Light signaling regulates root-knot nematode infection and development via HY5-SWEET signaling.

BACKGROUND: Meloidogyne incognita is one of the most important plant-parasitic nematodes and causes tremendous losses to the agricultural economy. Light is an important living factor for plants and pathogenic organisms, and sufficient light promotes root-knot nematode infection, but the underlying mechanism is still unclear. RESULTS: Expression level and genetic analyses revealed that the photoreceptor genes PHY, CRY, and PHOT have a negative impact on nematode infection. Interestingly, ELONGATED HYPOCOTYL5 (HY5), a downstream gene involved in the regulation of light signaling, is associated with photoreceptor-mediated negative regulation of root-knot nematode resistance. ChIP and yeast one-hybrid assays supported that HY5 participates in plant-to-root-knot nematode responses by directly binding to the SWEET negative regulatory factors involved in root-knot nematode resistance. CONCLUSIONS: This study elucidates the important role of light signaling pathways in plant resistance to nematodes, providing a new perspective for RKN resistance research.

CRISPR/Cas9-induced knockout of an amino acid permease gene (AAP6) reduced Arabidopsis thaliana susceptibility to Meloidogyne incognita.

BACKGROUND: Plant-parasitic root-knot nematode (Meloidogyne incognita) causes global yield loss in agri- and horticultural crops. Nematode management options rely on chemical method. However, only a handful of nematicides are commercially available. Resistance breeding efforts are not sustainable because R gene sources are limited and nematodes have developed resistance-breaking populations against the commercially available Mi-1.2 gene-expressing tomatoes. RNAi crops that manage nematode infection are yet to be commercialized because of the regulatory hurdles associated with transgenic crops. The deployment of the CRISPR/Cas9 system to improve nematode tolerance (by knocking out the susceptibility factors) in plants has emerged as a feasible alternative lately. RESULTS: In the present study, a M. incognita-responsive susceptibility (S) gene, amino acid permease (AAP6), was characterized from the model plant Arabidodpsis thaliana by generating the AtAAP6 overexpression line, followed by performing the GUS reporter assay by fusing the promoter of AtAAP6 with the ß-glucuronidase (GUS) gene. Upon challenge inoculation with M. incognita, overexpression lines supported greater nematode multiplication, and AtAAP6 expression was inducible to the early stage of nematode infection. Next, using CRISPR/Cas9, AtAAP6 was selectively knocked out without incurring any growth penalty in the host plant. The 'Cas9-free' homozygous T3 line was challenge inoculated with M. incognita, and CRISPR-edited A. thaliana plants exhibited considerably reduced susceptibility to nematode infection compared to the non-edited plants. Additionally, host defense response genes were unaltered between edited and non-edited plants, implicating the direct role of AtAAP6 towards nematode susceptibility. CONCLUSION: The present findings enrich the existing literature on CRISPR/Cas9 research in plant-nematode interactions, which is quite limited currently while compared with the other plant-pathogen interaction systems.

The nematode effector Mj-NEROSs interacts with Rieske's iron-sulfur protein influencing plastid ROS production to suppress plant immunity.

Root-knot nematodes (RKN; Meloidogyne species) are plant pathogens that introduce several effectors in their hosts to facilitate infection. The actual targets and functioning mechanism of these effectors largely remain unexplored. This study illuminates the role and interplay of the Meloidogyne javanica nematode effector ROS suppressor (Mj-NEROSs) within the host plant environment. Mj-NEROSs suppresses INF1-induced cell death as well as flg22-induced callose deposition and reactive oxygen species (ROS) production. A transcriptome analysis highlighted the downregulation of ROS-related genes upon Mj-NEROSs expression. NEROSs interacts with the plant Rieske's iron-sulfur protein (ISP) as shown by yeast-two-hybrid and bimolecular fluorescence complementation. Secreted from the subventral pharyngeal glands into giant cells, Mj-NEROSs localizes in the plastids where it interacts with ISP, subsequently altering electron transport rates and ROS production. Moreover, our results demonstrate that isp Arabidopsis thaliana mutants exhibit increased susceptibility to M. javanica, indicating ISP importance for plant immunity. The interaction of a nematode effector with a plastid protein highlights the possible role of root plastids in plant defense, prompting many questions on the details of this process.

NRPS-like ATRR in Plant-Parasitic Nematodes Involved in Glycine Betaine Metabolism to Promote Parasitism.

