. Many studies have suggested that the function of plant ICSs is regulated at the transcriptional level. In Arabidopsis thaliana, the expression of AtICS1 is induced by stress conditions in parallel with SA synthesis, and AtICS1 is required for SA synthesis. In contrast, the expression of NtICS is not induced when SA synthesis is activated in tobacco, and it is unlikely to be involved in SA synthesis. Studies on the biochemical properties of plant ICSs are limited, compared with those on transcriptional regulation. We analyzed the biochemical properties of four plant ICSs: AtICS1, NtICS, NbICS from Nicotiana benthamiana, and OsICS from rice. Multiple sequence alignment analysis revealed that their primary structures were well conserved, and predicted key residues for ICS activity were almost completely conserved. However, AtICS1 showed much higher activity than the other ICSs when expressed in Escherichia coli and N. benthamiana leaves. Moreover, the levels of AtICS1 protein expression in N. benthamiana leaves were higher than the other ICSs. Construction and analysis of chimeras between AtICS1 and OsICS revealed that the putative chloroplast transit peptides (TPs) significantly affected the levels of protein accumulation in N. benthamiana leaves. Chimeric and point-mutation analyses revealed that Thr531, Ser537, and Ile550 of AtICS1 are essential for its high activity. These distinct biochemical properties of plant ICSs may suggest different roles in their respective plant species.

PROHIBITIN3 Forms Complexes with ISOCHORISMATE SYNTHASE1 to Regulate Stress-Induced Salicylic Acid Biosynthesis in Arabidopsis

Salicylic acid (SA) is a major defense signal in plants. In Arabidopsis (Arabidopsis thaliana), the chloroplast-localized isochorismate pathway is the main source of SA biosynthesis during abiotic stress or pathogen infections. In the first step of the pathway, the enzyme ISOCHORISMATE SYNTHASE1 (ICS1) converts chorismate to isochorismate. An unknown enzyme subsequently converts isochorismate to SA. Here, we show that ICS1 protein levels increase during UV-C stress. To identify proteins that may play roles in SA production by regulating ICS1, we analyzed proteins that coimmunoprecipitated with ICS1 via mass spectrometry. The ICS1 complexes contained a large number of peptides from the PROHIBITIN (PHB) protein family, with PHB3 the most abundant. PHB proteins have diverse biological functions that include acting as scaffolds for protein complex formation and stabilization. PHB3 was reported previously to localize to mitochondria. Using fractionation, protease protection, and live imaging, we show that PHB3 also localizes to chloroplasts, where ICS1 resides. Notably, loss of PHB3 function led to decreased ICS1 protein levels in response to UV-C stress. However, ICS1 transcript levels remain unchanged, indicating that ICS1 is regulated posttranscriptionally. The phb3 mutant displayed reduced levels of SA, the SA-regulated protein PR1, and hypersensitive cell death in response to UV-C and avirulent strains of Pseudomonas syringae and, correspondingly, supported increased growth of P. syringae The expression of a PHB3 transgene in the phb3 mutant complemented all of these phenotypes. We suggest a model in which the formation of PHB3-ICS1 complexes stabilizes ICS1 to promote SA production in response to stress.