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

Switching perspectives: The roles of plant cellular reprogramming during nematode parasitism

We propose exploring plant biotrophic parasitism from both a pathogen‐centred and a plant‐centred perspective. This can generate novel research questions and reveal common plant mitigation strategies in response to biotrophic pathogens.

A lateral organ boundaries domain transcription factor acts downstream of the auxin response factor 2 to control nodulation and root architecture in Medicago truncatula

Legume plants develop two types of root postembryonic organs, lateral roots and symbiotic nodules, using shared regulatory components. The module composed by the microRNA390, the Trans-Acting SIRNA3 (TAS3) RNA and the Auxin Response Factors (ARF)2, ARF3, and ARF4 (miR390/TAS3/ARFs) mediates the control of both lateral roots and symbiotic nodules in legumes. Here, a transcriptomic approach identified a member of the Lateral Organ Boundaries Domain (LBD) family of transcription factors in Medicago truncatula, designated MtLBD17/29a, which is regulated by the miR390/TAS3/ARFs module. ChIP-PCR experiments evidenced that MtARF2 binds to an Auxin Response Element present in the MtLBD17/29a promoter. MtLBD17/29a is expressed in root meristems, lateral root primordia, and noninfected cells of symbiotic nodules. Knockdown of MtLBD17/29a reduced the length of primary and lateral roots and enhanced lateral root formation, whereas overexpression of MtLBD17/29a produced the opposite phenotype. Interestingly, both knockdown and overexpression of MtLBD17/29a reduced nodule number and infection events and impaired the induction of the symbiotic genes Nodulation Signaling Pathway (NSP) 1 and 2. Our results demonstrate that MtLBD17/29a is regulated by the miR390/TAS3/ARFs module and a direct target of MtARF2, revealing a new lateral root regulatory hub recruited by legumes to act in the root nodule symbiotic program.

Functional studies of plant transcription factors and their relevance in the plant root-knot nematode interaction

Root-knot nematodes are polyphagous parasitic nematodes that cause severe losses in the agriculture worldwide. They enter the root in the elongation zone and subtly migrate to the root meristem where they reach the vascular cylinder and establish a feeding site called gall. Inside the galls they induce a group of transfer cells that serve to nurture them along their parasitic stage, the giant cells. Galls and giant cells develop through a process of post-embryogenic organogenesis that involves manipulating different genetic regulatory networks within the cells, some of them through hijacking some molecular transducers of established plant developmental processes, such as lateral root formation or root regeneration. Galls/giant cells formation involves different mechanisms orchestrated by the nematode´s effectors that generate diverse plant responses in different plant tissues, some of them include sophisticated mechanisms to overcome plant defenses. Yet, the plant-nematode interaction is normally accompanied to dramatic transcriptomic changes within the galls and giant cells. It is therefore expected a key regulatory role of plant-transcription factors, coordinating both, the new organogenesis process induced by the RKNs and the plant response against the nematode. Knowing the role of plant-transcription factors participating in this process becomes essential for a clear understanding of the plant-RKNs interaction and provides an opportunity for the future development and design of directed control strategies. In this review, we present the existing knowledge of the TFs with a functional role in the plant-RKN interaction through a comprehensive analysis of current scientific literature and available transcriptomic data.

Transcription factor WOX11 modulates tolerance to cyst nematodes via adventitious lateral root formation

The transcription factor WUSCHEL-RELATED HOMEOBOX 11 (WOX11) in Arabidopsis (Arabidopsis thaliana) initiates the formation of adventitious lateral roots upon mechanical injury in primary roots. Root-invading nematodes also induce de novo root organogenesis leading to excessive root branching, but it is not known if this symptom of disease involves mediation by WOX11 and if it benefits the plant. Here, we show with targeted transcriptional repression and reporter gene analyses in Arabidopsis that the beet cyst nematode Heterodera schachtii activates WOX11-mediated adventitious lateral rooting from primary roots close to infection sites. The activation of WOX11 in nematode-infected roots occurs downstream of jasmonic acid-dependent damage signaling via ETHYLENE RESPONSE FACTOR109, linking adventitious lateral root formation to nematode damage to host tissues. By measuring different root system components, we found that WOX11-mediated formation of adventitious lateral roots compensates for nematode-induced inhibition of primary root growth. Our observations further demonstrate that WOX11-mediated rooting reduces the impact of nematode infections on aboveground plant development and growth. Altogether, we conclude that the transcriptional regulation by WOX11 modulates root system plasticity under biotic stress, which is one of the key mechanisms underlying the tolerance of Arabidopsis to cyst nematode infections.

The root-knot nematode effector Mi2G02 hijacks a host plant trihelix transcription factor to promote nematode parasitism

Root-knot nematodes (RKNs) cause huge agricultural losses every year. They secrete a repertoire of effectors to facilitate parasitism through the induction of plant-derived giant feeding cells, which serve as their sole source of nutrients. However, the mode of action of these effectors and their targeted host proteins remain largely unknown. In this study, we investigated the role of the effector Mi2G02 in Meloidogyne incognita parasitism. Host-derived Mi2G02 RNA interference in Arabidopsis thaliana affected giant cell development, whereas ectopic expression of Mi2G02 promoted root growth and increased plant susceptibility to M. incognita. We used various combinations of approaches to study the specific interactions between Mi2G02 and A. thaliana GT-3a, a trihelix transcription factor. GT-3a knockout in A. thaliana affected feeding-site development, resulting in production of fewer egg masses, whereas GT-3a overexpression in A. thaliana increased susceptibility to M. incognita and also root growth. Moreover, we demonstrated that Mi2G02 plays a role in maintaining GT-3a protein stabilization by inhibiting the 26S proteasome-dependent pathway, leading to suppression of TOZ and RAD23C expression and thus promoting nematode parasitism. This work enhances our understanding of how a pathogen effector manipulates the role and regulation of a transcription factor by interfering with a proteolysis pathway to reprogram gene expression for development of nematode feeding cells.

Metabolic variations in root tissues and rhizosphere soils of weak host plants potently lead to distinct host status and chemotaxis regulation of Meloidogyne incognita in intercropping

Root-knot nematodes (RKNs) inflict extensive damage to global agricultural production. Intercropping has been identified as a viable agricultural tool for combating RKNs, but the mechanisms by which intercropped plants modulate RKN parasitism are still not well understood. Here, we focus on the cucumber-amaranth intercropping system. We used a range of approaches, including the attraction assay, in vitro RNA interference (RNAi), untargeted metabolomics, and hairy root transformation, to unveil the mechanisms by which weak host plants regulate Meloidogyne incognita chemotaxis towards host plants and control infection. Amaranth roots showed a direct repellence to M. incognita through disrupting its chemotaxis. The in vitro RNAi assay demonstrated that the Mi-flp-1 and Mi-flp-18 genes (encoding FMRFamide-like peptides) regulated M. incognita chemotaxis towards cucumber and controlled infection. Moreover, M. incognita infection stimulated cucumber and amaranth to accumulate distinct metabolites in both root tissues and rhizosphere soils. In particular, naringenin and salicin, enriched specifically in amaranth rhizosphere soils, inhibited the expression of Mi-flp-1 and Mi-flp-18. In addition, overexpression of genes involved in the biosynthesis of pantothenic acid and phloretin, both of which were enriched specifically in amaranth root tissues, delayed M. incognita development in cucumber hairy roots. Together, our results reveal that both the distinct host status and disruption of chemotaxis contribute to M. incognita inhibition in intercropping.

