Induction of the Hahsp17.7G4 promoter by root-knot nematodes: involvement of heat-shock elements in promoter activity in giant cells

A nematode-inducible promoter can effectively drives RNAi construct to confer Meloidogyne incognita resistance in tomato

KEY MESSAGE

Heterologous expression of a nematode-responsive promoter in tomato successfully driven the RNAi constructs to impart root-knot nematode resistance. The root-knot nematode Meloidogyne incognita seriously afflicts the global productivity of tomatoes. Nematode management options are extremely reliant on chemical methods, however, only a handful of nematicides are commercially available. Additionally, nematodes have developed resistance-breaking phenotypes against the commercially available Mi gene-expressing tomatoes. Nematode resistance in crop plants can be enhanced using the bio-safe RNAi technology, in which plants are genetically modified to express nematode gene-specific dsRNA/siRNA molecules. However, the majority of the RNAi crops conferring nematode tolerance have used constitutive promoters, which have many limitations. In the present study, using promoter-GUS fusion, we functionally validated two nematode-inducible root-specific promoters (pAt1g74770 and pAt2g18140, identified from Arabidopsis thaliana) in the Solanum lycopersicum-M. incognita pathosystem. pAt2g18140 was found to be nematode-responsive during 10-21 days post-inoculation (dpi) and became non-responsive during the late infection stage (28 dpi). In contrast, pAt1g74770 remained nematode-responsive for a longer duration (10-28 dpi). Next, a number of transgenic lines were developed that expressed RNAi constructs (independently targeting the M. incognita integrase and splicing factor genes) driven by the pAt1g74770 promoter. M. incognita parasitic success (measured by multiplication factor ratio) in pAt1g74770:integrase and pAt1g74770:splicing factor RNAi lines were significantly reduced by 60.83-74.93% and 69.34-75.31%, respectively, compared to the control. These data were comparable with the RNAi lines having CaMV35S as the promoter. Further, a long-term RNAi effect was evident, because females extracted from transgenic lines were of deformed shape with depleted transcripts of integrase and splicing factor genes. We conclude that pAt1g74770 can be an attractive alternative to drive localized expression of RNAi constructs rather than using a constitutive promoter. The pAt1g74770-driven gene silencing system can be expanded into different plant-nematode interaction models.

The status of the CRISPR/Cas9 research in plant-nematode interactions

MAIN CONCLUSION

As an important biotic stressor, plant-parasitic nematodes afflict global crop productivity. Deployment of CRISPR/Cas9 system that selectively knock out host susceptibility genes conferred improved nematode tolerance in crop plants. As an important biotic stressor, plant-parasitic nematodes cause a considerable yield decline in crop plants that eventually contributes to a negative impact on global food security. Being obligate plant parasites, the root-knot and cyst nematodes maintain an intricate and sophisticated relationship with their host plants by hijacking the host's physiological and metabolic pathways for their own benefit. Significant progress has been made toward developing RNAi-based transgenic crops that confer nematode resistance. However, the strategy of host-induced gene silencing that targets nematode effectors is likely to fail because the induced silencing of effectors (which interact with plant R genes) may lead to the development of nematode phenotypes that break resistance. Lately, the CRISPR/Cas9-based genome editing system has been deployed to achieve host resistance against bacteria, fungi, and viruses. In these studies, host susceptibility (S) genes were knocked out to achieve resistance via loss of susceptibility. As the S genes are recessively inherited in plants, induced mutations of the S genes are likely to be long-lasting and confer broad-spectrum resistance. A number of S genes contributing to plant susceptibility to nematodes have been identified in Arabidopsis thaliana, rice, tomato, cucumber, and soybean. A few of these S genes were targeted for CRISPR/Cas9-based knockout experiments to improve nematode tolerance in crop plants. Nevertheless, the CRISPR/Cas9 system was mostly utilized to interrogate the molecular basis of plant-nematode interactions rather than direct research toward achieving tolerance in crop plants. The current standalone article summarizes the progress made so far on CRISPR/Cas9 research in plant-nematode interactions.