Plant-parasitic nematodes (PPNs) are among the most serious phytopathogens and cause widespread and serious damage in major crops. In this study, using a genome mining method, we identified nonribosomal peptide synthetase (NRPS)-like enzymes in genomes of plant-parasitic nematodes, which are conserved with two consecutive reducing domains at the N-terminus (A-T-R1-R2) and homologous to fungal NRPS-like ATRR. We experimentally investigated the roles of the NRPS-like enzyme (MiATRR) in nematode (Meloidogyne incognita) parasitism. Heterologous expression of Miatrr in Saccharomyces cerevisiae can overcome the growth inhibition caused by high concentrations of glycine betaine. RT-qPCR detection shows that Miatrr is significantly upregulated at the early parasitic life stage (J2s in plants) of M. incognita. Host-derived Miatrr RNA interference (RNAi) in Arabidopsis thaliana can significantly decrease the number of galls and egg masses of M. incognita, as well as retard development and reduce the body size of the nematode. Although exogenous glycine betaine and choline have no obvious impact on the survival of free-living M. incognita J2s (pre-parasitic J2s), they impact the performance of the nematode in planta, especially in Miatrr-RNAi plants. Following application of exogenous glycine betaine and choline in the rhizosphere soil of A. thaliana, the numbers of galls and egg masses were obviously reduced by glycine betaine but increased by choline. Based on the knowledge about the function of fungal NRPS-like ATRR and the roles of glycine betaine in host plants and nematodes, we suggest that MiATRR is involved in nematode-plant interaction by acting as a glycine betaine reductase, converting glycine betaine to choline. This may be a universal strategy in plant-parasitic nematodes utilizing NRPS-like ATRR to promote their parasitism on host plants.

A root-knot nematode effector targets the Arabidopsis cysteine protease RD21A for degradation to suppress plant defense and promote parasitism.

Meloidogyne incognita is one of the most widely distributed plant-parasitic nematodes and causes severe economic losses annually. The parasite produces effector proteins that play essential roles in successful parasitism. Here, we identified one such effector named MiCE108, which is exclusively expressed within the nematode subventral esophageal gland cells and is upregulated in the early parasitic stage of M. incognita. A yeast signal sequence trap assay showed that MiCE108 contains a functional signal peptide for secretion. Virus-induced gene silencing of MiCE108 impaired the parasitism of M. incognita in Nicotiana benthamiana. The ectopic expression of MiCE108 in Arabidopsis suppressed the deposition of callose, the generation of reactive oxygen species, and the expression of marker genes for bacterial flagellin epitope flg22-triggered immunity, resulting in increased susceptibility to M. incognita, Botrytis cinerea, and Pseudomonas syringae pv. tomato (Pst) DC3000. The MiCE108 protein physically associates with the plant defense protease RD21A and promotes its degradation via the endosomal-dependent pathway, or 26S proteasome. Consistent with this, knockout of RD21A compromises the innate immunity of Arabidopsis and increases its susceptibility to a broad range of pathogens, including M. incognita, strongly indicating a role in defense against this nematode. Together, our data suggest that M. incognita deploys the effector MiCE108 to target Arabidopsis cysteine protease RD21A and affect its stability, thereby suppressing plant innate immunity and facilitating parasitism.

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.

Functional analysis of a susceptibility gene (HIPP27) in the Arabidopsis thaliana-Meloidogyne incognita pathosystem by using a genome editing strategy.

BACKGROUND: Plant-parasitic root-knot nematodes cause immense yield declines in crop plants that ultimately obviate global food security. They maintain an intimate relationship with their host plants and hijack the host metabolic machinery to their own advantage. The existing resistance breeding strategies utilizing RNAi and resistance (R) genes might not be particularly effective. Alternatively, knocking out the susceptibility (S) genes in crop plants appears to be a feasible approach, as the induced mutations in S genes are likely to be long-lasting and may confer broad-spectrum resistance. This could be facilitated by the use of CRISPR/Cas9-based genome editing technology that precisely edits the gene of interest using customizable guide RNAs (gRNAs) and Cas9 endonuclease. RESULTS: Initially, we characterized the nematode-responsive S gene HIPP27 from Arabidopsis thaliana by generating HIPP27 overexpression lines, which were inoculated with Meloidogyne incognita. Next, two gRNAs (corresponding to the HIPP27 gene) were artificially synthesized using laboratory protocols, sequentially cloned into a Cas9 editor plasmid, mobilized into Agrobacterium tumefaciens strain GV3101, and transformed into Arabidopsis plants using the floral dip method. Apart from 1-3 bp deletions and 1 bp insertions adjacent to the PAM site, a long deletion of approximately 161 bp was documented in the T0 generation. Phenotypic analysis of homozygous, 'transgene-free' T2 plants revealed reduced nematode infection compared to wild-type plants. Additionally, no growth impairment was observed in gene-edited plants. CONCLUSION: Our results suggest that the loss of function of HIPP27 in A. thaliana by CRISPR/Cas9-induced mutagenesis can improve host resistance to M. incognita.