Transcriptome Analysis of GmPUB20A Overexpressing and RNA-Interferencing Transgenic Hairy Roots Reveals Underlying Negative Role in Soybean Resistance to Cyst Nematode

Ubiquitination genes are key components of plant responses to biotic stress. GmPUB20A, a ubiquitination gene, plays a negative role in soybean resistance to soybean cyst nematode (SCN). In this study, we employed high-throughput sequencing to investigate transcriptional changes in GmPUB20A overexpressing and RNA-interfering transgenic hairy roots. Totally, 7661 differentially expressed genes (DEGs) were identified. Gene Ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses revealed that DEGs were significantly enriched in disease resistance and signal transduction pathways. In addition, silencing Glyma.15G021600 and Glyma.09G284700 by siRNA, the total number of nematodes was decreased by 33.48% and 27.47% than control plants, respectively. Further, GUS activity and reactive oxygen species (ROS) assays revealed that GmPUB20A, Glyma.15G021600, and Glyma.09G284700 respond to SCN parasitism and interfere with the accumulation of ROS in plant roots, respectively. Collectively, our study provides insights into the molecular mechanism of GmPUB20A in soybean resistance to SCN.

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.

Genome-wide identification and analysis of Lateral Organ Boundaries Domain (LBD) transcription factor gene family in melon (Cucumis melo L.)

Background

Lateral Organ Boundaries Domain (LBD) transcription factor (TF) gene family members play very critical roles in several biological processes like plant-spesific development and growth process, tissue regeneration, different biotic and abiotic stress responses in plant tissues and organs. The LBD genes have been analyzed in various species. Melon (Cucumis melo L.), a member of the Cucurbitaceae family, is economically important and contains important molecules for nutrition and human health such as vitamins A and C, β-carotenes, phenolic acids, phenolic acids, minerals and folic acid. However, no studies have been reported so far about LBD genes in melon hence this is the first study for LBD genes in this plant.

Results

In this study, 40 melon CmLBD TF genes were identified, which were separated into seven groups through phylogenetic analysis. Cis-acting elements showed that these genes were associated with plant growth and development, phytohormone and abiotic stress responses. Gene Ontology (GO) analysis revealed that of CmLBD genes especially function in regulation and developmental processes. The in silico and qRT-PCR expression patterns demonstrated that CmLBD01 and CmLBD18 are highly expressed in root and leaf tissues, CmLBD03 and CmLBD14 displayed a high expression in male-female flower and ovary tissues.

Conclusions

These results may provide important contributions for future research on the functional characterization of the melon LBD gene family and the outputs of this study can provide information about the evolution and characteristics of melon LBD gene family for next studies.

AUXIN RESPONSIVE FACTOR8 regulates development of the feeding site induced by root-knot nematodes in tomato

Root-knot nematodes (RKN) from the genus Meloidogyne induce the dedifferentiation of root vascular cells into giant multinucleate feeding cells. These feeding cells result from an extensive reprogramming of gene expression, and auxin is known to be a key player in their development. However, little is known about how the auxin signal is transmitted during giant cell development. Integrative analyses combining transcriptome and small non-coding RNA datasets with the specific sequencing of cleaved transcripts identified genes targeted by miRNAs in tomato (Solanum lycopersicum) galls. The two auxin-responsive transcription factors ARF8A and ARF8B, and their miRNA167 regulators, were identified as robust gene-miRNA pair candidates to be involved in the tomato response to M. incognita. Spatiotemporal expression analysis using promoter-β-glucuronidase (GUS) fusions showed the up-regulation of ARF8A and ARF8B in RKN-induced feeding cells and surrounding cells. The generation and phenotyping of CRISPR (clustered regularly interspaced palindromic repeats) mutants demonstrated the role of ARF8A and ARF8B in giant cell development and allowed the characterization of their downstream regulated genes.

Investigation of the Effect of the Auxin Antagonist PEO-IAA on Cannabinoid Gene Expression and Content in Cannabis sativa L. Plants under In Vitro Conditions

The in vitro shoot propagation of Cannabis sativa L. is an emerging research area for large-scale plant material production. However, how in vitro conditions influence the genetic stability of maintained material, as well as whether changes in the concentration and composition of secondary metabolites can be expected are aspects that need to be better understood. These features are essential for the standardised production of medicinal cannabis. This work aimed to find out whether the presence of the auxin antagonist α-(2-oxo-2-phenylethyl)-1H-indole-3-acetic acid (PEO-IAA) in the culture media influenced the relative gene expression (RGE) of the genes of interest (OAC, CBCA, CBDA, THCA) and the concentrations of studied cannabinoids (CBCA, CBDA, CBC, ∆⁹-THCA, and ∆⁹-THC). Two C. sativa cultivars, 'USO-31' and 'Tatanka Pure CBD', were cultivated by in vitro conditions with PEO-IAA presence and then analysed. The RT-qPCR results indicated that even though some changes in the RGE profiles could be observed, no differences were statistically significant compared with the control variant. The results of the phytochemical analyses demonstrate that although there were some differences from the control variant, only the cultivar 'Tatanka Pure CBD' showed a statistically significant increase (at a statistical significance level α = 0.05) in the concentration of the cannabinoid CBDA. In conclusion, it would appear that using PEO-IAA in the culture medium is a suitable approach to improve in vitro cannabis multiplication.

Peat-based hairy root transformation using Rhizobium rhizogenes as a rapid and efficient tool for easily exploring potential genes related to root-knot nematode parasitism and host response

BACKGROUND

Root-knot nematodes (RKNs) pose a worldwide threat to agriculture of many crops including cucumber. Genetic transformation (GT) has emerged as a powerful tool for exploration of plant-RKN interactions and genetic improvement of RKN resistance. However, it is usually difficult to achieve a highly efficient and stable GT protocol for most crops due to the complexity of this process.

RESULTS

Here we firstly applied the hairy root transformation system in exploring root-RKN interactions in cucumber plants and developed a rapid and efficient tool transformation using Rhizobium rhizogenes strain K599. A solid-medium-based hypocotyl-cutting infection (SHI) method, a rockwool-based hypocotyl-cutting infection (RHI) method, and a peat-based cotyledon-node injection (PCI) method was evaluated for their ability to induce transgenic roots in cucumber plants. The PCI method generally outperformed the SHI and RHI methods for stimulating more transgenic roots and evaluating the phenotype of roots during nematode parasitism. Using the PCI method, we generated the CRISPR/Cas9-mediated malate synthase (MS) gene (involved in biotic stress responses) knockout plant and the LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16, a potential host susceptibility gene for RKN) promoter-driven GUS expressing plant. Knockout of MS in hairy roots resulted in effective resistance against RKNs, while nematode infection induced a strong expression of LBD16-driven GUS in root galls. This is the first report of a direct link between these genes and RKN performance in cucumber.

CONCLUSION

Taken together, the present study demonstrates that the PCI method allows fast, easy and efficient in vivo studies of potential genes related to root-knot nematode parasitism and host response.

Divergent regulation of auxin responsive genes in root-knot and cyst nematodes feeding sites formed in Arabidopsis

Cysts (CNs) and root-knot nematodes (RKNs) induce specialized feeding cells, syncytia, and giant cells (GCs), respectively, within plant roots. The plant tissues around the GCs usually by respond forming a root swelling called a gall that contains the GCs. The ontogenesis of feeding cells is different. GC formation is a process of new organogenesis from vascular cells, which are still not well characterized, that differentiate into GCs. In contrast, syncytia formation involves the fusion of adjacent cells that have already differentiated. Nonetheless, both feeding sites show an auxin maximum pertinent to feeding site formation. However, data on the molecular divergences and similarities between the formation of both feeding sites regarding auxin-responsive genes are still scarce. We studied genes from the auxin transduction pathways that are crucial during gall and lateral root (LR) development in the CN interaction by using promoter-reporter (GUS/LUC)transgenic lines, as well as loss of function lines of Arabidopsis. The promoters pGATA23 and several deletions of pmiR390a were active in syncytia, as were in galls, but pAHP6 or putative up-stream regulators as ARF5/7/19 were not active in syncytia. Additionally, none of these genes seemed to play a key role during cyst nematode establishment in Arabidopsis, as the infection rates in loss of function lines did not show significant differences compared to control Col-0 plants. Furthermore, the presence of only canonical AuxRe elements in their proximal promoter regions is highly correlated with their activation in galls/GCs (AHP6, LBD16), but those promoters active in syncytia (miR390, GATA23) carry AuxRe overlapping core cis-elements for other transcription factor families (i.e., bHLH, bZIP). Strikingly, in silico transcriptomic analysis showed very few genes upregulated by auxins common to those induced in GCs and syncytia, despite the high number of upregulated IAA responsive genes in syncytia and galls. The complex regulation of auxin transduction pathways, where different members of the auxin response factor (ARF) family may interact with other factors, and the differences in auxin sensitivity, as indicated by the lower induction of the DR5 sensor in syncytia than galls, among other factors, may explain the divergent regulation of auxin responsive genes in the two types of nematode feeding sites.