Gall-specific promoter, an alternative to the constitutive CaMV35S promoter, drives host-derived RNA interference targeting Mi-msp2 gene to confer effective nematode resistance

One of the major obligate plant parasites causing massive economic crop losses belongs to the class of root-knot nematodes (RKNs). Targeting of major nematode parasitism genes via Host Delivered-RNAi (HD-RNAi) to confer silencing is established as one of the most effective approaches to curb nematode infection. Utilizing nematode-responsive root-specific (NRRS) promoters to design a dsRNA molecule targeting approach to hamper nematode parasitism. Here, a previously validated peroxidase gall specific promoter, pAt2g18140, from Arabidopsis was employed to express the dsRNA construct of the nematode effector gene Mi-msp2 from Meloidogyne incognita. Arabidopsis RNAi lines of CaMV35S::Mi-msp2-RNAi and pAt2g18140::Mi-msp2-RNAi were compared with control plants to assess the decrease in plant nematode infection. When subjected to infection, the maximum reductions in the numbers of galls, females and egg masses in the CaMV35S::Mi-msp2-RNAi lines were 61%, 66% and 95%, respectively, whereas for the pAt2g18140::Mi-msp2-RNAi lines, they were 63%, 68% and 100%, respectively. The reduction in transcript level ranged from 79%-82% for CaMV35S::Mi-msp2-RNAi and 72%-79% for the pAt2g18140::Mi-msp2-RNAi lines. Additionally, a reduction in female size and a subsequent reduction in next-generation fecundity demonstrate the efficacy and potential of the gall specific promoter pAt2g18140 for utilization in the development of HD-RNAi constructs against RKN, as an excellent alternative to the CaMV35S promoter.

Essential Roles of Cupredoxin Family Proteins in Soybean Cyst Nematode Resistance

Soybean cyst nematode (SCN, Heterodera glycines), one of the most devastating soybean pathogens, causes a significant yield loss in soybean production. One of the most effective ways to manage SCN is to grow resistant cultivars. Therefore, comparative study using resistant and susceptible soybean cultivars provides a powerful tool to identify new genes involved in soybean SCN resistance. In the present study, a transcriptome analysis was carried out using both the resistant (PI88788) and susceptible (Williams 82) soybean cultivars to characterize the responses to nematode infection. Various defense-related genes and different pathways involved in nematode resistance were recognized as being highly expressed in resistant cultivar. Promoter-GUS analysis was conducted to monitor the spatial expression pattern of the genes highly induced by nematode infection. Two nematode-inducible promoters for Glyma.05g147000 (encoding caffeoyl-CoA O-methyltransferase) and Glyma.06g036700 (encoding cupredoxin superfamily protein) were characterized, and the promoters could efficiently drive the expression of known nematode resistance genes (α-SNAPRhg1HC or GmSHMT) to affect soybean SCN resistance. Interestingly, expression of the cupredoxin family genes was upregulated not only by SCN, but also by jasmonic acid treatment. DNA sequence analysis identified that a conserved motif (GGTGCATG) with high similarity to SCNbox1 and GC-rich element is enriched in their promoter regions, suggesting its potential to serve as a nematode-responsive regulatory element. Overexpression of Glyma.06g036700 significantly enhanced soybean resistance to cyst nematode. Overall, our findings not only highlight the essential role of cupredoxin family genes in SCN resistance, but also offer potential functional tools to develop nematode resistance in crops.

