ABA is required for Leptosphaeria maculans resistance via ABI1- and ABI4-dependent signaling

Physiological, metabolic and hormonal responses of two Pinus spp. with contrasting susceptibility to brown-spot needle blight disease

Needle blights are serious fungal diseases affecting European natural and planted pine forests. Brown-spot needle blight (BSNB) disease, caused by the fungus Lecanosticta acicola, causes canopy defoliation and severe productivity losses, with consequences depending on host susceptibility. To gain new insights into BSNB plant-pathogen interactions, constitutive and pathogen-induced traits were assessed in two host species with differential disease susceptibility. Six-month-old Pinus radiata D. Don (susceptible) and Pinus pinea L. (more resistant) seedlings were needle inoculated with L. acicola under controlled conditions. Eighty days after inoculation, healthy-looking needles from symptomatic plants were assessed for physiological parameters and sampled for biochemical analysis. Disease progression, plant growth, leaf gas-exchanges and biochemical parameters were complemented with hormonal and untargeted primary metabolism analysis and integrated for a holistic analysis. Constitutive differences between pine species were observed. Pinus pinea presented higher stomatal conductance and transpiration rate and higher amino and organic acids, abscisic acid as well as putrescine content than P. radiata. Symptoms from BSNB disease were observed in 54.54% of P. radiata and 45.45% of P. pinea seedlings, being more pronounced and generalized in P. radiata. For both species, plant height, sub-stomatal CO2 concentration and water-use efficiency were impacted by infection. In P. radiata, total soluble sugars, starch and total flavonoids content increased after infection. No differences in hormone content after infection were observed. However, secondary metabolism was induced in P. pinea visible through total phenolics, flavonoids and putrescine accumulation. Overall, the observed results suggest that P. pinea constitutive and induced traits may function as two layers of a defence strategy which contributed to an increased BSNB resistance in comparison with P. radiata. This is the first integrative study linking plant physiological and molecular traits in Pinus-Lecanosticta acicola pathosystem, contributing to a better understanding of the underlying resistance mechanisms to BSNB disease in pines.

The F-box protein ZmFBL41 negatively regulates disease resistance to Rhizoctonia solani by degrading the abscisic acid synthase ZmNCED6 in maize

KEY MESSAGE

The maize F-box protein ZmFBL41 targets abscisic acid synthase 9-cis-epoxycarotenoid dioxygenase 6 for degradation, and this regulatory module is exploited by Rhizoctonia solani to promote infection. F-box proteins are crucial regulators of plant growth, development, and responses to abiotic and biotic stresses. Previous research identified the F-box gene ZmFBL41 as a negative regulator of maize (Zea mays) defenses against Rhizoctonia solani. However, the precise mechanisms by which F-box proteins mediate resistance to R. solani remain poorly understood. In this study, we show that ZmFBL41 interacts with an abscisic acid (ABA) synthase, 9-cis-epoxycarotenoid dioxygenase 6 (ZmNCED6), promoting its degradation via the ubiquitination pathway. We discovered that the ectopic overexpression of ZmNCED6 in rice (Oryza sativa) inhibited R. solani infection by activating stomatal closure, callose deposition, and jasmonic acid (JA) biosynthesis, indicating that ZmNCED6 enhances plant immunity against R. solani. Natural variation at ZmFBL41 across different maize haplotypes did not affect the ZmFBL41-ZmNCED6 interaction. These findings suggest that ZmFBL41 targets ZmNCED6 for degradation, leading to a decrease in ABA levels in maize, in turn, inhibiting ABA-mediated disease resistance pathways, such as stomatal closure, callose deposition, and JA biosynthesis, ultimately facilitating R. solani infection.

Plant Stomata: An Unrealized Possibility in Plant Defense against Invading Pathogens and Stress Tolerance

Stomata are crucial structures in plants that play a primary role in the infection process during a pathogen's attack, as they act as points of access for invading pathogens to enter host tissues. Recent evidence has revealed that stomata are integral to the plant defense system and can actively impede invading pathogens by triggering plant defense responses. Stomata interact with diverse pathogen virulence factors, granting them the capacity to influence plant susceptibility and resistance. Moreover, recent studies focusing on the environmental and microbial regulation of stomatal closure and opening have shed light on the epidemiology of bacterial diseases in plants. Bacteria and fungi can induce stomatal closure using pathogen-associated molecular patterns (PAMPs), effectively preventing entry through these openings and positioning stomata as a critical component of the plant's innate immune system; however, despite this defense mechanism, some microorganisms have evolved strategies to overcome stomatal protection. Interestingly, recent research supports the hypothesis that stomatal closure caused by PAMPs may function as a more robust barrier against pathogen infection than previously believed. On the other hand, plant stomatal closure is also regulated by factors such as abscisic acid and Ca2+-permeable channels, which will also be discussed in this review. Therefore, this review aims to discuss various roles of stomata during biotic and abiotic stress, such as insects and water stress, and with specific context to pathogens and their strategies for evading stomatal defense, subverting plant resistance, and overcoming challenges faced by infectious propagules. These pathogens must navigate specific plant tissues and counteract various constitutive and inducible resistance mechanisms, making the role of stomata in plant defense an essential area of study.

Flagellin and mannitol modulate callose biosynthesis and deposition in soybean seedlings

Callose is a polymer deposited on the cell wall and is necessary for plant growth and development. Callose is synthesized by genes from the glucan synthase-like family (GSL) and dynamically responds to various types of stress. Callose can inhibit pathogenic infection, in the case of biotic stresses, and maintain cell turgor and stiffen the plant cell wall in abiotic stresses. Here, we report the identification of 23 GSL genes (GmGSL) in the soybean genome. We performed phylogenetic analyses, gene structure prediction, duplication patterns, and expression profiles on several RNA-Seq libraries. Our analyses show that WGD/Segmental duplication contributed to expanding this gene family in soybean. Next, we analyzed the callose responses in soybean under abiotic and biotic stresses. The data show that callose is induced by both osmotic stress and flagellin 22 (flg22) and is related to the activity of β-1,3-glucanases. By using RT-qPCR, we evaluated the expression of GSL genes during the treatment of soybean roots with mannitol and flg22. The GmGSL23 gene was upregulated in seedlings treated with osmotic stress or flg22, showing the essential role of this gene in the soybean defense response to pathogenic organisms and osmotic stress. Our results provide an important understanding of the role of callose deposition and regulation of GSL genes in response to osmotic stress and flg22 infection in soybean seedlings.

Genome-wide characterization of the PP2C gene family in peanut (Arachis hypogaea L.) and the identification of candidate genes involved in salinity-stress response

Plant protein phosphatase 2C (PP2C) play important roles in response to salt stress by influencing metabolic processes, hormone levels, growth factors, etc. Members of the PP2C family have been identified in many plant species. However, they are rarely reported in peanut. In this study, 178 PP2C genes were identified in peanut, which were unevenly distributed across the 20 chromosomes, with segmental duplication in 78 gene pairs. AhPP2Cs could be divided into 10 clades (A-J) by phylogenetic analysis. AhPP2Cs had experienced segmental duplications and strong purifying selection pressure. 22 miRNAs from 14 different families were identified, targeting 57 AhPP2C genes. Gene structures and motifs analysis exhibited PP2Cs in subclades AI and AII had high structural and functional similarities. Phosphorylation sites of AhPP2C45/59/134/150/35/121 were predicted in motifs 2 and 4, which located within the catalytic site at the C-terminus. We discovered multiple MYB binding factors and ABA response elements in the promoter regions of the six genes (AhPP2C45/59/134/150/35/121) by cis-elements analysis. GO and KEGG enrichment analysis confirmed AhPP2C-A genes in protein binding, signal transduction, protein modification process response to abiotic stimulus through environmental information processing. Based on RNA-Seq data of 22 peanut tissues, clade A AhPP2Cs showed a varying degree of tissue specificity, of which, AhPP2C35 and AhPP2C121 specifically expressed in seeds, while AhPP2C45/59/134/150 expressed in leaves and roots. qRT-PCR indicated that AhPP2C45 and AhPP2C134 displayed significantly up-regulated expression in response to salt stress. These results indicated that AhPP2C45 and AhPP2C134 could be candidate PP2Cs conferring salt tolerance. These results provide further insights into the peanut PP2C gene family and indicate PP2Cs potentially involved in the response to salt stress, which can now be further investigated in peanut breeding efforts to obtain cultivars with improved salt tolerance.