Root architecture plasticity in response to endoparasitic cyst nematodes is mediated by damage signaling

Plant root architecture plasticity in response to biotic stresses has not been thoroughly investigated. Infection by endoparasitic cyst nematodes induces root architectural changes that involve the formation of secondary roots at infection sites. However, the molecular mechanisms regulating secondary root formation in response to cyst nematode infection remain largely unknown. We first assessed whether secondary roots form in a nematode density-dependent manner by challenging wild-type Arabidopsis plants with increasing numbers of cyst nematodes (Heterodera schachtii). Next, using jasmonate-related reporter lines and knockout mutants, we tested whether tissue damage by nematodes triggers jasmonate-dependent secondary root formation. Finally, we verified whether damage-induced secondary root formation depends on local auxin biosynthesis at nematode infection sites. Intracellular host invasion by H. schachtii triggers a transient local increase in jasmonates, which activates the expression of ERF109 in a COI1-dependent manner. Knockout mutations in COI1 and ERF109 disrupt the nematode density-dependent increase in secondary roots observed in wild-type plants. Furthermore, ERF109 regulates secondary root formation upon H. schachtii infection via local auxin biosynthesis. Host invasion by H. schachtii triggers secondary root formation via the damage-induced jasmonate-dependent ERF109 pathway. This points at a novel mechanism underlying plant root plasticity in response to biotic stress.

Local auxin synthesis mediated by YUCCA4 induced during root-knot nematode infection positively regulates gall growth and nematode development

Parasites and pathogens are known to manipulate the host's endogenous signaling pathways to facilitate the infection process. In particular, plant-parasitic root-knot nematodes (RKN) are known to elicit auxin response at the infection sites, to aid the development of root galls as feeding sites for the parasites. Here we describe the role of local auxin synthesis induced during RKN infection. Exogenous application of auxin synthesis inhibitors decreased RKN gall formation rates, gall size and auxin response in galls, while auxin and auxin analogues produced the opposite effects, re-enforcing the notion that auxin positively regulates RKN gall formation. Among the auxin biosynthesis enzymes, YUCCA4 (YUC4) was found to be dramatically up-regulated during RKN infection, suggesting it may be a major contributor to the auxin accumulation during gall formation. However, yuc4-1 showed only very transient decrease in gall auxin levels and did not show significant changes in RKN infection rates, implying the loss of YUC4 is likely compensated by other auxin sources. Nevertheless, yuc4-1 plants produced significantly smaller galls with fewer mature females and egg masses, confirming that auxin synthesized by YUC4 is required for proper gall formation and RKN development within. Interestingly, YUC4 promoter was also activated during cyst nematode infection. These lines of evidence imply auxin biosynthesis from multiple sources, one of them being YUC4, is induced upon plant endoparasitic nematode invasion and likely contribute to their infections. The coordination of these different auxins adds another layer of complexity of hormonal regulations during plant parasitic nematode interaction.

Comparative Transcriptome Analysis Reveals the Specific Activation of Defense Pathways Against Globodera pallida in Gpa2 Resistant Potato Roots

Cyst nematodes are considered a dominant threat to yield for a wide range of major food crops. Current control strategies are mainly dependent on crop rotation and the use of resistant cultivars. Various crops exhibit single dominant resistance (R) genes that are able to activate effective host-specific resistance to certain cyst nematode species and/or populations. An example is the potato R gene Gpa2, which confers resistance against the potato cyst nematode (PCN), Globodera pallida population D383. Activation of Gpa2 results in a delayed resistance response, which is characterized by a layer of necrotic cells formed around the developing nematode feeding structure. However, knowledge about the Gpa2-induced defense pathways is still lacking. Here, we uncover the transcriptional changes and gene expression network induced upon Gpa2 activation in potato roots infected with G. pallida. To this end, in vitro-grown Gpa2-resistant potato roots were infected with the avirulent population D383 and virulent population Rookmaker. Infected root segments were harvested at 3 and 6 dpi and sent for RNA sequencing. Comparative transcriptomics revealed a total of 1,743 differentially expressed genes (DEGs) upon nematode infection, of which 559 DEGs were specifically regulated in response to D383 infection. D383-specific DEGs associated with Gpa2-mediated defense mainly relates to calcium-binding activity, salicylic acid (SA) biosynthesis, and systemic acquired resistance (SAR). These data reveal that cyst nematode resistance in potato roots depends on conserved downstream signaling pathways involved in plant immunity, which are also known to contribute to R genes-mediated resistance against other pathogens with different lifestyles.

Genome-Wide Analysis of LBD Transcription Factor Genes in Dendrobiumcatenatum

The LATERAL ORGAN BOUNDARIES DOMAIN (LBD) gene family comprises plant-specific transcription factors that control cell proliferation and differentiation during growth and development in many plant species. However, to date, no studies of the LBD gene family in Dendrobium catenatum have been reported. In this study, a genome-wide analysis of LBD genes was performed in D. catenatum and 24 LBD genes were identified. The genes were classified into two classes (I and II) based on phylogenetic relationships and motif structure. Subcellular localization analysis for DcaLBD6 and DcaLBD18 from class I and DcaLBD37 and DcaLBD41 from class II revealed that the proteins were localized in the nucleus. Transient expression analysis of DcaLBD6, DcaLBD18, DcaLBD37, and DcaLBD41 indicated that class I and class II members have opposite roles in regulating VASCULAR-RELATED NAC-DOMAIN 7 (VND7) expression. DcaLBD genes showed diverse expression patterns in response to different phytohormone treatments. Heat maps revealed diverse patterns of DcaLBD gene expression in different organs. These results lay the foundation for further detailed studies of the LBD gene family in D. catenatum.

PUCHI Regulates Giant Cell Morphology During Root-Knot Nematode Infection in Arabidopsis thaliana

Parasitic root-knot nematodes transform the host's vascular cells into permanent feeding giant cells (GCs) to withdraw nutrients from the host plants. GCs are multinucleated metabolically active cells with distinctive cell wall structures; however, the genetic regulation of GC formation is largely unknown. In this study, the functions of the Arabidopsis thaliana transcription factor PUCHI during GC development were investigated. PUCHI expression was shown to be induced in early developing galls, suggesting the importance of the PUCHI gene in gall formation. Despite the puchi mutant not differing significantly from the wild type in nematode invasion and reproduction rates, puchi GC cell walls appeared to be thicker and lobate when compared to the wild type, while the cell membrane sometimes formed invaginations. In three-dimensional (3D) reconstructions of puchi GCs, they appeared to be more irregularly shaped than those in the wild type, with noticeable cell-surface protrusions and folds. Interestingly, the loss-of-function mutant of 3-KETOACYL-COA SYNTHASE 1 showed GC morphology and cell wall defects similar to those of the puchi mutant, suggesting that PUCHI may regulate GC development via very long chain fatty acid synthesis.