The soybean gene GmHsp22.4 is involved in the resistance response to Meloidogyne javanica in Arabidopsis thaliana

BACKGROUND

Small heat shock proteins (sHSPs) belong to the class of molecular chaperones that respond to biotic and abiotic stresses in plants. A previous study has showed strong induction of the gene GmHsp22.4 in response to the nematode Meloidogyne javanica in a resistant soybean genotype, while repression in a susceptible one. This study aimed to investigate the functional involvement of this small chaperone in response to M. javanica in Arabidopsis thaliana. First, it was evaluated the activation of the promoter region after the nematode inoculation, and the occurrence of polymorphisms between resistant and susceptible re-sequenced soybean accessions. Then functional analysis using A. thaliana lines overexpressing the soybean GmHsp22.4 gene, and knocked-out mutants were challenged with M. javanica infestation.

RESULTS

High expression levels of the GFP gene marker in transformed A. thaliana plants revealed that the promoter region of GmHsp22.4 was strongly activated after nematode inoculation. Moreover, the multiplication of the nematode was significantly reduced in plants overexpressing GmHsp22.4 gene in A. thaliana compared to the wild type. Additionally, the multiplication of M. javanica in the A. thaliana mutants was significantly increased mainly in the event athsp22.0-2. This increase was not that evident in the event athsp22.0-1, the one that preserved a portion of the promoter region, including the HSEs in the region around - 83 bp. However, structural analysis at sequence level among soybean resistant and susceptible genotypes did not detect any polymorphisms in the whole gene model.

CONCLUSIONS

The soybean chaperone GmHsp22.4 is involved in the defense response to root-knot nematode M. javanica in A. thaliana. Specifically, the promoter region covering until - 191 from the transcriptional start site (TSS) is necessary to promoter activation after nematode infection in Arabidopsis. No polymorphisms that could explain these differences in the defense response were detected in the GmHsp22.4 gene between resistant and susceptible soybean genotypes. Therefore, further investigation is needed to elucidate the triggering factor of the plant's defense mechanism, both at the sequence level of the soybean genotypes presenting contrasting reaction to root-knot nematode and by detecting cis-elements that are essential for the activation of the GmHsp22.4 gene promoter.

Root-knot nematodes induce gall formation by recruiting developmental pathways of post-embryonic organogenesis and regeneration to promote transient pluripotency

Root-knot nematodes (RKNs; Meloidogyne spp.) induce new post-embryogenic organs within the roots (galls) where they stablish and differentiate nematode feeding cells, giant cells (GCs). The developmental programmes and functional genes involved remain poorly defined. Arabidopsis root apical meristem (RAM), lateral root (LR) and callus marker lines, SHORT-ROOT/SHR, SCARECROW/SCR, SCHIZORIZA/SCZ, WUSCHEL-RELATED-HOMEOBOX-5/WOX5, AUXIN-RESPONSIVE-FACTOR-5/ARF5, ARABIDOPSIS-HISTIDINE PHOSPHOTRANSFER-PROTEIN-6/AHP6, GATA-TRANSCRIPTION FACTOR-23/GATA23 and S-PHASE-KINASE-ASSOCIATED-PROTEIN2B/SKP2B, were analysed for nematode-dependent expression. Their corresponding loss-of-function lines, including those for LR upstream regulators, SOLITARY ROOT/SLR/IAA14, BONDELOS/BDL/IAA12 and INDOLE-3-ACETIC-ACID-INDUCIBLE-28/IAA28, were tested for RKN resistance/tolerance. LR genes, for example ARF5 (key factor for root stem-cell niche regeneration), GATA23 (which specifies pluripotent founder cells) and AHP6 (cytokinin-signalling-inhibitor regulating pericycle cell-divisions orientation), show a crucial function during gall formation. RKNs do not compromise the number of founder cells or LR primordia but locally induce gall formation possibly by tuning the auxin/cytokinin balance in which AHP6 might be necessary. Key RAM marker genes were induced and functional in galls. Therefore, the activation of plant developmental programmes promoting transient-pluripotency/stemness leads to the generation of quiescent-centre and meristematic-like cell identities within the vascular cylinder of galls. Nematodes enlist developmental pathways of new organogenesis and/or root regeneration in the vascular cells of galls. This should determine meristematic cell identities with sufficient transient pluripotency for gall organogenesis.