Exploiting breakdown in nonhost effector-target interactions to boost host disease resistance.

Plants are resistant to most microbial species due to nonhost resistance (NHR), providing broad-spectrum and durable immunity. However, the molecular components contributing to NHR are poorly characterised. We address the question of whether failure of pathogen effectors to manipulate nonhost plants plays a critical role in NHR. RxLR (Arg-any amino acid-Leu-Arg) effectors from two oomycete pathogens, Phytophthora infestans and Hyaloperonospora arabidopsidis, enhanced pathogen infection when expressed in host plants (Nicotiana benthamiana and Arabidopsis, respectively) but the same effectors performed poorly in distantly related nonhost pathosystems. Putative target proteins in the host plant potato were identified for 64 P. infestans RxLR effectors using yeast 2-hybrid (Y2H) screens. Candidate orthologues of these target proteins in the distantly related non-host plant Arabidopsis were identified and screened using matrix Y2H for interaction with RxLR effectors from both P. infestans and H. arabidopsidis. Few P. infestans effector-target protein interactions were conserved from potato to candidate Arabidopsis target orthologues (cAtOrths). However, there was an enrichment of H. arabidopsidis RxLR effectors interacting with cAtOrths. We expressed the cAtOrth AtPUB33, which unlike its potato orthologue did not interact with P. infestans effector PiSFI3, in potato and Nicotiana benthamiana. Expression of AtPUB33 significantly reduced P. infestans colonization in both host plants. Our results provide evidence that failure of pathogen effectors to interact with and/or correctly manipulate target proteins in distantly related non-host plants contributes to NHR. Moreover, exploiting this breakdown in effector-nonhost target interaction, transferring effector target orthologues from non-host to host plants is a strategy to reduce disease.

Water deficit changes the relationships between epidemiological traits of Cauliflower mosaic virus across diverse Arabidopsis thaliana accessions.

Changes in plant abiotic environments may alter plant virus epidemiological traits, but how such changes actually affect their quantitative relationships is poorly understood. Here, we investigated the effects of water deficit on Cauliflower mosaic virus (CaMV) traits (virulence, accumulation, and vectored-transmission rate) in 24 natural Arabidopsis thaliana accessions grown under strictly controlled environmental conditions. CaMV virulence increased significantly in response to water deficit during vegetative growth in all A. thaliana accessions, while viral transmission by aphids and within-host accumulation were significantly altered in only a few. Under well-watered conditions, CaMV accumulation was correlated positively with CaMV transmission by aphids, while under water deficit, this relationship was reversed. Hence, under water deficit, high CaMV accumulation did not predispose to increased horizontal transmission. No other significant relationship between viral traits could be detected. Across accessions, significant relationships between climate at collection sites and viral traits were detected but require further investigation. Interactions between epidemiological traits and their alteration under abiotic stresses must be accounted for when modelling plant virus epidemiology under scenarios of climate change.

Plant Viruses Can Alter Aphid-Triggered Calcium Elevations in Infected Leaves.

Alighting aphids probe a new host plant by intracellular test punctures for suitability. These induce immediate calcium signals that emanate from the punctured sites and might be the first step in plant recognition of aphid feeding and the subsequent elicitation of plant defence responses. Calcium is also involved in the transmission of non-persistent plant viruses that are acquired by aphids during test punctures. Therefore, we wanted to determine whether viral infection alters calcium signalling. For this, calcium signals triggered by aphids were imaged on transgenic Arabidopsis plants expressing the cytosolic FRET-based calcium reporter YC3.6-NES and infected with the non-persistent viruses cauliflower mosaic (CaMV) and turnip mosaic (TuMV), or the persistent virus, turnip yellows (TuYV). Aphids were placed on infected leaves and calcium elevations were recorded by time-lapse fluorescence microscopy. Calcium signal velocities were significantly slower in plants infected with CaMV or TuMV and signal areas were smaller in CaMV-infected plants. Transmission tests using CaMV-infected Arabidopsis mutants impaired in pathogen perception or in the generation of calcium signals revealed no differences in transmission efficiency. A transcriptomic meta-analysis indicated significant changes in expression of receptor-like kinases in the BAK1 pathway as well as of calcium channels in CaMV- and TuMV-infected plants. Taken together, infection with CaMV and TuMV, but not with TuYV, impacts aphid-induced calcium signalling. This suggests that viruses can modify plant responses to aphids from the very first vector/host contact.