Tomato Divinyl Ether-Biosynthesis Pathway Is Implicated in Modulating of Root-Knot Nematode Meloidogyne javanica's Parasitic Ability

The role of the 9-lipoxygenase (9-LOX)-derived oxylipins in plant defense is mainly known in solanaceous plants. In this work, we identify the functional role of the tomato divinyl ether synthase (LeDES) branch, which exclusively converts 9-hydroperoxides to the 9-divinyl ethers (DVEs) colneleic acid (CA) and colnelenic acid (CnA), during infection by the root-knot nematode Meloidogyne javanica. Analysis of LeDES expression in roots indicated a concurrent response to nematode infection, demonstrating a sharp increase in expression during the molting of third/fourth-stage juveniles, 15 days after inoculation. Spatiotemporal expression analysis using an LeDES promoter:GUS tomato line showed high GUS activity associated with the developing gall; however the GUS signal became more constricted as infection progressed to the mature nematode feeding sites, and eventually disappeared. Wounding did not activate the LeDES promoter, but auxins and methyl salicylate triggered LeDES expression, indicating a hormone-mediated function of DVEs. Heterologous expression of LeDES in Arabidopsis thaliana rendered the plants more resistant to nematode infection and resulted in a significant reduction in third/fourth-stage juveniles and adult females as compared to a vector control and the wild type. To further evaluate the nematotoxic activity of the DVEs CA and CnA, recombinant yeast that catalyzes the formation of CA and CnA from 9-hydroperoxides was generated. Transgenic yeast accumulating CnA was tested for its impact on M. javanica juveniles, indicating a decrease in second-stage juvenile motility. Taken together, our results suggest an important role for LeDES as a determinant in the defense response during M. javanica parasitism, and indicate two functional modes: directly via DVE motility inhibition effect and through signal molecule-mediated defense reactions to nematodes that depend on methyl salicylate.

Genome-wide identification and characterization of the Lateral Organ Boundaries Domain (LBD) gene family in polyploid wheat and related species

BACKGROUND

Wheat (Triticum aestivum) originated from three different diploid ancestral grass species and experienced two rounds of polyploidization. Exploring how certain wheat gene subfamilies have expanded during the evolutionary process is of great importance. The Lateral Organ Boundaries Domain (LBD) gene family encodes plant-specific transcription factors that share a highly conserved LOB domain and are prime candidates for this, as they are involved in plant growth, development, secondary metabolism and stress in various species.

METHODS

Using a genome-wide analysis of high-quality polyploid wheat and related species genome sequences, a total of 228 LBD members from five Triticeae species were identified, and phylogenetic relationship analysis of LBD members classified them into two main classes (classes I and II) and seven subgroups (classes I a-e, II a and II b).

RESULTS

The gene structure and motif composition analyses revealed that genes that had a closer phylogenetic relationship in the same subgroup also had a similar gene structure. Macrocollinearity and microcollinearity analyses of Triticeae species suggested that some LBD genes from wheat produced gene pairs across subgenomes of chromosomes 4A and 5A and that the complex evolutionary history of TaLBD4B-9 homologs was a combined result of chromosome translocation, polyploidization, gene loss and duplication events. Public RNA-seq data were used to analyze the expression patterns of wheat LBD genes in various tissues, different developmental stages and following abiotic and biotic stresses. Furthermore, qRT-PCR results suggested that some TaLBDs in class II responded to powdery mildew, regulated reproductive growth and were involved in embryo sac development in common wheat.

Genome-wide identification and expression analysis of LBD transcription factor genes in Moso bamboo (Phyllostachys edulis)

BACKGROUND

Moso bamboo, the fastest growing plant on earth, is an important source for income in large areas of Asia, mainly cultivated in China. Lateral organ boundaries domain (LBD) proteins, a family of transcription factors unique to plants, are involved in multiple transcriptional regulatory pathways and play important roles in lateral organ development, pathogen response, secondary growth, and hormone response. The LBD gene family has not previously been characterized in moso bamboo (Phyllostachys edulis).

RESULTS

In this study, we identified 55 members of the LBD gene family from moso bamboo and found that they were distributed non-uniformly across its 18 chromosomes. Phylogenetic analysis showed that the moso bamboo LBD genes could be divided into two classes. LBDs from the same class share relatively conserved gene structures and sequences encoding similar amino acids. A large number of hormone response-associated cis-regulatory elements were identified in the LBD upstream promoter sequences. Synteny analysis indicated that LBDs in the moso bamboo genome showed greater collinearity with those of O. sativa (rice) and Zea mays (maize) than with those of Arabidopsis and Capsicum annuum (pepper). Numerous segmental duplicates were found in the moso bamboo LBD gene family. Gene expression profiles in four tissues showed that the LBD genes had different spatial expression patterns. qRT-PCR assays with the Short Time-series Expression Miner (STEM) temporal expression analysis demonstrated that six genes (PeLBD20, PeLBD29, PeLBD46, PeLBD10, PeLBD38, and PeLBD06) were consistently up-regulated during the rapid growth and development of bamboo shoots. In addition, 248 candidate target genes that function in a variety of pathways were identified based on consensus LBD binding motifs.

CONCLUSIONS

In the current study, we identified 55 members of the moso bamboo transcription factor LBD and characterized for the first time. Based on the short-time sequence expression software and RNA-seq data, the PeLBD gene expression was analyzed. We also investigated the functional annotation of all PeLBDs, including PPI network, GO, and KEGG enrichment based on String database. These results provide a theoretical basis and candidate genes for studying the molecular breeding mechanism of rapid growth of moso bamboo.

Sedentary Plant-Parasitic Nematodes Alter Auxin Homeostasis via Multiple Strategies

Sedentary endoparasites such as cyst and root-knot nematodes infect many important food crops and are major agro-economical pests worldwide. These plant-parasitic nematodes exploit endogenous molecular and physiological pathways in the roots of their host to establish unique feeding structures. These structures function as highly active transfer cells and metabolic sinks and are essential for the parasites' growth and reproduction. Plant hormones like indole-3-acetic acid (IAA) are a fundamental component in the formation of these feeding complexes. However, their underlying molecular and biochemical mechanisms are still elusive despite recent advances in the field. This review presents a comprehensive overview of known functions of various auxins in plant-parasitic nematode infection sites, based on a systematic analysis of current literature. We evaluate multiple aspects involved in auxin homeostasis in plants, including anabolism, catabolism, transport, and signalling. From these analyses, a picture emerges that plant-parasitic nematodes have evolved multiple strategies to manipulate auxin homeostasis to establish a successful parasitic relationship with their host. Additionally, there appears to be a potential role for auxins other than IAA in plant-parasitic nematode infections that might be of interest to be further elucidated.

CsLOB1 regulates susceptibility to citrus canker through promoting cell proliferation in citrus

Citrus sinensis lateral organ boundary 1 (CsLOB1) was previously identified as a critical disease susceptibility gene for citrus bacterial canker, which is caused by Xanthomonas citri subsp. citri (Xcc). However, the molecular mechanisms of CsLOB1 in citrus response to Xcc are still elusive. Here, we constructed transgenic plants overexpressing and RNAi-silencing of CsLOB1 using the canker-disease susceptible 'wanjincheng' orange (C. sinensis Osbeck) as explants. CsLOB1-overexpressing plants exhibited dwarf phenotypes with smaller and thicker leaf, increased branches and adventitious buds clustered on stems. These phenotypes were followed by a process of pustule- and canker-like development that exhibited enhanced cell proliferation. Pectin depolymerization and expansin accumulation were enhanced by CsLOB1 overexpression, while cellulose and hemicellulose synthesis were increased by CsLOB1 silence. Whilst overexpression of CsLOB1 increased susceptibility, RNAi-silencing of CsLOB1 enhanced resistance to canker disease without impairing pathogen entry. Transcriptome analysis revealed that CsLOB1 positively regulated cell wall degradation and modification processes, cytokinin metabolism, and cell division. Additionally, 565 CsLOB1-targeted genes were identified in chromatin immunoprecipitation-sequencing (ChIP-seq) experiments. Motif discovery analysis revealed that the most highly overrepresented binding sites had a conserved 6-bp 'GCGGCG' consensus DNA motif. RNA-seq and ChIP-seq data suggested that CsLOB1 directly activates the expression of four genes involved in cell wall remodeling, and three genes that participate in cytokinin and brassinosteroid hormone pathways. Our findings indicate that CsLOB1 promotes cell proliferation by mechanisms depending on cell wall remodeling and phytohormone signaling, which may be critical to citrus canker development and bacterial growth in citrus.