Heat Shock Proteins: Dynamic Biomolecules to Counter Plant Biotic and Abiotic Stresses

Due to the present scenario of climate change, plants have to evolve strategies to survive and perform under a plethora of biotic and abiotic stresses, which restrict plant productivity. Maintenance of plant protein functional conformation and preventing non-native proteins from aggregation, which leads to metabolic disruption, are of prime importance. Plant heat shock proteins (HSPs), as chaperones, play a pivotal role in conferring biotic and abiotic stress tolerance. Moreover, HSP also enhances membrane stability and detoxifies the reactive oxygen species (ROS) by positively regulating the antioxidant enzymes system. Additionally, it uses ROS as a signal to molecules to induce HSP production. HSP also enhances plant immunity by the accumulation and stability of pathogenesis-related (PR) proteins under various biotic stresses. Thus, to unravel the entire plant defense system, the role of HSPs are discussed with a special focus on plant response to biotic and abiotic stresses, which will be helpful in the development of stress tolerance in plant crops.

Expression and Localization Patterns of a Small Heat Shock Protein that Interacts with the Helicase Domain of Cucumber Green Mottle Mosaic Virus

Cucumber green mottle mosaic virus (CGMMV), a member of the genus Tobamovirus (family Virgaviridae), is an economically important virus that has detrimental effects on cucurbit crops worldwide. Understanding the interaction between host factors and CGMMV viral proteins will facilitate the design of new strategies for disease control. In this study, a yeast two-hybrid assay revealed that the CGMMV helicase (HEL) domain interacts with a Citrullus lanatus small heat shock protein (sHSP), and we verified this observation by performing in vitro GST pull-down and in vivo coimmunoprecipitation assays. Measurement of the levels of accumulated sHSP transcript revealed that sHSP is upregulated on initial CGMMV infection in both Nicotiana benthamiana and C. lanatus plants, although not in the systemically infected leaves. We also found that the subcellular localization of the sHSP was altered after CGMMV infection. To further validate the role of sHSP in CGMMV infection, we produced and assayed N. benthamiana transgenic plants with up- and down-regulated sHSP expression. Overexpression of sHSP inhibited viral RNA accumulation and retarded disease development, whereas sHSP silencing had no marked effect on CGMMV infection. Therefore, we postulate that the identified sHSP may be one of the factors modulating host defense mechanisms in response to CGMMV infection and that the HEL domain interaction may inhibit this sHSP function to promote viral infection.

Identification, Validation and Utilization of Novel Nematode-Responsive Root-Specific Promoters in Arabidopsis for Inducing Host-Delivered RNAi Mediated Root-Knot Nematode Resistance

The root-knot nematode (RKN), Meloidogyne incognita, is an obligate, sedentary endoparasite that infects a large number of crops and severely affects productivity. The commonly used nematode control strategies have their own limitations. Of late, RNA interference (RNAi) has become a popular approach for the development of nematode resistance in plants. Transgenic crops capable of expressing dsRNAs, specifically in roots for disrupting the parasitic process, offer an effective and efficient means of producing resistant crops. We identified nematode-responsive and root-specific (NRRS) promoters by using microarray data from the public domain and known conserved cis-elements. A set of 51 NRRS genes was identified which was narrowed down further on the basis of presence of cis-elements combined with minimal expression in the absence of nematode infection. The comparative analysis of promoters from the enriched NRRS set, along with earlier reported nematode-responsive genes, led to the identification of specific cis-elements. The promoters of two candidate genes were used to generate transgenic plants harboring promoter GUS constructs and tested in planta against nematodes. Both promoters showed preferential expression upon nematode infection, exclusively in the root in one and galls in the other. One of these NRRS promoters was used to drive the expression of splicing factor, a nematode-specific gene, for generating host-delivered RNAi-mediated nematode-resistant plants. Transgenic lines expressing dsRNA of splicing factor under the NRRS promoter exhibited upto a 32% reduction in number of galls compared to control plants.