Deciphering immune responses primed by a bacterial lipopeptide in wheat towards Zymoseptoria tritici

Plant immunity induction with natural biocontrol compounds is a valuable and promising ecofriendly tool that fits with sustainable agriculture and healthy food. Despite the agroeconomic significance of wheat, the mechanisms underlying its induced defense responses remain obscure. We reveal here, using combined transcriptomic, metabolomic and cytologic approach, that the lipopeptide mycosubtilin from the beneficial bacterium Bacillus subtilis, protects wheat against Zymoseptoria tritici through a dual mode of action (direct and indirect) and that the indirect one relies mainly on the priming rather than on the elicitation of plant defense-related mechanisms. Indeed, the molecule primes the expression of 80 genes associated with sixteen functional groups during the early stages of infection, as well as the accumulation of several flavonoids during the period preceding the fungal switch to the necrotrophic phase. Moreover, genes involved in abscisic acid (ABA) biosynthesis and ABA-associated signaling pathways are regulated, suggesting a role of this phytohormone in the indirect activity of mycosubtilin. The priming-based bioactivity of mycosubtilin against a biotic stress could result from an interaction of the molecule with leaf cell plasma membranes that may mimic an abiotic stress stimulus in wheat leaves. This study provides new insights into induced immunity in wheat and opens new perspectives for the use of mycosubtilin as a biocontrol compound against Z. tritici.

The Emerging Role of PP2C Phosphatases in Tomato Immunity

The antagonistic effect of plant immunity on growth likely drove evolution of molecular mechanisms that prevent accidental initiation and prolonged activation of plant immune responses. Signaling networks of pattern-triggered and effector-triggered immunity, the two main layers of plant immunity, are tightly regulated by the activity of protein phosphatases that dephosphorylate their protein substrates and reverse the action of protein kinases. Members of the PP2C class of protein phosphatases have emerged as key negative regulators of plant immunity, primarily from research in the model plant Arabidopsis thaliana, revealing the potential to employ PP2C proteins to enhance plant disease resistance. As a first step towards focusing on the PP2C family for both basic and translational research, we analyzed the tomato genome sequence to ascertain the complement of the tomato PP2C family, identify conserved protein domains and signals in PP2C amino acid sequences, and examine domain combinations in individual proteins. We then identified tomato PP2Cs that are candidate regulators of single or multiple layers of the immune signaling network by in-depth analysis of publicly available RNA-seq datasets. These included expression profiles of plants treated with fungal or bacterial pathogen-associated molecular patterns, with pathogenic, nonpathogenic, and disarmed bacteria, as well as pathogenic fungi and oomycetes. Finally, we discuss the possible use of immunity-associated PP2Cs to better understand the signaling networks of plant immunity and to engineer durable and broad disease resistance in crop plants. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

MADS2 regulates priming defence in postharvest peach through combined salicylic acid and abscisic acid signaling

MADS-box genes play well-documented roles in plant development, but relatively little is known regarding their involvement in defence responses. In this study, pre-treatment of peach (Prunus persica) fruit with β-aminobutyric acid (BABA) activated resistance against Rhizopus stolonifer, leading to a significant delay in the symptomatic appearance of disease. This was associated with an integrated defence response that included a H2O2 burst, ABA accumulation, and callose deposition. cDNA library screening identified nucleus-localized MADS2 as an interacting partner with NPR1, and this was further confirmed by yeast two-hybrid, luciferase complementation imaging, and co-immunoprecipitation assays. The DNA-binding activity of NPR1 conferred by the NPR1-MADS2 complex was required for the transcription of SA-dependent pathogenesis-related (PR) and ABA-inducible CalS genes in order to gain the BABA-induced resistance, in which MAPK1-induced post-translational modification of MADS2 was also involved. In accordance with this, overexpression of PpMADS2 in Arabidopsis potentiated the transcription of a group of PR genes and conferred fungal resistance in the transgenic plants. Conversely, Arabidopsis mads2-knockout lines showed high sensitivity to the fungal pathogen. Our results indicate that MADS2 positively participates in BABA-elicited defence in peach through a combination of SA-dependent NPR1 activation and ABA signaling-induced callose accumulation, and that this defence is also related to the post-translational modification of MADS2 by MAPK1 for signal amplification.

NDW, encoding a receptor-like protein kinase, regulates plant growth, cold tolerance and susceptibility to Botrytis cinerea in tomato

Plants utilize different mechanisms to respond and adapt to continuously changing environmental factors. Receptor-like protein kinases (RLKs) comprise one of the largest families of plant transmembrane signaling proteins, which play critical and diverse roles in plant growth, development, and stress response. Here, we identified the necrotic dwarf (ndw) mutant introgression line (IL) 6-2, which demonstrated stunting, leaf curl, and progressive necrosis at low temperatures. Based on map-based cloning and transgenic analysis, we determined that the phenotype of ndw mutant is caused by decreased expression of NDW, which encodes an RLK. NDW is a plasma membrane and cytoplasmic located protein. Overexpression of NDW can restore both of the semi-dwarf and necrotic phenotype in IL6-2 at low temperatures, further we found that NDW could significantly reduce susceptibility to Botrytis cinerea. On the contrary, knockdown NDW in M82 plants could increase the sensitivity to B. cinerea. Furthermore, transcriptional expression analysis showed that NDW affects the expression of genes related to the abscisic acid (ABA) signaling pathway. Taken together, these results indicate that NDW plays an important role in regulating plant growth, cold tolerance and mitigating susceptibility to Botrytis cinerea.

Aspergillus alliaceus infection fatally shifts Orobanche hormones and phenolic metabolism

In this study, the physio pathological effects of Aspergillus alliaceus (Aa, fungi, biocontrol agent) on Orobanche (parasitic plant) were investigated by hormone and phenolic substance tests. In experimental group, Orobanches were treated with the fungi, considering control group was fungus-free. Based on the hormonal tests, in the experimental group, salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA) and gibberellic acid (GA) levels significantly decreased, and only indole acetic acid (IAA) hormone levels were fairly higher than the control group. According to phenolic substance tests, it was found that only gallic acid, syringic acid and caffeic acid values significantly increased compared with control, and catechin and p-coumaric acid values were significantly lower. Consequently, it was determined that Aa pathogenesis (1) considerably reduces the effects of all defence hormones (JA, ABA, SA), (2) operates an inadequate defence based solely on the IAA hormone and several phenolic substances (gallic acid, syringic acid and caffeic acid), (3) and inevitably the fungi lead the Orobanche to a slow and continuous death. The results were evaluated in detail in the light of similar recent article and current literature in terms of biocontrol and pathology.

Effects of Abscisic Acid and Salicylic Acid on Gene Expression in the Antiviral RNA Silencing Pathway in Arabidopsis

The RNA silencing pathways modulate responses to certain stresses, and can be partially tuned by several hormones such as salicylic acid (SA) and abscisic acid (ABA). Although SA and ABA are often antagonistic and often modulate different stress responses, they have similar effects on virus resistance, which are partially achieved through the antiviral RNA silencing pathway. Whether they play similar roles in regulating the RNA silencing pathway is unclear. By employing coexpression and promoter analyses, we found that some ABA- and SA-related transcription factors (TFs) are coexpressed with several AGO, DCL, and RDR genes, and have multiple binding sites for the identified TFs in the queried promoters. ABA and SA are antagonistic with respect to the expression of AGO1 and RDRs because ABA was able to induce these genes only in the SA mutant. Nevertheless, both hormones showed similarities in the regulation of other genes, for example, the induction of AGO2 by ABA was SA-dependent, indicating that ABA acts upstream of SA in this regulation. We inferred that the similar effects of ABA and SA on some genes resulted in the redundancy of their roles in resistance to bamboo mosaic virus, but that the two hormones are antagonistic with respect to other genes unrelated to their biosynthesis pathways.