Identification of genes involved in Meloidogyne incognita-induced gall formation processes in Arabidopsis thaliana

Root-knot nematodes (RKN; Meloidogyne incognita) are phytoparasitic nematodes that cause significant damage to crop plants worldwide. Recent studies have revealed that RKNs disrupt various physiological processes in host plant cells to induce gall formation. However, little is known about the molecular mechanisms of gall formation induced by nematodes. We have previously found that RNA expression levels of some of genes related to micro-RNA, cell division, membrane traffic, vascular formation, and meristem maintenance system were modified by nematode infection. Here we evaluated these genes importance during nematode infection by using Arabidopsis mutants and/or β-glucronidase (GUS) marker genes, particularly after inoculation with nematodes, to identify the genes involved in successful nematode infection. Our results provide new insights not only for the basic biology of plant-nematode interactions but also to improve nematode control in an agricultural setting.

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.

Phylogeny and Functions of LOB Domain Proteins in Plants

Lateral organ boundaries (LOB) domain (LBD) genes, a gene family encoding plant-specific transcription factors, play important roles in plant growth and development. At present, though there have been a number of genome-wide analyses on LBD gene families and functional studies on individual LBD proteins, the diverse functions of LBD family members still confuse researchers and an effective strategy is required to summarize their functional diversity. To further integrate and improve our understanding of the phylogenetic classification, functional characteristics and regulatory mechanisms of LBD proteins, we review and discuss the functional characteristics of LBD proteins according to their classifications under a phylogenetic framework. It is proved that this strategy is effective in the anatomy of diverse functions of LBD family members. Additionally, by phylogenetic analysis, one monocot-specific and one eudicot-specific subclade of LBD proteins were found and their biological significance in monocot and eudicot development were also discussed separately. The review will help us better understand the functional diversity of LBD proteins and facilitate further studies on this plant-specific transcription factor family.

Genome-wide identification and characterization of the lateral organ boundaries domain gene family in Brassica rapa var. rapa

The Lateral Organ Boundaries Domain (LBD) genes encode highly conserved plant-specific LOB domain proteins which regulate growth and development in various species. However, members of the LBD gene family have yet to be identified in Brassica rapa var. rapa. In the present study, fifty-nine LBD genes were identified and distributed on 10 chromosomes. The BrrLBD proteins are predicted to encode hydrophobic polypeptides between 118 and 394 amino acids in length and with molecular weights ranging from 13.31 to 44.24 kDa; the theoretical pI for these proteins varies from 4.83 to 9.68. There were 17 paralogous gene pairs in the BrrLBD family, suggesting that the amplification of the BrrLBD gene family involved large-scale gene duplication events. Members of the BrrLBD family were divided into 7 subclades (class I a to e, class II a and b). Analysis of gene structure and conserved domains revealed that most BrrLBD genes of the same subclade had similar gene structures and protein motifs. The expression profiles of 59 BrrLBD genes were determined through Quantitative Real-time fluorescent PCR (qRT-PCR). Most BrrLBD genes in the same subclade had similar gene expression profiles. However, the expression patterns of 7 genes differed from their duplicates, indicating that although the gene function of most BrrLBD genes has been conserved, some BrrLBD genes may have undergone evolutionary change.

A role for ALF4 during gall and giant cell development in the biotic interaction between Arabidopsis and Meloidogyne spp

Root-knot nematodes (RKNs; Meloidogyne spp.) are a major pest for the agriculture worldwide. RKNs induce specialized feeding cells (giant cells, GCs) inside galls which are de novo formed pseudo-organs in the roots that share similarities with other developmental processes as lateral root (LR) and callus formation or grafting involving new vascular development or pericycle proliferation. Hence, it is pertinent to study the molecular mechanisms directing the plant-nematode interaction. In this respect, ALF4 is a key gene during LR formation, vascular vessels reconnection in grafting, hormone-induced callus formation or de novo root organogenesis from leaf explants. Our results show that ALF4 is also induced in galls at early infection stages in an auxin-independent way. Furthermore, ALF4 activity is necessary for the formation of proper galls and GCs, as the mutant alf4-1 presents aberrant galls and GCs with severe structural abnormalities leading to a dramatic reduction in the nematode egg production. However, a low-reproduction rate is maintained, that might be explained by the local auxin maximum build by the nematodes in galls, partially rescuing alf4-1 phenotype. This would be similar to the partial rescue described for LR formation with exogenous auxins and also agrees with the LR emergence from alf4-1 galls but not from uninfected roots. In addition, ALF4 is also induced in syncytia formed by cyst nematodes. All these data support a pivotal role for ALF4 during de novo organogenesis processes induced by endoparasitic nematodes, in addition to its role in LR formation, callus development or vessel reconnection during grafting.

Pivotal role of LBD16 in root and root-like organ initiation

In the post-embryonic stage of Arabidopsis thaliana, roots can be initiated from the vascular region of the existing roots or non-root organs; they are designated as lateral roots (LRs) and adventitious roots (ARs), respectively. Some root-like organs can also be initiated from the vasculature. In tissue culture, auxin-induced callus, which is a group of pluripotent root-primordium-like cells, is formed via the rooting pathway. The formation of feeding structures from the vasculature induced by root-knot nematodes also borrows the rooting pathway. In this review, we summarize and discuss recent progress on the role of LATERAL ORGAN BOUNDARIES DOMAIN16 (LBD16; also known as ASYMMETRIC LEAVES2-LIKE18, ASL18), a member of the LBD/ASL gene family encoding plant-specific transcription factors, in roots and root-like organ initiation. Different root and root-like organ initiation processes have distinct priming mechanisms to specify founder cells. All these priming mechanisms converge to activate LBD16 expression in the primed founder cells. The activation of LBD16 expression leads to organ initiation via promotion of cell division and establishment of root-primordium identity. Therefore, LBD16 might play a common and pivotal role in root and root-like organ initiation.

The root-knot nematode Meloidogyne incognita produces a functional mimic of the Arabidopsis INFLORESCENCE DEFICIENT IN ABSCISSION signaling peptide

INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) is a signaling peptide that regulates cell separation in Arabidopsis including floral organ abscission and lateral root emergence. IDA is highly conserved in dicotyledonous flowering plant genomes. IDA-like sequences were also found in the genomic sequences of root-knot nematodes, Meloidogyne spp., which are globally deleterious pathogens of agriculturally important plants, but the role of these genes is unknown. Exogenous treatment of the Arabidopsis ida mutant with synthetic peptide identical to the M. incognita IDA-like 1 (MiIDL1) protein sequence minus its N-terminal signal peptide recovered both the abscission and root architecture defects. Constitutive expression of the full-length MiIDL1 open reading frame in the ida mutant substantially recovered the delayed floral organ abscission phenotype whereas transformants expressing a construct missing the MiIDL1 signal peptide retained the delayed abscission phenotype. Importantly, wild-type Arabidopsis plants harboring an MiIDL1-RNAi construct and infected with nematodes had approximately 40% fewer galls per root than control plants. Thus, the MiIDL1 gene produces a functional IDA mimic that appears to play a role in successful gall development on Arabidopsis roots.