Novel Functions of the Fatty Acid and Retinol Binding Protein (FAR) Gene Family Revealed by Fungus-Mediated RNAi in the Parasitic Nematode, Aphelenchoides besseyi.

RNA interference (RNAi) is a powerful tool for the analysis of gene function in nematodes. Fatty acid and retinol binding protein (FAR) is a protein that only exists in nematodes and plays an important role in their life activities. The rice white-tip nematode (RWTN), Aphelenchoides besseyi, is a migratory endoparasitic plant nematode that causes serious damage in agricultural production. In this study, the expression levels of eight RWTN genes were effectively decreased when RWTN was fed Ab-far-n (n: 1-8) hairpin RNA transgenic Botrytis cinerea (ARTBn). These functions of the far gene family were identified to be consistent and diverse through phenotypic changes after any gene was silenced. Such consistency indicates that the body lengths of the females were significantly shortened after silencing any of the eight Ab-far genes. The diversities were mainly manifested as follows: (1) Reproduction of nematodes was clearly inhibited after Ab-far-1 to Ab-far-4 were silenced. In addition, silencing Ab-far-2 could inhibit the pathogenicity of nematodes to Arabidopsis; (2) gonad length of female nematodes was significantly shortened after Ab-far-2 and Ab-far-4 were silenced; (3) proportion of male nematodes significantly increased in the adult population after Ab-far-1, Ab-far-3, and Ab-far-5 were silenced, whereas the proportion of adult nematodes significantly decreased in the nematode population after Ab-far-4 were silenced. (4) Fat storage of nematodes significantly decreased after Ab-far-3, Ab-far-4, and Ab-far-7 were silenced. To our knowledge, this is the first study to demonstrate that Ab-far genes affect sex formation and lipid metabolism in nematodes, which provides valuable data for further study and control of RWTNs.

Parasitic modulation of host development by ubiquitin-independent protein degradation.

Certain obligate parasites induce complex and substantial phenotypic changes in their hosts in ways that favor their transmission to other trophic levels. However, the mechanisms underlying these changes remain largely unknown. Here we demonstrate how SAP05 protein effectors from insect-vectored plant pathogenic phytoplasmas take control of several plant developmental processes. These effectors simultaneously prolong the host lifespan and induce witches' broom-like proliferations of leaf and sterile shoots, organs colonized by phytoplasmas and vectors. SAP05 acts by mediating the concurrent degradation of SPL and GATA developmental regulators via a process that relies on hijacking the plant ubiquitin receptor RPN10 independent of substrate ubiquitination. RPN10 is highly conserved among eukaryotes, but SAP05 does not bind insect vector RPN10. A two-amino-acid substitution within plant RPN10 generates a functional variant that is resistant to SAP05 activities. Therefore, one effector protein enables obligate parasitic phytoplasmas to induce a plethora of developmental phenotypes in their hosts.

Marasmius oreades agglutinin enhances resistance of Arabidopsis against plant-parasitic nematodes and a herbivorous insect.

BACKGROUND: Plant-parasitic nematodes and herbivorous insects have a significant negative impact on global crop production. A successful approach to protect crops from these pests is the in planta expression of nematotoxic or entomotoxic proteins such as crystal proteins from Bacillus thuringiensis (Bt) or plant lectins. However, the efficacy of this approach is threatened by emergence of resistance in nematode and insect populations to these proteins. To solve this problem, novel nematotoxic and entomotoxic proteins are needed. During the last two decades, several cytoplasmic lectins from mushrooms with nematicidal and insecticidal activity have been characterized. In this study, we tested the potential of Marasmius oreades agglutinin (MOA) to furnish Arabidopsis plants with resistance towards three economically important crop pests: the two plant-parasitic nematodes Heterodera schachtii and Meloidogyne incognita and the herbivorous diamondback moth Plutella xylostella. RESULTS: The expression of MOA does not affect plant growth under axenic conditions which is an essential parameter in the engineering of genetically modified crops. The transgenic Arabidopsis lines showed nearly complete resistance to H. schachtii, in that the number of female and male nematodes per cm root was reduced by 86-91 % and 43-93 % compared to WT, respectively. M. incognita proved to be less susceptible to the MOA protein in that 18-25 % and 26-35 % less galls and nematode egg masses, respectively, were observed in the transgenic lines. Larvae of the herbivorous P. xylostella foraging on MOA-expression lines showed a lower relative mass gain (22-38 %) and survival rate (15-24 %) than those feeding on WT plants. CONCLUSIONS: The results of our in planta experiments reveal a robust nematicidal and insecticidal activity of the fungal lectin MOA against important agricultural pests which may be exploited for crop protection.