A Reliable Protocol for In situ microRNAs Detection in Feeding Sites Induced by Root-Knot Nematodes

Galls induced by Meloidogyne spp. in plant roots are a complex organ formed by heterogeneous tissues; within them there are 5-8 giant cells (GCs) that root-knot nematodes use for their own nurturing. Subtle regulatory mechanisms likely mediate the massive gene repression described at early infection stages in galls, particularly in giant cells. Some of these mechanisms are mediated by microRNAs (miRNAs); hence we describe a reliable protocol to detect miRNAs abundance within the gall tissues induced by Meloidogyne spp. Some methods are available to determine the abundance of specific miRNAs in different plant parts; however, galls are complex organs formed by different tissues. Therefore, detection of miRNAs at the cellular level is particularly important to understand specific regulatory mechanisms operating within the GCs. In situ hybridization (ISH) is a classical, robust and accurate method that allows the localization of specific RNAs directly on plant tissues. We present for the first time an adapted and standardized ISH protocol to detect miRNAs in GCs induced by nematodes based on tissue embedded in paraffin and on-slide ISH of miRNAs. It can be adapted to any laboratory with no more requirements than a microtome and an optical microscope and it takes 10 days to perform once plant material has been collected. It showed to be very valuable for a quick detection of miRNAs expression pattern in tomato. We tested the protocol for miR390, as massive sequencing analysis showed that miR390 was induced at 3 dpi (days post-infection) in Arabidopsis galls and miR390 is 100% conserved between Arabidopsis and tomato. Successful localization of miR390 in tomato GCs constitutes a validation of this method that could be easily extended to other crops and/or syncytia induced by cyst nematodes. Finally, the protocol also includes guidance on troubleshooting.

Phenotyping nematode feeding sites: three-dimensional reconstruction and volumetric measurements of giant cells induced by root-knot nematodes in Arabidopsis

The control of plant parasitic nematodes is an increasing problem. A key process during the infection is the induction of specialized nourishing cells, called giant cells (GCs), in roots. Understanding the function of genes required for GC development is crucial to identify targets for new control strategies. We propose a standardized method for GC phenotyping in different plant genotypes, like those with modified genes essential for GC development. The method combines images obtained by bright-field microscopy from the complete serial sectioning of galls with TrakEM2, specialized three-dimensional (3D) reconstruction software for biological structures. The volumes and shapes from 162 3D models of individual GCs induced by Meloidogyne javanica in Arabidopsis were analyzed for the first time along their life cycle. A high correlation between the combined volume of all GCs within a gall and the total area occupied by all the GCs in the section/s where they show maximum expansion, and a proof of concept from two Arabidopsis transgenic lines (J0121 ≫ DTA and J0121 ≫ GFP) demonstrate the reliability of the method. We phenotyped GCs and developed a reliable simplified method based on a two-dimensional (2D) parameter for comparison of GCs from different Arabidopsis genotypes, which is also applicable to galls from different plant species and in different growing conditions, as thickness/transparency is not a restriction.

Temporal and spatial control of gene expression in horticultural crops

Biotechnology provides plant breeders an additional tool to improve various traits desired by growers and consumers of horticultural crops. It also provides genetic solutions to major problems affecting horticultural crops and can be a means for rapid improvement of a cultivar. With the availability of a number of horticultural genome sequences, it has become relatively easier to utilize these resources to identify DNA sequences for both basic and applied research. Promoters play a key role in plant gene expression and the regulation of gene expression. In recent years, rapid progress has been made on the isolation and evaluation of plant-derived promoters and their use in horticultural crops, as more and more species become amenable to genetic transformation. Our understanding of the tools and techniques of horticultural plant biotechnology has now evolved from a discovery phase to an implementation phase. The availability of a large number of promoters derived from horticultural plants opens up the field for utilization of native sequences and improving crops using precision breeding. In this review, we look at the temporal and spatial control of gene expression in horticultural crops and the usage of a variety of promoters either isolated from horticultural crops or used in horticultural crop improvement.