Differential Function of Endogenous and Exogenous Abscisic Acid during Bacterial Pattern-Induced Production of Reactive Oxygen Species in Arabidopsis

Abscisic acid (ABA) plays important roles in positively or negatively regulating plant disease resistance to pathogens. Here, we reassess the role of endogenous and exogenous ABA by using: 35S::ABA2, a previously reported transgenic Arabidopsis line with increased endogenous ABA levels; aba2-1, a previously reported ABA2 mutant with reduced endogenous ABA levels; and exogenous application of ABA. We found that bacterial susceptibility promoted by exogenous ABA was suppressed in 35S::ABA2 plants. The 35S::ABA2 and aba2-1 plants displayed elevated and reduced levels, respectively, of bacterial flagellin peptide (flg22)-induced H₂O₂. Surprisingly, ABA pre-treatment reduced flg22-induced H₂O₂ generation. Exogenous, but not endogenous ABA, increased catalase activity. Loss of nicotinamide adenine dinucleotide phosphate oxidase genes, RBOHD and RBOHF, restored exogenous ABA-promoted bacterial susceptibility of 35S::ABA2 transgenic plants. In addition, endogenous and exogenous ABA had similar effects on callose deposition and salicylic acid (SA) signaling. These results reveal an underlying difference between endogenous and exogenous ABA in regulating plant defense responses. Given that some plant pathogens are able to synthesize ABA and affect endogenous ABA levels in plants, our results highlight the importance of reactive oxygen species in the dual function of ABA during plant-pathogen interactions.

Recent Advances in Mechanisms of Plant Defense to Sclerotinia sclerotiorum

Sclerotinia sclerotiorum (Lib.) de Bary is an unusual pathogen which has the broad host range, diverse infection modes, and potential double feeding lifestyles of both biotroph and necrotroph. It is capable of infecting over 400 plant species found worldwide and more than 60 names have agriculturally been used to refer to diseases caused by this pathogen. Plant defense to S. sclerotiorum is a complex biological process and exhibits a typical quantitative disease resistance (QDR) response. Recent studies using Arabidopsis thaliana and crop plants have obtained new advances in mechanisms used by plants to cope with S. sclerotiorum infection. In this review, we focused on our current understanding on plant defense mechanisms against this pathogen, and set up a model for the defense process including three stages: recognition of this pathogen, signal transduction and defense response. We also have a particular interest in defense signaling mediated by diverse signaling molecules. We highlight the current challenges and unanswered questions in both the defense process and defense signaling. Essentially, we discussed candidate resistance genes newly mapped by using high-throughput experiments in important crops, and classified these potential gene targets into different stages of the defense process, which will broaden our understanding of the genetic architecture underlying quantitative resistance to S. sclerotiorum. We proposed that more powerful mapping population(s) will be required for accurate and reliable QDR gene identification.

Starch degradation, abscisic acid and vesicular trafficking are important elements in callose priming by indole-3-carboxylic acid in response to Plectosphaerella cucumerina infection

A fast callose accumulation has been shown to mediate defence priming in certain plant-pathogen interactions, but the events upstream of callose assembly following chemical priming are poorly understood, mainly because those steps comprise sugar transfer to the infection site. β-Amino butyric acid (BABA)-induced resistance in Arabidopsis against Plectosphaerella cucumerina is known to be mediated by callose priming. Indole-3-carboxylic acid (ICOOH, also known as I3CA) mediates BABA-induced resistance in Arabidopsis against P. cucumerina. This indolic compound is found in a common fingerprint of primed metabolites following treatments with various priming stimuli. In the present study, we show that I3CA induces resistance in Arabidopsis against P. cucumerina and primes enhancement of callose accumulation. I3CA treatment increased abscisic acid (ABA) levels before infection with P. cucumerina. An intact ABA synthesis pathway is needed to activate a starch amylase (BAM1) to trigger augmented callose deposition against P. cucumerina during I3CA-IR. To verify the relevance of the BAM1 amylase in I3CA-IR, knockdown mutants and overexpressors of the BAM1 gene were tested. The mutant bam1 was impaired to express I3CA-IR, but complemented 35S::BAM1-YFP lines in the background of bam1 restored an intact I3CA-IR and callose priming. Therefore, a more active starch metabolism is a committed step for I3CA-IR, inducing callose priming in adult plants. Additionally, I3CA treatments induced expression of the ubiquitin ligase ATL31 and syntaxin SYP131, suggesting that vesicular trafficking is relevant for callose priming. As a final element in the callose priming, an intact Powdery Mildew resistant4 (PMR4) gene is also essential to fully express I3CA-IR.

Defense responses of lentil (Lens culinaris) genotypes carrying non-allelic ascochyta blight resistance genes to Ascochyta lentis infection

Ascochyta blight of lentil is an important fungal disease in many lentil-producing regions of the world causing major yield and grain quality losses. Quick shifts in aggressiveness of the population of the causal agent Ascochyta lentis mandates developing germplasm with novel and durable resistance. In the absence of complete resistance, lentil genotypes CDC Robin and 964a-46 have frequently been used as sources of partial resistance to ascochyta blight and carry non-allelic ascochyta blight resistance genes. RNA-seq analysis was conducted to identify differences in the transcriptome of CDC Robin, 964a-46 and the susceptible check Eston after inoculation with A. lentis. Candidate defense genes differentially expressed among the genotypes had hypothetical functions in various layers of plant defense, including pathogen recognition, phytohormone signaling pathways and downstream defense responses. CDC Robin and 964a-46 activated cell surface receptors (e.g. receptor like kinases) tentatively associated with pathogen-associated molecular patterns (PAMP) recognition and nucleotide-binding site leucine-rich repeat (NBS-LRR) receptors associated with intracellular effector recognition upon A. lentis infection, and differed in their activation of salicylic acid, abscisic acid and jasmonic acid / ethylene signal transduction pathways. These differences were reflected in the differential expression of downstream defense responses such as pathogenesis-related proteins, and genes associated with the induction of cell death and cell-wall reinforcement. A significant correlation between expression levels of a selection of genes based on quantitative real-time PCR and their expression levels estimated through RNA-seq demonstrated the technical and analytical accuracy of RNA-seq for identification of genes differentially expressed among genotypes. The presence of different resistance mechanisms in 964a-46 and CDC Robin indicates their value for pyramiding gene leading to more durable resistance to ascochyta blight.

Reactive oxygen species signaling and stomatal movement in plant responses to drought stress and pathogen attack

Stomata, the pores formed by a pair of guard cells, are the main gateways for water transpiration and photosynthetic CO₂ exchange, as well as pathogen invasion in land plants. Guard cell movement is regulated by a combination of environmental factors, including water status, light, CO₂ levels and pathogen attack, as well as endogenous signals, such as abscisic acid and apoplastic reactive oxygen species (ROS). Under abiotic and biotic stress conditions, extracellular ROS are mainly produced by plasma membrane-localized NADPH oxidases, whereas intracellular ROS are produced in multiple organelles. These ROS form a sophisticated cellular signaling network, with the accumulation of apoplastic ROS an early hallmark of stomatal movement. Here, we review recent progress in understanding the molecular mechanisms of the ROS signaling network, primarily during drought stress and pathogen attack. We summarize the roles of apoplastic ROS in regulating stomatal movement, ABA and CO₂ signaling, and immunity responses. Finally, we discuss ROS accumulation and communication between organelles and cells. This information provides a conceptual framework for understanding how ROS signaling is integrated with various signaling pathways during plant responses to abiotic and biotic stress stimuli.

Muscadinia rotundifolia 'Noble' defense response to Plasmopara viticola inoculation by inducing phytohormone-mediated stilbene accumulation

Downy mildew (DM), one of the most devastating grape diseases worldwide, is caused by the biotrophic oomycete Plasmopara viticola (Pv). In general, grapevine responds to Pv infection with the accumulation of phytoalexins as part of the innate immune system, and diverse phytoalexins are induced on grapevines with different DM-resistance levels in response to Pv invasion. However, the regulation of phytoalexin biosynthesis during grapevine against Pv is still unclear. Herein, we detected stilbenes by UPLC-ESI-MS/MS and found that resveratrol was accumulated to higher level and earlier in the DM-immune Muscadinia rotundifolia 'Noble' than that in the DM-susceptible Vitis vinifera 'Thompson Seedless' after Pv inoculation. Additionally, a considerable amount of pterostilbene and ε-viniferin was found in 'Noble', while a little was detected in 'Thompson Seedless'. Resveratrol was glycosylated into piceid both in 'Noble' and 'Thompson Seedless' after Pv inoculation. The qPCR analysis of gene expression indicated that the resveratrol-synthesis gene (STS) was induced by Pv inoculation earlier in 'Noble' than that in 'Thompson Seedless', while the pterostilbene-synthesis gene (ROMT) was induced in 'Noble' but not in 'Thompson Seedless' at all. The piceid-synthesis gene (GT) was generally up-regulated in both cultivars. Sequence analysis of STS, ROMT, and GT promoters revealed that they contained cis-regulatory elements responsive to phytohormones and pathogens. Following Pv inoculation, the level of SA, MeJA, and ABA was found to be consistently higher in 'Noble' than those in 'Thompson Seedless'. The results of exogenous hormone elicitation further demonstrated that the accumulation of stilbenes was regulated by phytohormones. The earlier and higher accumulation of phytohormones and consequent induction of stilbene synthesis may play an important role in grapevine defense against downy mildew disease.