Transcriptome analysis of root-knot nematode (Meloidogyne incognita)-infected tomato (Solanum lycopersicum) roots reveals complex gene expression profiles and metabolic networks of both host and nematode during susceptible and resistance responses

Root-knot nematodes (RKNs, Meloidogyne incognita) are economically important endoparasites with a wide host range. We used a comprehensive transcriptomic approach to investigate the expression of both tomato and RKN genes in tomato roots at five infection time intervals from susceptible plants and two infection time intervals from resistant plants, grown under soil conditions. Differentially expressed genes during susceptible (1827, tomato; 462, RKN) and resistance (25, tomato; 160, RKN) interactions were identified. In susceptible responses, tomato genes involved in cell wall structure, development, primary and secondary metabolite, and defence signalling pathways, together with RKN genes involved in host parasitism, development and defence, are discussed. In resistance responses, tomato genes involved in secondary metabolite and hormone-mediated defence responses, together with RKN genes involved in starvation stress-induced apoptosis, are discussed. In addition, 40 novel differentially expressed RKN genes encoding secretory proteins were identified. Our findings provide novel insights into the temporal regulation of genes involved in various biological processes from tomato and RKN simultaneously during susceptible and resistance responses, and reveal the involvement of a complex network of biosynthetic pathways during disease development.

A role for the gene regulatory module microRNA172/TARGET OF EARLY ACTIVATION TAGGED 1/FLOWERING LOCUS T (miRNA172/TOE1/FT) in the feeding sites induced by Meloidogyne javanica in Arabidopsis thaliana

Root knot nematodes (RKNs) penetrate into the root vascular cylinder, triggering morphogenetic changes to induce galls, de novo formed 'pseudo-organs' containing several giant cells (GCs). Distinctive gene repression events observed in early gall/GCs development are thought to be mediated by post-transcriptional silencing via microRNAs (miRNAs), a process that is far from being fully characterized. Arabidopsis thaliana backgrounds with altered activities based on target 35S::MIMICRY172 (MIM172), 35S::TARGET OF EARLY ACTIVATION TAGGED 1 (TOE1)-miR172-resistant (35S::TOE1^R ) and mutant (flowering locus T-10 (ft-10)) lines were used for functional analysis of nematode infective and reproductive parameters. The GUS-reporter lines, MIR172A-E::GUS, treated with auxin (IAA) and an auxin-inhibitor (a-(phenyl ethyl-2-one)-indole-3-acetic acid (PEO-IAA)), together with the MIR172C AuxRE::GUS line with two mutated auxin responsive elements (AuxREs), were assayed for nematode-dependent gene expression. Arabidopsis thaliana backgrounds with altered expression of miRNA172, TOE1 or FT showed lower susceptibility to the RKNs and smaller galls and GCs. MIR172C-D::GUS showed restricted promoter activity in galls/GCs that was regulated by auxins through auxin-responsive factors. IAA induced their activity in galls while PEO-IAA treatment and mutations in AuxRe motifs abolished it. The results showed that the regulatory module miRNA172/TOE1/FT plays an important role in correct GCs and gall development, where miRNA172 is modulated by auxins.

LBD14/ASL17 Positively Regulates Lateral Root Formation and is Involved in ABA Response for Root Architecture in Arabidopsis

The LATERAL ORGAN BOUNDARIES (LOB) DOMAIN/ASYMMETRIC LEAVES2-LIKE (LBD/ASL) gene family members play key roles in diverse aspects of plant development. Previous studies have shown that LBD16, 18, 29 and 33 are critical for integrating the plant hormone auxin to control lateral root development in Arabidopsis thaliana. In the present study, we show that LBD14 is expressed exclusively in the root where it promotes lateral root (LR) emergence. Repression of LBD14 expression by ABA correlates with the inhibitory effects of ABA on LR emergence. Transient gene expression assays with Arabidopsis protoplasts demonstrated that LBD14 is a nuclear-localized transcriptional activator. The knock-down of LBD14 expression by RNA interference (RNAi) resulted in reduced LR formation by delaying both LR primordium development and LR emergence, whereas overexpression of LBD14 in Arabidopsis enhances LR formation. We show that ABA (but not other plant hormones such as auxin, brassinosteroids and cytokinin) specifically down-regulated β-glucuronidase (GUS) expression under the control of the LBD14 promoter in transgenic Arabidopsis during LR development from initiation to emergence and endogenous LBD14 transcript levels in the root. Moreover, RNAi of LBD14 enhanced the LR suppression in response to ABA, whereas LBD14 overexpression did not alter the ABA-mediated suppression of LR formation. Taken together, these results suggest that LBD14 promoting LR formation is one of the critical factors regulated by ABA to inhibit LR growth, contributing to the regulation of the Arabidopsis root system architecture in response to ABA.

The LATERAL ORGAN BOUNDARIES Domain gene family in grapevine: genome-wide characterization and expression analyses during developmental processes and stress responses

LATERAL ORGAN BOUNDARIES (LOB) DOMAIN (LBD) constitute a family of plant-specific transcription factors with key roles in the regulation of plant organ development, pollen development, plant regeneration, pathogen response, and anthocyanin and nitrogen metabolisms. However, the role of LBDs in fruit ripening and in grapevine (Vitis vinifera L.) development and stress responses is poorly documented. By performing a model curation of LBDs in the latest genome annotation 50 genes were identified. Phylogenetic analysis showed that LBD genes can be grouped into two classes mapping on 16 out of the 19 V. vinifera chromosomes. New gene subclasses were identified that have not been characterized in other species. Segmental and tandem duplications contributed significantly to the expansion and evolution of the LBD gene family in grapevine as noticed for other species. The analysis of cis-regulatory elements and transcription factor binding sites in the VviLBD promoter regions suggests the involvement of several hormones in the regulation of LBDs expression. Expression profiling suggest the involvement of LBD transcription factors in grapevine development, berry ripening and stress responses. Altogether this study provides valuable information and robust candidate genes for future functional analysis aiming to clarify mechanisms responsible for the onset of fruit ripening and fruit defense strategies.

Anatomical Alterations in Plant Tissues Induced by Plant-Parasitic Nematodes

Plant-parasitic nematodes (PPNs) interact with plants in different ways, for example, through subtle feeding behavior, migrating destructively through infected tissues, or acting as virus-vectors for nepoviruses. They are all obligate biotrophic parasites as they derive their nutrients from living cells which they modify using pharyngeal gland secretions prior to food ingestion. Some of them can also shield themselves against plant defenses to sustain a relatively long lasting interaction while feeding. This paper is centered on cell types or organs that are newly induced in plants during PPN parasitism, including recent approaches to their study based on molecular biology combined with cell biology-histopathology. This issue has already been reviewed extensively for major PPNs (i.e., root-knot or cyst nematodes), but not for other genera (viz. Nacobbus aberrans, Rotylenchulus spp.). PPNs have evolved with plants and this co-evolution process has allowed the induction of new types of plant cells necessary for their parasitism. There are four basic types of feeding cells: (i) non-hypertrophied nurse cells; (ii) single giant cells; (iii) syncytia; and (iv) coenocytes. Variations in the structure of these cells within each group are also present between some genera depending on the nematode species viz. Meloidogyne or Rotylenchulus. This variability of feeding sites may be related in some way to PPN life style (migratory ectoparasites, sedentary ectoparasites, migratory ecto-endoparasites, migratory endoparasites, or sedentary endoparasites). Apart from their co-evolution with plants, the response of plant cells and roots are closely related to feeding behavior, the anatomy of the nematode (mainly stylet size, which could reach different types of cells in the plant), and the secretory fluids produced in the pharyngeal glands. These secretory fluids are injected through the stylet into perforated cells where they modify plant cytoplasm prior to food removal. Some species do not produce specialized feeding sites (viz. Ditylenchus, Subanguina), but may develop a specialized modification of the root system (e.g., unspecialized root galls or a profusion of roots). This review introduces new data on cell types and plant organs stimulated by PPNs using sources varying from traditional histopathology to new holistic methodologies.