A role for LATERAL ORGAN BOUNDARIES-DOMAIN 16 during the interaction Arabidopsis-Meloidogyne spp. provides a molecular link between lateral root and root-knot nematode feeding site development

Plant endoparasitic nematodes induce the formation of their feeding cells by injecting effectors from the esophageal glands into root cells. Although vascular cylinder cells seem to be involved in the formation of root-knot nematode (RKN) feeding structures, molecular evidence is scarce. We address the role during gall development of LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16), a key component of the auxin pathway leading to the divisions in the xylem pole pericycle (XPP) for lateral root (LR) formation. Arabidopsis T-DNA tagged J0192 and J0121 XPP marker lines, LBD16 and DR5::GUS promoter lines, and isolated J0192 protoplasts were assayed for nematode-dependent gene expression. Infection tests in LBD16 knock-out lines were used for functional analysis. J0192 and J0121 lines were activated in early developing galls and giant cells (GCs), resembling the pattern of the G2/M-transition specific ProC yc B 1;1 :CycB1;1(NT)-GUS line. LBD16 was regulated by auxins in galls as in LRs, and induced by RKN secretions. LBD16 loss of function mutants and a transgenic line with defective XPP cells showed a significantly reduced infection rate. The results show that genes expressed in the dividing XPP, particularly LBD16, are important for gall formation, as they are for LR development.

Genome-wide analysis of the Hsp20 gene family in soybean: comprehensive sequence, genomic organization and expression profile analysis under abiotic and biotic stresses

BACKGROUND

The Hsp20 genes are associated with stress caused by HS and other abiotic factors, but have recently been found to be associated with the response to biotic stresses. These genes represent the most abundant class among the HSPs in plants, but little is known about this gene family in soybean. Because of their apparent multifunctionality, these proteins are promising targets for developing crop varieties that are better adapted to biotic and abiotic stresses. Thus, in the present study an in silico identification of GmHsp20 gene family members was performed, and the genes were characterized and subjected to in vivo expression analysis under biotic and abiotic stresses.

RESULTS

A search of the available soybean genome databases revealed 51 gene models as potential GmHsp20 candidates. The 51 GmHsp20 genes were distributed across a total of 15 subfamilies where a specific predicted secondary structure was identified. Based on in vivo analysis, only 47 soybean Hsp20 genes were responsive to heat shock stress. Among the GmHsp20 genes that were potentials HSR, five were also cold-induced, and another five, in addition to one GmAcd gene, were responsive to Meloidogyne javanica infection. Furthermore, one predicted GmHsp20 was shown to be responsive only to nematode infection; no expression change was detected under other stress conditions. Some of the biotic stress-responsive GmHsp20 genes exhibited a divergent expression pattern between resistant and susceptible soybean genotypes under M. javanica infection. The putative regulatory elements presenting some conservation level in the GmHsp20 promoters included HSE, W-box, CAAT box, and TA-rich elements. Some of these putative elements showed a unique occurrence pattern among genes responsive to nematode infection.

CONCLUSIONS

The evolution of Hsp20 family in soybean genome has most likely involved a total of 23 gene duplications. The obtained expression profiles revealed that the majority of the 51 GmHsp20 candidates are induced under HT, but other members of this family could also be involved in normal cellular functions, unrelated to HT. Some of the GmHsp20 genes might be specialized to respond to nematode stress, and the predicted promoter structure of these genes seems to have a particular conserved pattern related to their biological function.