Transcriptome analysis of the Brassica napus-Leptosphaeria maculans pathosystem identifies receptor, signaling and structural genes underlying plant resistance

The hemibiotrophic fungal pathogen Leptosphaeria maculans is the causal agent of blackleg disease in Brassica napus (canola, oilseed rape) and causes significant loss of yield worldwide. While genetic resistance has been used to mitigate the disease by means of traditional breeding strategies, there is little knowledge about the genes that contribute to blackleg resistance. RNA sequencing and a streamlined bioinformatics pipeline identified unique genes and plant defense pathways specific to plant resistance in the B. napus-L. maculans LepR1-AvrLepR1 interaction over time. We complemented our temporal analyses by monitoring gene activity directly at the infection site using laser microdissection coupled to quantitative PCR. Finally, we characterized genes involved in plant resistance to blackleg in the Arabidopsis-L. maculans model pathosystem. Data reveal an accelerated activation of the plant transcriptome in resistant host cotyledons associated with transcripts coding for extracellular receptors and phytohormone signaling molecules. Functional characterization provides direct support for transcriptome data and positively identifies resistance regulators in the Brassicaceae. Spatial gradients of gene activity were identified in response to L. maculans proximal to the site of infection. This dataset provides unprecedented spatial and temporal resolution of the genes required for blackleg resistance and serves as a valuable resource for those interested in host-pathogen interactions.

PacMYBA, a sweet cherry R2R3-MYB transcription factor, is a positive regulator of salt stress tolerance and pathogen resistance

Plant R2R3-MYB transcription factors play crucial roles in stress responses. We previously isolated a R2R3-MYB homolog from sweet cherry cv. Hong Deng, designated PacMYBA (GenBank accession No. KF974774). To explore the role of PacMYBA in the plant stress response, we heterologously expressed PacMYBA in transgenic Arabidopsis thaliana plants. In a previous study, we demonstrated that PacMYBA is mainly localized to the nucleus and could be induced by abscisic acid (ABA). Analysis of the promoter sequence of PacMYBA revealed that it contains several stress-related cis-elements. QPCR results showed that PacMYBA is induced by salt, salicylic (SA), and jasmonic acid (JA) in sweet cherry leaves. Transgenic Arabidopsis plants heterologously expressing PacMYBA exhibited enhanced salt-tolerance and increased resistance to Pseudomonas syringe pv. tomato (Pst) DC3000 infection. Overexpression of PacMYBA decreased the osmotic potential (OP), increased the free proline content, and increased the peroxidase content in transgenic Arabidopsis plants. Furthermore, overexpression of PacMYBA also affected the expression levels of salt stress- and pathogen defense-related genes in the transgenic plants. These results indicate that PacMYBA is a positive regulator of salt stress tolerance and pathogen resistance.

Late seed maturation: drying without dying

Besides the deposition of storage reserves, seed maturation is characterized by the acquisition of functional traits including germination, desiccation tolerance, dormancy, and longevity. After seed filling, seed longevity increases up to 30-fold, concomitant with desiccation that brings the embryo to a quiescent state. The period that we define as late maturation phase can represent 10-78% of total seed development time, yet it remains overlooked. Its importance is underscored by the fact that in the seed production chain, the stage of maturity at harvest is the primary factor that influences seed longevity and seedling establishment. This review describes the major events and regulatory pathways underlying the acquisition of seed longevity, focusing on key indicators of maturity such as chlorophyll degradation, accumulation of raffinose family oligosaccharides, late embryogenesis abundant proteins, and heat shock proteins. We discuss how these markers are correlated with or contribute to seed longevity, and highlight questions that merit further attention. We present evidence suggesting that molecular players involved in biotic defence also have a regulatory role in seed longevity. We also explore how the concept of plasticity can help understand the acquisition of longevity.

Abscisic Acid as Pathogen Effector and Immune Regulator

Abscisic acid (ABA) is a sesquiterpene signaling molecule produced in all kingdoms of life. To date, the best known functions of ABA are derived from its role as a major phytohormone in plant abiotic stress resistance. Different organisms have developed different biosynthesis and signal transduction pathways related to ABA. Despite this, there are also intriguing common themes where ABA often suppresses host immune responses and is utilized by pathogens as an effector molecule. ABA also seems to play an important role in compatible mutualistic interactions such as mycorrhiza and rhizosphere bacteria with plants, and possibly also the animal gut microbiome. The frequent use of ABA in inter-species communication could be a possible reason for the wide distribution and re-invention of ABA as a signaling molecule in different organisms. In humans and animal models, it has been shown that ABA treatment or nutrient-derived ABA is beneficial in inflammatory diseases like colitis and type 2 diabetes, which confer potential to ABA as an interesting nutraceutical or pharmacognostic drug. The anti-inflammatory activity, cellular metabolic reprogramming, and other beneficial physiological and psychological effects of ABA treatment in humans and animal models has sparked an interest in this molecule and its signaling pathway as a novel pharmacological target. In contrast to plants, however, very little is known about the ABA biosynthesis and signaling in other organisms. Genes, tools and knowledge about ABA from plant sciences and studies of phytopathogenic fungi might benefit biomedical studies on the physiological role of endogenously generated ABA in humans.

ABA Suppresses Botrytis cinerea Elicited NO Production in Tomato to Influence H2O2 Generation and Increase Host Susceptibility

Abscisic acid (ABA) production has emerged a susceptibility factor in plant-pathogen interactions. This work examined the interaction of ABA with nitric oxide (NO) in tomato following challenge with the ABA-synthesizing pathogen, Botrytis cinerea. Trace gas detection using a quantum cascade laser detected NO production within minutes of challenge with B. cinerea whilst photoacoustic laser detection detected ethylene production - an established mediator of defense against this pathogen - occurring after 6 h. Application of the NO generation inhibitor N-Nitro-L-arginine methyl ester (L-NAME) suppressed both NO and ethylene production and resistance against B. cinerea. The tomato mutant sitiens fails to accumulate ABA, shows increased resistance to B. cinerea and we noted exhibited elevated NO and ethylene production. Exogenous application of L-NAME or ABA reduced NO production in sitiens and reduced resistance to B. cinerea. Increased resistance to B. cinerea in sitiens have previously been linked to increased reactive oxygen species (ROS) generation but this was reduced in both L-NAME and ABA-treated sitiens. Taken together, our data suggests that ABA can decreases resistance to B. cinerea via reduction of NO production which also suppresses both ROS and ethylene production.

The nuclear protein GmbZIP110 has transcription activation activity and plays important roles in the response to salinity stress in soybean

Plant basic-leucine zipper (bZIP) transcription factors play important roles in many biological processes and are involved in the regulation of salt stress tolerance. Previously, our lab generated digital gene expression profiling (DGEP) data to identify differentially expressed genes in a salt-tolerant genotype of Glycine soja (STGoGS) and a salt-sensitive genotype of Glycine max (SSGoGM). This DGEP data revealed that the expression (log2 ratio) of GmbZIP110 was up-regulated 2.76-fold and 3.38-fold in SSGoGM and STGoGS, respectively. In the present study, the salt inducible gene GmbZIP110 was cloned and characterized through phylogenetic analysis, subcellular localization and in silico transcript abundance analysis in different tissues. The functional role of this gene in salt tolerance was studied through transactivation analysis, DNA binding ability, expression in soybean composite seedlings and transgenic Arabidopsis, and the effect of GmbZIP110 on the expression of stress-related genes in transgenic Arabidopsis was investigated. We found that GmbZIP110 could bind to the ACGT motif, impact the expression of many stress-related genes and the accumulation of proline, Na(+) and K(+), and enhanced the salt tolerance of composite seedlings and transgenic Arabidopsis. Integrating all these results, we propose that GmbZIP110 plays a critical role in the response to salinity stress in soybean and has high potential usefulness in crop improvement.