Bacillus firmus I-1582 promotes plant growth and impairs infection and development of the cyst nematode Heterodera schachtii over two generations.

Plant-parasitic nematodes wreak havoc on crops by root parasitism worldwide. An approach to combat nematode root parasitism is the application of antagonistic microbes like the rhizobacterium Bacillus firmus I-1582 which is promoted as biological control agent. Although B. firmus is a known nematode antagonist in general, the underlying mechanisms about its interaction with nematodes and plants have not yet been elucidated. Therefore, we explored the influence of B. firmus I-1582 as well as its extracellular and secreted molecules on plant-nematode interaction utilizing the plant-pathogen system Arabidopsis thaliana-Heterodera schachtii. We demonstrated that B. firmus I-1582 is attracted by A. thaliana root exudates, particularly by those of young plants. The bacterium colonized the root and showed a strictly pH-dependent development and plant growth promotion effect. Our results revealed that root colonization by B. firmus I-1582 significantly protected A. thaliana from infestation by the beet cyst nematode whereas dead bacterial cells or the culture supernatant were not effective. The bacterium also negatively affected nematode reproduction as well as pathogenicity and development of next generation nematodes. The obtained results highlight B. firmus I-1582 as a promising biocontrol agent that is well suited as an element of integrated control management strategies in sustainable agriculture.

Arabidopsis thaliana Myb59 Gene Is Involved in the Response to Heterodera schachtii Infestation, and Its Overexpression Disturbs Regular Development of Nematode-Induced Syncytia.

Transcription factors are proteins that directly bind to regulatory sequences of genes to modulate and adjust plants' responses to different stimuli including biotic and abiotic stresses. Sedentary plant parasitic nematodes, such as beet cyst nematode, Heterodera schachtii, have developed molecular tools to reprogram plant cell metabolism via the sophisticated manipulation of genes expression, to allow root invasion and the induction of a sequence of structural and physiological changes in plant tissues, leading to the formation of permanent feeding sites composed of modified plant cells (commonly called a syncytium). Here, we report on the AtMYB59 gene encoding putative MYB transcription factor that is downregulated in syncytia, as confirmed by RT-PCR and a promoter pMyb59::GUS activity assays. The constitutive overexpression of AtMYB59 led to the reduction in A. thaliana susceptibility, as indicated by decreased numbers of developed females, and to the disturbed development of nematode-induced syncytia. In contrast, mutant lines with a silenced expression of AtMYB59 were more susceptible to this parasite. The involvement of ABA in the modulation of AtMYB59 gene transcription appears feasible by several ABA-responsive cis regulatory elements, which were identified in silico in the gene promoter sequence, and experimental assays showed the induction of AtMYB59 transcription after ABA treatment. Based on these results, we suggest that AtMYB59 plays an important role in the successful parasitism of H. schachtii on A. thaliana roots.

Redox responses of Arabidopsis thaliana to the green peach aphid, Myzus persicae.

The green peach aphid (Myzus persicae) is a phloem-feeding insect that causes economic damage on a wide array of crops. Using a luminol-based assay, a superoxide-responsive reporter gene (Zat12::luciferase), and a probe specific to hydrogen peroxide (HyPer), we demonstrated that this aphid induces accumulation of reactive oxygen species (ROS) in Arabidopsis thaliana. Similar to the apoplastic oxidative burst induced by pathogens, this response to aphids was rapid and transient, with two peaks occurring within 1 and 4 hr after infestation. Aphid infestation also induced an oxidative response in the cytosol and peroxisomes, as measured using a redox-sensitive variant of green fluorescent protein (roGFP2). This intracellular response began within minutes of infestation but persisted 20 hr or more after inoculation, and the response of the peroxisomes appeared stronger than the response in the cytosol. Our results suggest that the oxidative response to aphids involves both apoplastic and intracellular sources of ROS, including ROS generation in the peroxisomes, and these different sources of ROS may potentially differ in their impacts on host suitability for aphids.