Activation of geminivirus V-sense promoters in roots is restricted to nematode feeding sites

Obligate sedentary endoparasitic nematodes, such as the root-knot and cyst nematodes, elicit the differentiation of specialized nematode nurse or feeding cells [nematode feeding sites (NFS), giant cells and syncytia, respectively]. During NFS differentiation, marked changes in cell cycle progression occur, partly similar to those induced by some geminiviruses. In this work, we describe the activation of V-sense promoters from the Maize streak virus (MSV) and Wheat dwarf virus (WDV) in NFS formed by root-knot and cyst nematodes. Both promoters were transiently active in microinjection experiments. In tobacco and Arabidopsis transgenic lines carrying promoter-beta-glucuronidase fusions, the MSV V-sense promoter was activated in the vascular tissues of aerial plant parts, primarily leaf and cotyledon phloem tissue and some floral structures. Interestingly, in roots, promoter activation was restricted to syncytia and giant cells tested with four different nematode populations, but undetectable in the rest of the root system. As the activity of the promoter in transgenic rootstocks should be restricted to NFS only, the MSV promoter may have utility in engineering grafted crops for nematode control. Therefore, this study represents a step in the provision of some of the much needed additional data on promoters with restricted activation in NFS useful in biotechnological nematode control strategies.

Early transcriptomic events in microdissected Arabidopsis nematode-induced giant cells

Root-knot nematodes differentiate highly specialized feeding cells in roots (giant cells, GCs), through poorly characterized mechanisms that include extensive transcriptional changes. While global transcriptome analyses have used galls, which are complex root structures that include GCs and surrounding tissues, no global gene expression changes specific to GCs have been described. We report on the differential transcriptome of GCs versus root vascular cells, induced in Arabidopsis by Meloidogyne javanica at a very early stage of their development, 3 days after infection (d.p.i.). Laser microdissection was used to capture GCs and root vascular cells for microarray analysis, which was validated through qPCR and by a promoter-GUS fusion study. Results show that by 3 d.p.i., GCs exhibit major gene repression. Although some genes showed similar regulation in both galls and GCs, the majority had different expression patterns, confirming the molecular distinctiveness of the GCs within the gall. Most of the differentially regulated genes in GCs have no previously assigned function. Comparisons with other transcriptome analyses revealed similarities between GCs and cell suspensions differentiating into xylem cells. This suggests a molecular link between GCs and developing vascular cells, which represent putative GC stem cells. Gene expression in GCs at 3 d.p.i. was also found to be similar to crown galls induced by Agrobacterium tumefaciens, a specialized root biotroph.

Functional characterization of transcripts expressed in early-stage Meloidogyne javanica-induced giant cells isolated by laser microdissection

The root-knot nematode Meloidogyne javanica induces giant cells and feeds from them during its development and reproduction. To study the cellular processes underlying the formation of giant cells, laser microdissection was used to isolate the contents of early-stage giant cells 4 and 7 days post-infection (dpi) from tomato, and cDNA libraries from both stages were generated with 87 [250 expressed sequence tag (EST) clones] and 54 (309 EST clones) individual transcripts identified, respectively. These transcripts have roles in metabolism, stress response, protein synthesis, cell division and morphogenesis, transport, signal transduction, protein modification and fate, and regulation of cellular processes. The expression of 25 selected transcripts was studied further by real-time quantitative reverse transcriptase-polymerase chain reaction. Among them, 13 showed continuous up-regulation in giant cells from 4 to 7 dpi. The expression of two transcripts was higher than in controls at 4 dpi and remained at the same level at 7 dpi; a further five transcripts were highly expressed only at 7 dpi. The Phi-1 protein gene, a cell cycle-related homologue in tobacco, was expressed 8.5 times more strongly in giant cells than in control cells at 4 dpi, but was reduced to 6.7 times at 7 dpi. Using in situ hybridization, the expression of the Phi-1 gene was preferentially localized in the cytoplasm of giant cells at 4 dpi, together with a pectinesterase U1 precursor gene. The identification of highly expressed transcripts in developing giant cells adds to the knowledge of the plant genes responsive to nematode infection, and may provide candidate genes for nematode control strategies.