Soybean C2H2-Type Zinc Finger Protein GmZFP3 with Conserved QALGGH Motif Negatively Regulates Drought Responses in Transgenic Arabidopsis

Plant response to environmental stresses is regulated by a complicated network of regulatory and functional genes. In this study, we isolated the putative stress-associated gene GmZFP3 (a C2H2-type Zinc finger protein gene) based on the previous finding that it was one of two genes located in the QTL region between the Satt590 and Satt567 markers related to soybean tolerance to drought. Temporal and spatial expression analysis using quantitative real-time PCR indicated that GmZFP3 was primarily expressed in roots, stems and leaf organs and was expressed at low levels in flowers and soybean pods. Moreover, GmZFP3 expression increased in response to polyethylene glycol (PEG) and Abscisic acid (ABA) treatments. In addition, subcellular localization analysis indicated that GmZFP3 was ubiquitously distributed in plant cells. Transgenic experiments indicated that GmZFP3 played a negative role in plant tolerance to drought. Analysis of ABA-related marker gene expression in Arabidopsis suggested that GmZFP3 might be involved in the ABA-dependent pathway during the drought stress response. Taken together, these results suggest that soybean GmZFP3 negatively regulates the drought response.

Molecular and physiological stages of priming: how plants prepare for environmental challenges

Being sessile organisms, plants must respond to various challenges in the environment. The priming process consists of three clear stages. The first stage includes all the cellular changes in the absence of the challenge so-called pre-challenge priming stage. These changes are expected to be rather subtle, affecting the preparation of the plant to properly manage subsequent responses to pathogens with no major fitness costs. Most of the research that has been conducted at this stage has been dedicated to the study of changes in gene expression and protein phosphorylation. However, the metabolic changes that occur during the pre-challenge priming stage are poorly understood. The second stage affects the early to late stages of the defence response, which occurs after the interaction with a pathogen has been established. Most studies involving priming are dedicated to the molecular events that take place during this stage. Most studies have shown that defence priming is strongly hormonally regulated; however, there is also evidence of the involvement of phenolic derivative compounds and many other secondary metabolites, leading to stronger and faster plant responses. The third priming phase ranges from long lasting defence priming to trans-generational acquired resistance. Long-term metabolic transitions, that occur in the offspring of primed plants, remain to be elucidated. Here we review existing information in the literature that relates to the metabolic changes that occur during all three defence priming stages and highlight the metabolic transitions that are associated with the stimulation of priming and the characteristics of the pathogens whenever possible.

Characterization of evolutionarily conserved motifs involved in activity and regulation of the ABA-INSENSITIVE (ABI) 4 transcription factor

In recent years, the transcription factor ABI4 has emerged as an important node of integration for external and internal signals such as nutrient status and hormone signaling that modulates critical transitions during the growth and development of plants. For this reason, understanding the mechanism of action and regulation of this protein represents an important step towards the elucidation of crosstalk mechanisms in plants. However, this understanding has been hindered due to the negligible levels of this protein as a result of multiple posttranscriptional regulations. To better understand the function and regulation of the ABI4 protein in this work, we performed a functional analysis of several evolutionarily conserved motifs. Based on these conserved motifs, we identified ortholog genes of ABI4 in different plant species. The functionality of the putative ortholog from Theobroma cacao was demonstrated in transient expression assays and in complementation studies in plants. The function of the highly conserved motifs was analyzed after their deletion or mutagenesis in the Arabidopsis ABI4 sequence using mesophyll protoplasts. This approach permitted us to immunologically detect the ABI4 protein and identify some of the mechanisms involved in its regulation. We identified sequences required for the nuclear localization (AP2-associated motif) as well as those for transcriptional activation function (LRP motif). Moreover, this approach showed that the protein stability of this transcription factor is controlled through protein degradation and subcellular localization and involves the AP2-associated and the PEST motifs. We demonstrated that the degradation of ABI4 protein through the PEST motif is mediated by the 26S proteasome in response to changes in the sugar levels.

Overexpression of cotton GhMKK4 enhances disease susceptibility and affects abscisic acid, gibberellin and hydrogen peroxide signalling in transgenic Nicotiana benthamiana

Mitogen-activated protein kinase (MAPK) cascades are involved in plant development, stress responses and hormonal signal transduction. MAPK kinases (MAPKKs), as the key nodes in these cascades, link MAPKs and MAPKK kinases (MAPKKKs). In this study, GhMKK4, a novel group C MAPKK gene from cotton (Gossypium hirsutum), was isolated and identified. Its expression can be induced by various stresses and signalling molecules. The overexpression of GhMKK4 in Nicotiana benthamiana enhanced its susceptibility to bacterial and fungal pathogens, but had no significant effects on salt or drought tolerance. Notably, the overexpressing plants showed increased sensitivity to abscisic acid (ABA) and gibberellin A3 (GA3), and ABA and gibberellin (GA) signalling were affected on infection with Ralstonia solanacearum bacteria. Furthermore, the overexpressing plants showed more reactive oxygen species (ROS) accumulation and stronger inhibition of catalase (CAT), a ROS-scavenging enzyme, than control plants after salicylic acid (SA) treatment. Interestingly, two genes encoding ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMDC), the key enzymes in polyamine synthesis, exhibited reduced R. solanacearum-induced expression in overexpressing plants. These findings broaden our knowledge about the functions of MAPKKs in diverse signalling pathways and the negative regulation of disease resistance in the cotton crop.

A novel role of PR2 in abscisic acid (ABA) mediated, pathogen-induced callose deposition in Arabidopsis thaliana

Pathogenesis-related protein 2 (PR2) is known to play a major role in plant defense and general stress responses. Resistance against the fungal pathogen Leptosphaeria maculans in Arabidopsis requires abscisic acid (ABA), which promotes the deposition of callose, a β-1,3-glucan polymer. Here, we examined the role of PR2 in callose deposition in relation to ABA treatment and challenge with L. maculans and Pseudomonas syringae. Characterization of PR2-overexpressing plants and the knockout line indicated that PR2 negatively affects callose deposition. Recombinant PR2 purified from Pichia pastoris showed callose-degrading activity, and a considerable reduction in the callose-degrading activity was observed in the leaf extract of the PR2 knockout line compared with the wild-type. ABA pretreatment before challenge with L. maculans concomitantly repressed PR2 and enhanced callose accumulation. Likewise, overexpression of an ABA biosynthesis gene NCED3 resulted in reduced PR2 expression and increased callose deposition. We propose that ABA promotes callose deposition through the transcriptional repression of PR2 in Arabidopsis challenged by L. maculans and P. syringae. Callose by itself is likely to act antagonistically on salicylic acid (SA) defense signaling, suggesting that PR2 may function as a modulator of callose- and SA-dependent defense responses.

Vitamin C and the abscisic acid-insensitive 4 transcription factor are important determinants of aphid resistance in Arabidopsis

AIMS

Aphids, like other insects, are probably unable to synthesize vitamin C (ascorbic acid), which is therefore an essential dietary nutrient that has to be obtained from the host plant. Plant responses to aphids involve hormones such as salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA), but hormone/redox interactions remain poorly characterized. We therefore investigated hormone/redox signaling in the response of Arabidopsis thaliana to infestation by the aphid Myzus persicae, focusing on the interactions between ascorbic acid and ABA, together with the influence of altered ascorbate and ABA signaling on the SA- and JA-dependent pathways.

RESULTS

Whole-genome microarray analysis revealed highly dynamic transcriptional responses to aphid infestation with extensive differences between transcript profiles of infested and systemic leaves, revealing aphid-dependent effects on the suites of transcripts involved in the redox, SA, and ABA responses. Central roles for ascorbate, ABA-insensitive 4 (ABI4), and oxidative signal-inducible 1 in plant resistance to aphids were demonstrated by altered fecundity on respective mutants. However, ABA had a negative effect on aphid resistance, as did ABI4 or redox-responsive transcription factor 1. The decrease in aphid fecundity observed in mutants defective in ascorbate accumulation (vtc2) was absent from abi4vtc2 double mutants that are also deficient in ABA signaling (abi4). Aphid-dependent transcriptome responses reveal a role for ascorbate-regulated receptor-like kinases in plant defenses against aphids.

INNOVATION

Vitamin C deficiency enhances plant resistance to aphids through redox signaling pathways rather than dietary requirements.

CONCLUSION

ABI4 is a linchpin of redox regulation of the innate immune response to aphids.