Three-dimensional reconstructions of haustoria in two parasitic plant species in the Orobanchaceae.

Parasitic plants infect other plants by forming haustoria, specialized multicellular organs consisting of several cell types, each of which has unique morphological features and physiological roles associated with parasitism. Understanding the spatial organization of cell types is, therefore, of great importance in elucidating the functions of haustoria. Here, we report a three-dimensional (3-D) reconstruction of haustoria from two Orobanchaceae species, the obligate parasite Striga hermonthica infecting rice (Oryza sativa) and the facultative parasite Phtheirospermum japonicum infecting Arabidopsis (Arabidopsis thaliana). In addition, field-emission scanning electron microscopy observation revealed the presence of various cell types in haustoria. Our images reveal the spatial arrangements of multiple cell types inside haustoria and their interaction with host roots. The 3-D internal structures of haustoria highlight differences between the two parasites, particularly at the xylem connection site with the host. Our study provides cellular and structural insights into haustoria of S. hermonthica and P. japonicum and lays the foundation for understanding haustorium function.

LATE ELONGATED HYPOCOTYL potentiates resistance conferred by CIRCADIAN CLOCK ASSOCIATED1 to aphid by co-regulating the expression of indole glucosinolate biosynthetic genes.

CIRCADIAN CLOCK ASSOCIATED1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) are core components of the circadian clock in Arabidopsis thaliana that impacts plant response to biotic stresses. Their clock-regulating functions are believed to be partially redundant, and mutation of either gene leads to shortened periods of the circadian cycle. Our recent study has demonstrated that CCA1 promotes plant resistance to the green peach aphid (Myzus persicae) through modulation of indole glucosinolate biosynthesis, but the role of LHY remains to be elucidated. Here we showed that, similar to cca1-11, single mutant lhy-21 became more susceptible to aphid infestation. Damage to the cca1-11 lhy-21 double mutant by aphids was most pronounced, indicating that the defensive roles of CCA1 and LHY were not entirely redundant. Also, the cyclic expression pattern of key indole glucosinolate biosynthetic genes was considerably disturbed in both single mutants and this was more severe in the double mutant. Apparently, both CCA1 and LHY were necessary for circadian-regulated indole glucosinolate biosynthesis. Taken together, LHY-CCA1 coordination in transcriptional regulation of indole glucosinolate biosynthetic genes most likely contributed to plant defensive capacity against aphids.

Susceptibility factor RTP1 negatively regulates Phytophthora parasitica resistance via modulating UPR regulators bZIP60 and bZIP28.

The unfolded protein response (UPR) is a conserved stress adaptive signaling pathway in eukaryotic organisms activated by the accumulation of misfolded proteins in the endoplasmic reticulum (ER). UPR can be elicited in the course of plant defense, playing important roles in plant-microbe interactions. The major signaling pathways of plant UPR rely on the transcriptional activity of activated forms of ER membrane-associated stress sensors bZIP60 and bZIP28, which are transcription factors that modulate expression of UPR genes. In this study, we report the plant susceptibility factor Resistance to Phytophthora parasitica 1 (RTP1) is involved in ER stress sensing and rtp1-mediated resistance against P. parasitica is synergistically regulated with UPR, as demonstrated by the simultaneous strong induction of UPR and ER stress-associated immune genes in Arabidopsis thaliana rtp1 mutant plants during the infection by P. parasitica. We further demonstrate RTP1 contributes to stabilization of the ER membrane-associated bZIP60 and bZIP28 through manipulating the bifunctional protein kinase/ribonuclease IRE1-mediated bZIP60 splicing activity and interacting with bZIP28. Consequently, we find rtp1bzip60 and rtp1bzip28 mutant plants exhibit compromised resistance accompanied with attenuated induction of ER stress-responsive immune genes and reduction of callose deposition in response to P. parasitica infection. Taken together, we demonstrate RTP1 may exert negative modulating roles in the activation of key UPR regulators bZIP60 and bZIP28, which are required for rtp1-mediated plant resistance to P. parasitica. This facilitates our understanding of the important roles of stress adaptive UPR and ER stress in plant immunity.