Distinct heat-shock element arrangements that mediate the heat shock, but not the late-embryogenesis induction of small heat-shock proteins, correlate with promoter activation in root-knot nematode feeding cells

Genes coding small heat-shock proteins (sHSPs) show distinct behaviours with respect to environmental and developmental signals. Their transcriptional regulation depends on particular combinations of heat stress cis-elements (heat-shock elements; HSEs) but many aspects regarding their regulation remain unclear. Cyst and root-knot nematodes induce, in the roots of infected plants, the differentiation of special feeding cells with high metabolic activity (syncytia and giant cells, respectively), a process accompanied by extensive gene expression changes. The Hahsp17.7G4 (G4) promoter was active in giant cells and its HSE arrangements were crucial for this activation. In the present work, we provide further basis to associate giant cell expression with the heat-shock response of this gene class, by analysing additional promoters. The Hahsp17.6G1 (G1) promoter, not induced by heat shock, was silent in giant cells, while Hahsp18.6G2 (G2), which responds to heat shock, was specifically induced in giant cells. In addition, a mutated Hahsp17.7G4 promoter version (G4MutP) with a strong heat-shock induction was also induced in giant cells. The responses of the different promoters correlated with distinct HSE configurations, which might have implications on differential trans-activation. Furthermore, the shortest giant cell and heat-shock-inducible sHSP promoter version analysed in tobacco (-83pb Hahsp17.7G4) fully maintained its expression profile in Arabidopsis. Cyst nematodes did not induce the Hahsp17.7G4 promoter, revealing additional specificity in the nematode response. These findings, together with the fact that the class I sHSP products of endogenous genes accumulated specifically in tobacco giant cells, support the idea that these nematode-induced giant cells represent a transcriptional state very similar to that produced by heat shock regarding this class of genes. The high metabolic rate of giant cells may result in unfolded proteins requiring class I sHSPs as chaperones, which might, somehow, mimic heat-shock and/or other stress responses.

Genome-wide expression profiling of the host response to root-knot nematode infection in Arabidopsis

During a compatible interaction, root-knot nematodes (Meloidogyne spp.) induce the redifferentiation of root cells into multinucleate nematode feeding cells (giant cells). Hyperplasia and hypertrophy of the surrounding cells leads to the formation of a root gall. We investigated the plant response to root-knot nematodes by carrying out a global analysis of gene expression during gall formation in Arabidopsis, using giant cell-enriched root tissues. Among 22 089 genes monitored with the complete Arabidopsis transcriptome microarray gene-specific tag, we identified 3373 genes that display significant differential expression between uninfected root tissues and galls at different developmental stages. Quantitative PCR analysis and the use of promoter GUS fusions confirmed the changes in mRNA levels observed in our microarray analysis. We showed that a comparable number of genes were found to be up- and downregulated, indicating that gene downregulation might be essential to allow proper gall formation. Moreover, many genes belonging to the same family are differently regulated in feeding cells. This genome-wide overview of gene expression during plant-nematode interaction provides new insights into nematode feeding-cell formation, and highlights that the suppression of plant defence is associated with nematode feeding-site development.

Loss of susceptibility as an alternative for nematode resistance

Among plant pathogens, sedentary endoparasitic nematodes are one of the most damaging pests in global agriculture. These obligate parasites interact with their hosts in a quite unique and intriguing way. They induce the redifferentiation of root cells into specialized feeding cells essential for nematode growth and reproduction; thus, nematodes have evolved the ability to exploit plant genes and hijack host functions for their own requirements. Various approaches to engineer plants with resistance to parasitic nematodes have been pursued, most focusing on the introduction of resistance genes. An alternative strategy to achieve resistance is to exploit the susceptibility of plant disease. Better knowledge of the plant response during the compatible interaction should allow the identification of targets to engineer resistance to parasitic nematodes in crop species.