ABI4 downregulates expression of the sodium transporter HKT1;1 in Arabidopsis roots and affects salt tolerance

A plant's ability to cope with salt stress is highly correlated with their ability to reduce the accumulation of sodium ions in the shoot. Arabidopsis mutants affected in the ABSCISIC ACID INSENSITIVE (ABI) 4 gene display increased salt tolerance, whereas ABI4-overexpressors are hypersensitive to salinity from seed germination to late vegetative developmental stages. In this study we demonstrate that abi4 mutant plants accumulate lower levels of sodium ions and higher levels of proline than wild-type plants following salt stress. We show higher HKT1;1 expression in abi4 mutant plants and lower levels of expression in ABI4-overexpressing plants, resulting in reduced accumulation of sodium ions in the shoot of abi4 mutants. HKT1;1 encodes a sodium transporter which is known to unload sodium ions from the root xylem stream into the xylem parenchyma stele cells. We have shown recently that ABI4 is expressed in the root stele at various developmental stages and that it plays a key role in determining root architecture. Thus ABI4 and HKT1;1 are expressed in the same cells, which suggests the possibility of direct binding of ABI4 to the HKT1;1 promoter. In planta chromatin immunoprecipitation and in vitro electrophoresis mobility shift assays demonstrated that ABI4 binds two highly related sites within the HKT1;1 promoter. These sites, GC(C/G)GCTT(T), termed ABI4-binding element (ABE), have also been identified in other ABI4-repressed genes. We therefore suggest that ABI4 is a major modulator of root development and function.

Recognition of avirulence gene AvrLm1 from hemibiotrophic ascomycete Leptosphaeria maculans triggers salicylic acid and ethylene signaling in Brassica napus

Interaction of a plant with a fungal pathogen is an encounter with hundreds of molecules. In contrast to this, a single molecule often decides between the disease and resistance. In the present article, we describe the defense responses triggered by AvrLm1, an avirulence gene from a hemibiotrophic ascomycete, Leptosphaeria maculans, responsible for an incompatible interaction with Brassica napus. Using multiple hormone quantification and expression analysis of defense-related genes, we investigated signaling events in Rlm1 plants infected with two sister isolates of L. maculans differentiated by the presence or absence of AvrLm1. Infection with the isolate carrying AvrLm1 increased the biosynthesis of salicylic acid (SA) and induced expression of the SA-associated genes ICS1, WRKY70, and PR-1, a feature characteristic of responses to biotrophic pathogens and resistance gene-mediated resistance. In addition to SA-signaling elements, we also observed the induction of ASC2a, HEL, and CHI genes associated with ethylene (ET) signaling. Pharmacological experiments confirmed the positive roles of SA and ET in mediating resistance to L. maculans. The unusual cooperation of SA and ET signaling might be a response to the hemibiotrophic nature of L. maculans. Our results also demonstrate the profound difference between the natural host B. napus and the model plant Arabidopsis in their response to L. maculans infection.

Genomic advances will herald new insights into the Brassica: Leptosphaeria maculans pathosystem

The study of the relationship between plants and phytopathogenic fungi is one of the most rapidly moving fields in the plant sciences, the findings of which have contributed to the development of new strategies and technologies to protect crops. Plants employ sophisticated mechanisms to perceive and appropriately defend themselves against pathogens. A good example of plant and pathogen evolution is the gene-for-gene interaction between the fungal pathogen Leptosphaeria maculans, the causal agent of blackleg disease, and Brassica crops. This interaction has been studied at the genetic and physiological level due to its agro-economic importance. The newly available genome sequence for Brassica spp. and L. maculans will provide the resources to study the co-evolution of this plant and pathogen. Particularly, an understanding of the co-evolution of genes responsible for virulence and resistance will lead to improved plant protection strategies for Brassica canola and provide a model to understand plant-pathogen interactions in other major crops. This review summarises the research-to-date in the study of the Brassica-L. maculans gene-for-gene interaction, with a focus on the genetics of resistance in Brassica and the wealth of information to be gained from genome sequencing efforts.

The transcription factor ABI4 Is required for the ascorbic acid-dependent regulation of growth and regulation of jasmonate-dependent defense signaling pathways in Arabidopsis

Cellular redox homeostasis is a hub for signal integration. Interactions between redox metabolism and the ABSCISIC ACID-INSENSITIVE-4 (ABI4) transcription factor were characterized in the Arabidopsis thaliana vitamin c defective1 (vtc1) and vtc2 mutants, which are defective in ascorbic acid synthesis and show a slow growth phenotype together with enhanced abscisic acid (ABA) levels relative to the wild type (Columbia-0). The 75% decrease in the leaf ascorbate pool in the vtc2 mutants was not sufficient to adversely affect GA metabolism. The transcriptome signatures of the abi4, vtc1, and vtc2 mutants showed significant overlap, with a large number of transcription factors or signaling components similarly repressed or induced. Moreover, lincomycin-dependent changes in LIGHT HARVESTING CHLOROPHYLL A/B BINDING PROTEIN 1.1 expression were comparable in these mutants, suggesting overlapping participation in chloroplast to nucleus signaling. The slow growth phenotype of vtc2 was absent in the abi4 vtc2 double mutant, as was the sugar-insensitive phenotype of the abi4 mutant. Octadecanoid derivative-responsive AP2/ERF-domain transcription factor 47 (ORA47) and AP3 (an ABI5 binding factor) transcripts were enhanced in vtc2 but repressed in abi4 vtc2, suggesting that ABI4 and ascorbate modulate growth and defense gene expression through jasmonate signaling. We conclude that low ascorbate triggers ABA- and jasmonate-dependent signaling pathways that together regulate growth through ABI4. Moreover, cellular redox homeostasis exerts a strong influence on sugar-dependent growth regulation.

Arabidopsis ocp3 mutant reveals a mechanism linking ABA and JA to pathogen-induced callose deposition

In the present study, we evaluated the role of the defense-related gene OCP3 in callose deposition as a response to two necrotrophic fungal pathogens, Botrytis cinerea and Plectosphaerella cucumerina. ocp3 plants exhibited accelerated and intensified callose deposition in response to fungal infection associated with enhanced disease resistance to the two pathogens. A series of double mutant analyses showed potentiation of callose deposition and the heightened disease resistance phenotype in ocp3 plants required the plant hormone abscisic acid (ABA) and the PMR4 gene encoding a callose synthase. This finding was congruent with an observation that ocp3 plants exhibited increased ABA accumulation, and ABA was rapidly synthesized following fungal infection in wild-type plants. Furthermore, we determined that potentiation of callose deposition in ocp3 plants, including enhanced disease resistance, also required jasmonic acid (JA) recognition though a COI1 receptor, however JA was not required for basal callose deposition following fungal infection. In addition, potentiation of callose deposition in ocp3 plants appeared to follow a different mechanism than that proposed for callose β-amino-butyric acid (BABA)-induced resistance and priming, because ocp3 plants responded to BABA-induced priming for callose deposition and induced resistance of a magnitude similar to that observed in wild-type plants. Our results point to a model in which OCP3 represents a specific control point for callose deposition regulated by JA yet ultimately requiring ABA. These results provide new insights into the mechanism of callose deposition regulation in response to pathogen attack; however the complexities of the processes remain poorly understood.

HrpN Ea-induced deterrent effect on phloem feeding of the green peach aphid Myzus persicae requires AtGSL5 and AtMYB44 genes in Arabidopsis thaliana

In Arabidopsis thaliana (Arabidopsis) treated with the harpin protein HrpN Ea, resistance to the green peach aphid Myzus persicae, a generalist phloem-feeding insect, develops with induced expression of the AtMYB44 gene. Special GLUCAN SYNTHESIS-LIKE (GSL) genes and beta-1,3-glucan callose play an important role in plant defence responses to attacks by phloem-feeding insects. Here we report that AtGLS5 and AtMYB44 are both required for Hrp Ea-induced repression of M. persicae feeding from the phloem of Arabidopsis leaves. In 24 h successive surveys on large-scale aphid populations, the proportion of feeding aphids was much smaller in HrpN Ea-treated plants than in control plants, and aphids preferred to feed from the 37 tested atgsl mutants rather than the wild-type plant. The atgsl mutants were generated previously by mutagenesis in 12 identified AtGSL genes (AtGSL1 through AtGSL12); in the 24 h survey, both atgsl5 and atgsl6 tolerated aphid feeding, and atgsl5 was the most tolerant. Consistently, atgsl5 was also most inhibitive to the deterrent effect of HrpN Ea on the phloem-feeding activity of aphids as monitored by the electrical penetration graph technique. These results suggested an important role of the AtGSL5 gene in the effect of HrpN Ea. In response to HrpN Ea, AtGSL5 expression and callose deposition were induced in the wild-type plant but not in atgsl5. In response to HrpN Ea, moreover, the AtMYB44 gene known to be required for repression of aphid reproduction on the plant was also required for repression of the phloem-feeding activity. Small amounts of the AtGSL5 transcript and callose deposition were detected in the atmyb44 mutant, as in atgsl5. Both mutants performed similarly in tolerating the phloem-feeding activity and impairing the deterrent effect of HrpN Ea, suggesting that AtGSL5 and AtMYB44 both contributed to the effect.