Genomewide analysis of the lateral organ boundaries domain gene family in Vitis vinifera

In plants, the transcription factor families have been implicated in many important biological processes. These processes include morphogenesis, signal transduction and environmental stress responses. Proteins containing the lateral organ boundaries domain (LBD), which encodes a zinc finger-like domain are only found in plants. This finding indicates that this unique gene family regulates only plant-specific biological processes. LBD genes play crucial roles in the growth and development of plants such as Arabidopsis, Oryza sativa, Zea mays, poplar, apple and tomato. However, relatively little is known about the LBD genes in grape (Vitis vinifera). In this study, we identified 40 LBD genes in the grape genome. A complete overview of the chromosomal locations, phylogenetic relationships, structures and expression profiles of this gene family during development in grape is presented here. Phylogenetic analysis showed that the LBD genes could be divided into classes I and II, together with LBDs from Arabidopsis. We mapped the 40 LBD genes on the grape chromosomes (chr1-chr19) and found that 37 of the predicted grape LBD genes were distributed in different densities across 12 chromosomes. Grape LBDs were found to share a similar intron/exon structure and gene length within the same class. The expression profiles of grape LBD genes at different developmental stages were analysed using microarray data. Results showed that 21 grape LBD genes may be involved in grape developmental processes, including preveraison, veraison and ripening. Finally, we analysed the expression patterns of six LBD genes through quantitative real-time polymerase chain reation analysis. The six LBD genes showed differential expression patterns among the three representative grape tissues, and five of these genes were found to be involved in responses to mannitol, sodium chloride, heat stress and low temperature treatments. To our knowledge, this is the first study to analyse the LBD gene family in grape and provides valuable information for classification and functional investigation of this gene family.

Molecular Transducers from Roots Are Triggered in Arabidopsis Leaves by Root-Knot Nematodes for Successful Feeding Site Formation: A Conserved Post-Embryogenic De novo Organogenesis Program?

Root-knot nematodes (RKNs; Meloidogyne spp.) induce feeding cells (giant cells; GCs) inside a pseudo-organ (gall) from still unknown root cells. Understanding GCs ontogeny is essential to the basic knowledge of RKN-plant interaction and to discover novel and effective control strategies. Hence, we report for the first time in a model plant, Arabidopsis, molecular, and cellular features concerning ectopic de novo organogenesis of RKNs GCs in leaves. RKNs induce GCs in leaves with irregular shape, a reticulated cytosol, and fragmented vacuoles as GCs from roots. Leaf cells around the nematode enter G2-M shown by ProCycB1;1:CycB1;1(NT)-GUS expression, consistent to multinucleated GCs. In addition, GCs nuclei present irregular and varied sizes. All these characteristics mentioned, being equivalent to GCs in root-galls. RKNs complete their life cycle forming a gall/callus-like structure in the leaf vascular tissues resembling auxin-induced callus with an auxin-response maxima, indicated by high expression of DR5::GUS that is dependent on leaf auxin-transport. Notably, induction of leaves calli/GCs requires molecular components from roots crucial for lateral roots (LRs), auxin-induced callus and root-gall formation, i.e., LBD16. Hence, LBD16 is a xylem pole pericycle specific and local marker in LR primordia unexpectedly induced locally in the vascular tissue of leaves after RKN infection. LBD16 is also fundamental for feeding site formation as RKNs could not stablish in 35S::LBD16-SRDX leaves, and likely it is also a conserved molecular hub between biotic and developmental signals in Arabidopsis either in roots or leaves. Moreover, RKNs induce the ectopic development of roots from leaf and root-galls, also formed in mutants compromised in LR formation, arf7/arf19, slr, and alf4. Therefore, nematodes must target molecular signatures to induce post-embryogenic de novo organogenesis through the LBD16 callus formation pathway partially different from those prevalent during normal LR development.

Characterization of the LBD gene family in Brachypodium: a phylogenetic and transcriptional study

An unambiguous nomenclature is proposed for the twenty-eight-member LOB domain transcription factor family in Brachypodium . Expression analysis provides unique transcript patterns that are characteristic of a wide range of organs and plant parts. LOB (lateral organ boundaries)-domain proteins define a family of plant-specific transcription factors involved in developmental processes from embryogenesis to seed production. They play a crucial role in shaping the plant architecture through coordinating cell fate at meristem to organ boundaries. Despite their high potential importance, our knowledge of them is limited, especially in the case of monocots. In this study, we characterized LOB domain protein coding genes (LBDs) of Brachypodium distachyon, a model plant for grasses, and present their phylogenetic relationships and an overall spatial expression study. In the Brachypodium genome database, 28 LBDs were found and then classified based on the presence of highly conserved LOB domain motif. Their transcript amounts were measured via quantitative real-time RT-PCR in 37 different plant parts from root tip to generative organs. Comprehensive phylogenetic analysis suggests that there are neither Brachypodium- nor monocot-specific lineages among LBDs, but there are differences in terms of complexity of subclasses between monocots and dicots. Although LBDs in Brachypodium have wide variation of tissue-specific expression and relative transcript levels, overall expression patterns show similarity to their counterparts in other species. The varying transcript profiles we observed support the hypothesis that Brachypodium LBDs have diverse but conserved functions in plant organogenesis.

A Standardized Method to Assess Infection Rates of Root-Knot and Cyst Nematodes in Arabidopsis thaliana Mutants with Alterations in Root Development Related to Auxin and Cytokinin Signaling

Plant parasitic nematodes cause a great impact in agricultural systems. The search for effective control methods is partly based on the understanding of underlying molecular mechanisms leading to the formation of nematode feeding sites. In this respect, crosstalk of hormones such as auxins and cytokinins (IAA, CK) between the plant and the nematode seems to be crucial. Thence, the study of loss of function or overexpressing lines with altered IAA and CK functioning is entailed. Those lines frequently show developmental defects in the number, position and/or length of the lateral roots what could generate a bias in the interpretation of the nematode infection parameters. Here we present a protocol to assess differences in nematode infectivity with the lowest interference of root architecture phenotypes in the results. Thus, tailored growth conditions and normalization parameters facilitate the standardized phenotyping of nematode infection.

Root-Knot and Cyst Nematodes Activate Procambium-Associated Genes in Arabidopsis Roots

Developmental plasticity is one of the most striking features of plant morphogenesis, as plants are able to vary their shapes in response to environmental cues. Biotic or abiotic stimuli often promote organogenesis events in plants not observed under normal growth conditions. Root-knot nematodes (RKNs) are known to parasitize multiple species of rooting plants and to induce characteristic tissue expansion called galls or root-knots on the roots of their hosts by perturbing the plant cellular machinery. Galls contain giant cells (GCs) and neighboring cells, and the GCs are a source of nutrients for the parasitizing nematode. Highly active cell proliferation was observed in galls. However, the underlying mechanisms that regulate the symptoms triggered by the plant-nematode interaction have not yet been elucidated. In this study, we deciphered the molecular mechanism of gall formation with an in vitro infection assay system using RKN Meloidogyne incognita, and the model plant Arabidopsis thaliana. By taking advantages of this system, we performed next-generation sequencing-based transcriptome profiling, and found that the expression of procambium identity-associated genes were enriched during gall formation. Clustering analyses with artificial xylogenic systems, together with the results of expression analyses of the candidate genes, showed a significant correlation between the induction of gall cells and procambium-associated cells. Furthermore, the promoters of several procambial marker genes such as ATHB8, TDR and WOX4 were activated not only in M. incognita-induced galls, but similarly in M. javanica induced-galls and Heterodera schachtii-induced syncytia. Our findings suggest that phytoparasitic nematodes modulate the host's developmental regulation of the vascular stem cells during gall formation.