RLM3, a potential adaptor between specific TIR-NB-LRR receptors and DZC proteins

In our recent paper, we identified a TIR encoding gene, which is required for resistance against a broad range of necrotrophic fungi. Here we present this finding in a broader perspective and discuss the unique features of this gene which might explain its role as a general regulator of resistance responses against a class of pathogens that have previously not been associated to the classical resistance (R) gene type of defense.

Abscisic acid enhances resistance to Alternaria solani in tomato seedlings

The plant hormone abscisic acid (ABA) is an important regulator in many aspects of plant growth and development, as well as stress resistance. Here, we investigated the effects of exogenous ABA application on the interaction between tomato (Solanum lycopersicon L.) and Alternaria solani (early blight). Foliar spraying of 7.58 μM ABA was effective in reducing disease severity in tomato plants. Previously, increased activities of phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO) and peroxidase (POD) were observed in exogenous ABA-treated tomato leaves. Moreover, these enzyme activities were maintained at higher levels in ABA-pretreated and A. solani challenged tomato plants. Tomato defense genes, such as PR1, β-1, 3-glucanase (GLU), PPO, POD, and superoxide dismutase (SOD), were rapidly and significantly up-regulated by exogenous ABA treatment. Furthermore, a subsequent challenge of ABA-pretreated plants with the pathogen A. solani resulted in higher expression of defense genes, compared to water-treated or A. solani inoculated plants. Therefore, our results suggest that exogenous ABA could enhance disease resistance against A. solani infection in tomato through the activation of defense genes and via the enhancement of defense-related enzymatic activities.

Botrytis cinerea manipulates the antagonistic effects between immune pathways to promote disease development in tomato

Plants have evolved sophisticated mechanisms to sense and respond to pathogen attacks. Resistance against necrotrophic pathogens generally requires the activation of the jasmonic acid (JA) signaling pathway, whereas the salicylic acid (SA) signaling pathway is mainly activated against biotrophic pathogens. SA can antagonize JA signaling and vice versa. Here, we report that the necrotrophic pathogen Botrytis cinerea exploits this antagonism as a strategy to cause disease development. We show that B. cinerea produces an exopolysaccharide, which acts as an elicitor of the SA pathway. In turn, the SA pathway antagonizes the JA signaling pathway, thereby allowing the fungus to develop its disease in tomato (Solanum lycopersicum). SA-promoted disease development occurs through Nonexpressed Pathogen Related1. We also show that the JA signaling pathway required for tomato resistance against B. cinerea is mediated by the systemin elicitor. These data highlight a new strategy used by B. cinerea to overcome the plant's defense system and to spread within the host.

Expression of ABSCISIC ACID INSENSITIVE 4 (ABI4) in developing Arabidopsis seedlings

We have recently demonstrated that the transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4) mediates abscisic acid and cytokinin inhibition of lateral root formation by reducing polar auxin transport in Arabidopsis thaliana. ( 1) In that study, we provided a direct demonstration of ABI4 expression in phloem companion cells and parenchyma of the vascular system in the mature regions of the roots. Although also studied in mature plants, ABI4 has been studied primarily in germinating seedlings, and its expression has been assumed by some researchers to be restricted to early germination stages. We thus constructed transgenic Arabidopsis plants expressing an ABI4:GUS construct, and followed ABI4 promoter activity during seedling development, focusing on the roots.

Direct targets of the transcription factors ABA-Insensitive(ABI)4 and ABI5 reveal synergistic action by ABI4 and several bZIP ABA response factors

The plant hormone abscisic acid (ABA) is a key regulator of seed development. In addition to promoting seed maturation, ABA inhibits seed germination and seedling growth. Many components involved in ABA response have been identified, including the transcription factors ABA insensitive (ABI)4 and ABI5. The genes encoding these factors are expressed predominantly in developing and mature seeds, and are positive regulators of ABA mediated inhibition of seed germination and growth. The direct effects of ABI4 and ABI5 in ABA response remain largely undefined. To address this question, plants over-expressing ABI4 or ABI5 were used to allow identification of direct transcriptional targets. Ectopically expressed ABI4 and ABI5 conferred ABA-dependent induction of slightly over 100 genes in 11 day old plants. In addition to effector genes involved in seed maturation and reserve storage, several signaling proteins and transcription factors were identified as targets of ABI4 and/or ABI5. Although only 12% of the ABA- and ABI-dependent transcriptional targets were induced by both ABI factors in 11 day old plants, 40% of those normally expressed in seeds had reduced transcript levels in both abi4 and abi5 mutants. Surprisingly, many of the ABI4 transcriptional targets do not contain the previously characterized ABI4 binding motifs, the CE1 or S box, in their promoters, but some of these interact with ABI4 in electrophoretic mobility shift assays, suggesting that sequence recognition by ABI4 may be more flexible than known canonical sequences. Yeast one-hybrid assays demonstrated synergistic action of ABI4 with ABI5 or related bZIP factors in regulating these promoters, and mutant analyses showed that ABI4 and these bZIPs share some functions in plants.

Hormone crosstalk in plant disease and defense: more than just jasmonate-salicylate antagonism

Until recently, most studies on the role of hormones in plant-pathogen interactions focused on salicylic acid (SA), jasmonic acid (JA), and ethylene (ET). It is now clear that pathogen-induced modulation of signaling via other hormones contributes to virulence. A picture is emerging of complex crosstalk and induced hormonal changes that modulate disease and resistance, with outcomes dependent on pathogen lifestyles and the genetic constitution of the host. Recent progress has revealed intriguing similarities between hormone signaling mechanisms, with gene induction responses often achieved by derepression. Here, we report on recent advances, updating current knowledge on classical defense hormones SA, JA, and ET, and the roles of auxin, abscisic acid (ABA), cytokinins (CKs), and brassinosteroids in molding plant-pathogen interactions. We highlight an emerging theme that positive and negative regulators of these disparate hormone signaling pathways are crucial regulatory targets of hormonal crosstalk in disease and defense.

ABI4 mediates abscisic acid and cytokinin inhibition of lateral root formation by reducing polar auxin transport in Arabidopsis

Key steps in a plant's development and adaptation to the environment are the initiation and development of lateral roots (LRs). LR development is regulated by auxin, the major plant hormone promoting LR formation, its counteracting hormones cytokinin, and abscisic acid (ABA). Here, we show that mutating ABSCISIC ACID INSENSITIVE4 (ABI4), which encodes an ABA-regulated AP2 domain transcription factor, results in an increased number of LRs. We show that ABI4 is expressed in roots and that its overexpression impairs LR development. Root expression of ABI4 is enhanced by ABA, and cytokinin and is repressed by auxin. Using hormone response promoters, we show that ABI4 also affects auxin and cytokinin profiles in the root. Furthermore, LR development in abi4 mutants is not altered or inhibited by cytokinin or ABA. Expression of the auxin-efflux carrier protein PIN1 is reduced in ABI4 overexpressors, enhanced in abi4 mutants, and is less sensitive to inhibition by cytokinin and ABA in abi4 mutants than in wild-type plants. Transport levels of exogenously applied auxin were elevated in abi4 mutants and reduced in ABI4 overexpressors. We therefore suggest that ABI4 mediates ABA and cytokinin inhibition of LR formation via reduction of polar auxin transport and that the resulting decrease in root auxin leads to a reduction in LR development.

Necrotroph attacks on plants: wanton destruction or covert extortion?