Evolutionarily distant pathogens require the Arabidopsis phytosulfokine signalling pathway to establish disease

Secreted peptides and their specific receptors frequently orchestrate cell-to-cell communication in plants. Phytosulfokines (PSKs) are secreted tyrosine-sulphated peptide hormones, which trigger cellular dedifferentiation and redifferentiation upon binding to their membrane receptor. Biotrophic plant pathogens frequently trigger the differentiation of host cells into specialized feeding structures, which are essential for successful infection. We found that oomycete and nematode infections were characterized by the tissue-specific transcriptional regulation of genes encoding Arabidopsis PSKs and the PSK receptor 1 (PSKR1). Subcellular analysis of PSKR1 distribution showed that the plasma membrane-bound receptor internalizes after binding of PSK-α. Arabidopsis pskr1 knockout mutants were impaired in their susceptibility to downy mildew infection. Impaired disease susceptibility depends on functional salicylic acid (SA) signalling, but not on the massive up-regulation of SA-associated defence-related genes. Knockout pskr1 mutants also displayed a major impairment of root-knot nematode reproduction. In the absence of functional PSKR1, giant cells arrested their development and failed to fully differentiate. Our findings indicate that the observed restriction of PSK signalling to cells surrounding giant cells contributes to the isotropic growth and maturation of nematode feeding sites. Taken together, our data suggest that PSK signalling in Arabidopsis promotes the differentiation of host cells into specialized feeding cells.

Differentially expressed small RNAs in Arabidopsis galls formed by Meloidogyne javanica: a functional role for miR390 and its TAS3-derived tasiRNAs

Root-knot nematodes (RKNs) induce inside the vascular cylinder the giant cells (GCs) embedded in the galls. The distinctive gene repression in early-developing GCs could be facilitated by small RNAs (sRNA) such as miRNAs, and/or epigenetic mechanisms mediated by 24nt-sRNAs, rasiRNAs and 21-22nt-sRNAs. Therefore, the sRNA-population together with the role of the miR390/TAS3/ARFs module were studied during early gall/GC formation. Three sRNA libraries from 3-d-post-inoculation (dpi) galls induced by Meloidogyne javanica in Arabidopsis and three from uninfected root segments were sequenced following Illumina-Solexa technology. pMIR390a::GUS and pTAS3::GUS lines were assayed for nematode-dependent promoter activation. A sensor line indicative of TAS3-derived tasiRNAs binding to the ARF3 sequence (pARF3:ARF3-GUS) together with a tasiRNA-resistant ARF3 line (pARF3:ARF3m-GUS) were used for functional analysis. The sRNA population showed significant differences between galls and controls, with high validation rate and correspondence with their target expression: 21-nt sRNAs corresponding mainly to miRNAs were downregulated, whilst 24-nt-sRNAs from the rasiRNA family were mostly upregulated in galls. The promoters of MIR390a and TAS3, active in galls, and the pARF3:ARF3-GUS line, indicated a role of TAS3-derived-tasiRNAs in galls. The regulatory module miR390/TAS3 is necessary for proper gall formation possibly through auxin-responsive factors, and the abundance of 24-nt sRNAs (mostly rasiRNAs) constitutes a gall hallmark.

LOB Domain Proteins: Beyond Lateral Organ Boundaries

LATERAL ORGAN BOUNDARIES DOMAIN (LBD) proteins defined by a conserved LATERAL ORGAN BOUNDARIES (LOB) domain are key regulators of plant organ development. Recent studies have expanded their functional diversity beyond the definition of lateral organ boundaries to pollen development, plant regeneration, photomorphogenesis, pathogen response, and specific developmental functions in non-model plants, such as poplar and legumes. The identification of a range of upstream regulators, protein partners, and downstream targets of LBD family members has unraveled the molecular networks of LBD-dependent processes. Moreover, it has been demonstrated that LBD proteins have essential roles in integrating developmental changes in response to phytohormone signaling or environmental cues. As we discuss here, these novel discoveries of LBD functions and their molecular contexts promote a better understanding of this plant-specific transcription factor family.

Gall-forming root-knot nematodes hijack key plant cellular functions to induce multinucleate and hypertrophied feeding cells

Among plant-parasitic nematodes, the root-knot nematodes (RKNs) of the Meloidogyne spp. are the most economically important genus. RKN are root parasitic worms able to infect nearly all crop species and have a wide geographic distribution. During infection, RKNs establish and maintain an intimate relationship with the host plant. This includes the creation of a specialized nutritional structure composed of multinucleate and hypertrophied giant cells, which result from the redifferentiation of vascular root cells. Giant cells constitute the sole source of nutrients for the nematode and are essential for growth and reproduction. Hyperplasia of surrounding root cells leads to the formation of the gall or root-knot, an easily recognized symptom of plant infection by RKNs. Secreted effectors produced in nematode salivary glands and injected into plant cells through a specialized feeding structure called the stylet play a critical role in the formation of giant cells. Here, we describe the complex interactions between RKNs and their host plants. We highlight progress in understanding host plant responses, focusing on how RKNs manipulate key plant processes and functions, including cell cycle, defence, hormones, cellular scaffold, metabolism and transport.

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.

Long-Term In Vitro System for Maintenance and Amplification of Root-Knot Nematodes in Cucumis sativus Roots

Root-knot nematodes (RKN) are polyphagous plant-parasitic roundworms that produce large crop losses, representing a relevant agricultural pest worldwide. After infection, they induce swollen root structures called galls containing giant cells (GCs) indispensable for nematode development. Among efficient control methods are biotechnology-based strategies that require a deep knowledge of underlying molecular processes during the plant-nematode interaction. Methods of achieving this knowledge include the application of molecular biology techniques such as transcriptomics (as massive sequencing or microarray hybridization), proteomics or metabolomics. These require aseptic experimental conditions, as undetected contamination with other microorganisms could compromise the interpretation of the results. Herein, we present a simple, efficient and long-term method for nematode amplification on cucumber roots grown in vitro. Amplification of juveniles (J2) from the starting inoculum is around 40-fold. The method was validated for three Meloidogyne species (Meloidogyne javanica, M. incognita, and M. arenaria), producing viable and robust freshly hatched J2s. These J2s can be used for further in vitro infection of different plant species such as Arabidopsis, tobacco and tomato, as well as to maintain and amplify the population. The method allowed maintenance of around 90 Meloidogyne sp. generations (one every 2 months) from a single initial female over 15 years.

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.

Genes co-regulated with LBD16 in nematode feeding sites inferred from in silico analysis show similarities to regulatory circuits mediated by the auxin/cytokinin balance in Arabidopsis

Plant endoparasitic nematodes, root-knot and cyst nematodes (RKNs and CNs) induce within the root vascular cylinder transfer cells used for nourishing, termed giant cells (GCs) and syncytia. Understanding the molecular mechanisms behind this process is essential to develop tools for nematode control. Based on the crucial role in gall development of LBD16, also a key component of the auxin pathway leading to the divisions in the xylem pole pericycle during lateral root (LR) formation, we investigated genes co-regulated with LBD16 in different transcriptomes and analyzed their similarities and differences with those of RKNs and CNs feeding sites (FS). This analysis confirmed LBD16 and its co-regulated genes, integrated in signaling cascades mediated by auxins during LR and callus formation, as a particular feature of RKN-FS distinct to CNs. However, LBD16, and its positively co-regulated genes, were repressed in syncytia, suggesting a selective down- regulation of the LBD16 auxin mediated pathways in CNs-FS. Interestingly, cytokinin-induced genes are enriched in syncytia and we encountered similarities between the transcriptome of shoot regeneration from callus, modulated by cytokinins, and that of syncytia. These findings establish differences in the regulatory networks leading to both FS formation, probably modulated by the auxin/cytokinin balance.

Integrated signaling networks in plant responses to sedentary endoparasitic nematodes: a perspective

Sedentary plant endoparasitic nematodes can cause detrimental yield losses in crop plants making the study of detailed cellular, molecular, and whole plant responses to them a subject of importance. In response to invading nematodes and nematode-secreted effectors, plant susceptibility/resistance is mainly determined by the coordination of different signaling pathways including specific plant resistance genes or proteins, plant hormone synthesis and signaling pathways, as well as reactive oxygen signals that are generated in response to nematode attack. Crosstalk between various nematode resistance-related elements can be seen as an integrated signaling network regulated by transcription factors and small RNAs at the transcriptional, posttranscriptional, and/or translational levels. Ultimately, the outcome of this highly controlled signaling network determines the host plant susceptibility/resistance to nematodes.