Necrotrophic pathogens cause major pre- and post-harvest diseases in numerous agronomic and horticultural crops inflicting significant economic losses. In contrast to biotrophs, obligate plant parasites that infect and feed on living cells, necrotrophs promote the destruction of host cells to feed on their contents. This difference underpins the divergent pathogenesis strategies and plant immune responses to biotrophic and necrotrophic infections. This chapter focuses on Arabidopsis immunity to necrotrophic pathogens. The strategies of infection, virulence and suppression of host defenses recruited by necrotrophs and the variation in host resistance mechanisms are highlighted. The multiplicity of intraspecific virulence factors and species diversity in necrotrophic organisms corresponds to variations in host resistance strategies. Resistance to host-specific necrotophs is monogenic whereas defense against broad host necrotrophs is complex, requiring the involvement of many genes and pathways for full resistance. Mechanisms and components of immunity such as the role of plant hormones, secondary metabolites, and pathogenesis proteins are presented. We will discuss the current state of knowledge of Arabidopsis immune responses to necrotrophic pathogens, the interactions of these responses with other defense pathways, and contemplate on the directions of future research.

Nitric oxide participates in the complex interplay of defense-related signaling pathways controlling disease resistance to Sclerotinia sclerotiorum in Arabidopsis thaliana

Studies of the interaction between Arabidopsis thaliana and the necrotrophic fungal pathogen Sclerotinia sclerotiorum have been hampered by the extreme susceptibility of this model plant to the fungus. In addition, analyses of the plant defense response suggested the implication of a complex interplay of hormonal and signaling pathways. To get a deeper insight into this host-pathogen interaction, we first analyzed the natural variation in Arabidopsis for resistance to S. sclerotiorum. The results revealed a large variation of resistance and susceptibility in Arabidopsis, with some ecotypes, such as Ws-4, Col-0, and Rbz-1, being strongly resistant, and others, such as Shahdara, Ita-0, and Cvi-0, exhibiting an extreme susceptibility. The role of different signaling pathways in resistance was then determined by assessing the symptoms of mutants affected in the perception, production, or transduction of hormonal signals after inoculation with S. sclerotiorum. This analysis led to the conclusions that i) signaling of inducible defenses is predominantly mediated by jasmonic acid and abscisic acid, influenced by ethylene, and independent of salicylic acid; and ii) nitric oxide (NO) and reactive oxygen species are important signals required for plant resistance to S. sclerotiorum. Defense gene expression analysis supported the specific role of NO in defense activation.

Soybean Trihelix transcription factors GmGT-2A and GmGT-2B improve plant tolerance to abiotic stresses in transgenic Arabidopsis

BACKGROUND

Trihelix transcription factors play important roles in light-regulated responses and other developmental processes. However, their functions in abiotic stress response are largely unclear. In this study, we identified two trihelix transcription factor genes GmGT-2A and GmGT-2B from soybean and further characterized their roles in abiotic stress tolerance.

FINDINGS

Both genes can be induced by various abiotic stresses, and the encoded proteins were localized in nuclear region. In yeast assay, GmGT-2B but not GmGT-2A exhibits ability of transcriptional activation and dimerization. The N-terminal peptide of 153 residues in GmGT-2B was the minimal activation domain and the middle region between the two trihelices mediated the dimerization of the GmGT-2B. Transactivation activity of the GmGT-2B was also confirmed in plant cells. DNA binding analysis using yeast one-hybrid assay revealed that GmGT-2A could bind to GT-1bx, GT-2bx, mGT-2bx-2 and D1 whereas GmGT-2B could bind to the latter three elements. Overexpression of the GmGT-2A and GmGT-2B improved plant tolerance to salt, freezing and drought stress in transgenic Arabidopsis plants. Moreover, GmGT-2B-transgenic plants had more green seedlings compared to Col-0 under ABA treatment. Many stress-responsive genes were altered in GmGT-2A- and GmGT-2B-transgenic plants.

CONCLUSION

These results indicate that GmGT-2A and GmGT-2B confer stress tolerance through regulation of a common set of genes and specific sets of genes. GmGT-2B also affects ABA sensitivity.

Studies on the mechanism of resistance to Bipolaris sorokiniana in the barley lesion mimic mutant bst1

SUMMARY The Bipolaris sorokiniana tolerant 1 (bst1) barley mutant is derived from fast neutron-irradiated seeds of wild-type Bowman(Rph3). The induced mutation was genetically localized to a position on chromosome 5HL distal to the centromere using amplified fragment length polymorphism markers. In addition, the defence responses and related gene expression in the bst1 mutant after fungal challenge were compared with those occurring in wild-type plants. Hydrogen peroxide generation, determined by 3,3-diaminobenzidine staining, revealed a clearly reduced level of bst1, compared with the wild-type, during the entire experimental time: 8-120 h post-inoculation (hpi). At 48 hpi, the wild-type samples displayed twice as much fungal mass and three times greater H(2)O(2) production than bst1. At the same time, staining of B. sorokiniana showed less fungal growth in the spontaneous lesions of bst1 compared with the wild-type. Monitoring of defence-related genes at 48 hpi demonstrated strong expression of PR-1a, PR-2, PR-5 and PR-10 in bst1. A gene coding for a unique oxidoreductase enzyme, designated as HCP1, was expressed at much higher levels in inoculated leaves of the bst1 mutant than in those of the wild-type plant. Taken together, the results suggest that the defence to B. sorokiniana largely relies on salicylic acid-responsive pathogenesis-related (PR) genes, as well as selected reactive oxygen species and unknown HCP1-associated factors.

Abscisic acid has a key role in modulating diverse plant-pathogen interactions

We isolated an activation-tagged Arabidopsis (Arabidopsis thaliana) line, constitutive disease susceptibility2-1D (cds2-1D), that showed enhanced bacterial growth when challenged with various Pseudomonas syringae strains. Systemic acquired resistance and systemic PATHOGENESIS-RELATED GENE1 induction were also compromised in cds2-1D. The T-DNA insertion adjacent to NINE-CIS-EPOXYCAROTENOID DIOXYGENASE5 (NCED5), one of six genes encoding the abscisic acid (ABA) biosynthetic enzyme NCED, caused a massive increase in transcript level and enhanced ABA levels >2-fold. Overexpression of NCED genes recreated the enhanced disease susceptibility phenotype. NCED2, NCED3, and NCED5 were induced, and ABA accumulated strongly following compatible P. syringae infection. The ABA biosynthetic mutant aba3-1 showed reduced susceptibility to virulent P. syringae, and ABA, whether through exogenous application or endogenous accumulation in response to mild water stress, resulted in increased bacterial growth following challenge with virulent P. syringae, indicating that ABA suppresses resistance to P. syringae. Likewise ABA accumulation also compromised resistance to the biotrophic oomycete Hyaloperonospora arabidopsis, whereas resistance to the fungus Alternaria brassicicola was enhanced in cds2-1D plants and compromised in aba3-1 plants, indicating that ABA promotes resistance to this necrotroph. Comparison of the accumulation of salicylic acid and jasmonic acid in the wild type, cds2-1D, and aba3-1 plants challenged with P. syringae showed that ABA promotes jasmonic acid accumulation and exhibits a complex antagonistic relationship with salicylic acid. Our findings provide genetic evidence that the abiotic stress signal ABA also has profound roles in modulating diverse plant-pathogen interactions mediated at least in part by cross talk with the jasmonic acid and salicylic acid biotic stress signal pathways.

Cre-lox univector acceptor vectors for functional screening in protoplasts: analysis of Arabidopsis donor cDNAs encoding ABSCISIC ACID INSENSITIVE1-like protein phosphatases

The 14,200 available full length Arabidopsis thaliana cDNAs in the universal plasmid system (UPS) donor vector pUNI51 should be applied broadly and efficiently to leverage a "functional map-space" of homologous plant genes. We have engineered Cre-lox UPS host acceptor vectors (pCR701- 705) with N-terminal epitope tags in frame with the loxH site and downstream from the maize Ubiquitin promoter for use in transient protoplast expression assays and particle bombardment transformation of monocots. As an example of the utility of these vectors, we recombined them with several Arabidopsis cDNAs encoding Ser/Thr protein phosphatase type 2C (PP2Cs) known from genetic studies or predicted by hierarchical clustering meta-analysis to be involved in ABA and stress responses. Our functional results in Zea mays mesophyll protoplasts on ABA-inducible expression effects on the Late Embryogenesis Abundant promoter ProEm:GUS reporter were consistent with predictions and resulted in identification of novel activities of some PP2Cs. Deployment of these vectors can facilitate functional genomics and proteomics and identification of novel gene activities.