HvCEBiP, a gene homologous to rice chitin receptor CEBiP, contributes to basal resistance of barley to Magnaporthe oryzae
© Tanaka et al; licensee BioMed Central Ltd. 2010
Received: 9 May 2010
Accepted: 30 December 2010
Published: 30 December 2010
Rice CEBiP recognizes chitin oligosaccharides on the fungal cell surface or released into the plant apoplast, leading to the expression of plant disease resistance against fungal infection. However, it has not yet been reported whether CEBiP is actually required for restricting the growth of fungal pathogens. Here we evaluated the involvement of a putative chitin receptor gene in the basal resistance of barley to the ssd1 mutant of Magnaporthe oryzae, which induces multiple host defense responses.
The mossd1 mutant showed attenuated pathogenicity on barley and appressorial penetration was restricted by the formation of callose papillae at attempted entry sites. When conidial suspensions of mossd1 mutant were spotted onto the leaves of HvCEBiP-silenced plants, small brown necrotic flecks or blast lesions were produced but these lesions did not expand beyond the inoculation site. Wild-type M. oryzae also produced slightly more severe symptoms on the leaves of HvCEBiP-silenced plants. Cytological observation revealed that these lesions resulted from appressorium-mediated penetration into plant epidermal cells.
These results suggest that HvCEBiP is involved in basal resistance against appressorium-mediated infection and that basal resistance might be triggered by the recognition of chitin oligosaccharides derived from M. oryzae.
To resist attack by microbial pathogens, plants have evolved to recognize them, triggering the expression of diverse defense reactions. The currently accepted model is that plants recognize conserved pathogen-associated molecular patterns (PAMPs) through corresponding pattern recognition receptors (PRRs) which in turn trigger plant immune responses [1–3]. The involvement of PRRs in disease resistance against bacterial pathogens is well-documented. For example, the N-terminal amino acid sequence of bacterial flagellin (designated as flg22) can be recognized through the corresponding receptor FLS2 in Arabidopsis thaliana [4, 5]. In addition, the N-terminal sequence of bacterial translational elongation factor Tu (designated as elf18) can be recognized through the corresponding receptor EFR [6, 7].
In contrast to bacterial PAMP receptors, much less is known about the role of fungal PAMP receptors in plants. It is conceivable that oligosaccharides derived from chitin or glucan may function as PAMPs because they are major structural components of fungal cell walls and can induce the expression of several defense-related genes when they are applied to plants [8, 9]. The rice plasma membrane glycoprotein CEBiP (Chitin Elicitor Binding Protein) was shown to be an important component for chitin-derived signaling and is thought to be a receptor for fungal PAMPs . CEBiP was identified as a chitin-binding protein from suspension cultured rice cells and contains two LysM (lysin) domains which mediate binding to oligosaccharides. Physiological experiments suggest that CEBiP is required for the production of reactive oxygen species by rice plants in response to treatment with chitin elicitor . It is assumed that CEBiP recognizes chitin oligosaccharides present on the fungal cell surface or released into the plant apoplast, leading to the expression of plant disease resistance against fungal infection. However, it has not yet been reported whether CEBiP is actually required for restricting the growth of fungal pathogens in rice.
Magnaporthe oryzae is an ascomycete fungus that causes the devastating blast disease in rice . In the previous report, we have generated ssd1 mutants in M. oryzae and the cucumber anthracnose fungus Colletotrichum orbiculare, in which infection of their respective host plants was restricted by cellular defense responses . Subsequently, by inoculating the C. orbiculare ssd1 mutant onto Nicotiana benthamiana plants in which defense-related genes were silenced, we evaluated the involvement of those genes in basal defense. These experiments revealed that plants in which genes encoding specific MAPKK (MEK2) and MAPKs (SIPK/WIPK) had been silenced were susceptible to the ssd1 mutant, as well as the wild-type strain . Furthermore, we revealed that these MAPKs were activated by fungal cell surface components during infection and that the level of MAPK activation induced by the ssd1 mutant was higher than by the wild-type strain, suggesting that MAPK signaling is required for enhanced basal defense and restriction of fungal infection. In addition, use of the ssd1 mutant together with gene-silenced plants allowed us to critically evaluate the involvement of specific defense-related genes in basal resistance by assessing whether the ssd1 mutant could produce disease lesions on the silenced plants.
In plants, RNA interference (RNAi) is a powerful tool for the evaluation of gene function . For RNAi, it is necessary to generate transgenic plants that express a partial fragment of the target gene, but considerable time is required to obtain seeds from T1 transformants. In contrast, virus-induced gene silencing (VIGS) is a simple, rapid method to transiently generate knock-down plants that avoids the need for stable transformation . Although procedures for VIGS are not yet established for rice, there are reports that VIGS is applicable to barley through the use of barley stripe mosaic virus (BSMV) [16, 17]. Barley is a susceptible host plant for M. oryzae, so that interactions between M. oryzae and barley provide a model for the molecular analysis of compatible interactions between monocot plants and fungal pathogens .
In this study, we have exploited the barley-Magnaporthe pathosystem to evaluate the involvement in basal resistance of genes encoding a putative PAMP receptor, namely HvCEBiP, which is homologous to the rice chitin receptor CEBiP. For this, we used the M. oryzae ssd1 mutant and BSMV-mediated gene silencing. We present evidence that HvCEBiP contributes to basal defense against appressorium-mediated infection by M. oryzae in barley.
Magnaporthe oryzae SSD1is required for infection of barley
Virus-induced gene silencing of HvCEBiPusing barley stripe mosaic virus
HvCEBiP contributes to restricting infection by mossd1mutants
To determine whether the mossd1 mutant was able to develop infection hyphae and colonize barley tissues, we observed leaf inoculation sites in BSMV:HvCEBiP-treated plants at 96 hpi. At sites showing brown necrotic flecks (Type II symptom), appressoria were present on the leaf surface, and infection hyphae developed from appressoria inside the initially infected epidermal cell, which appeared to undergo a cell death reaction (Figure 5C). However, when we observed inoculation sites at 7 dpi, fungal hyphae had not colonized the neighboring host cells and hyphae were entirely confined to the first infected cell (data not shown). These observations suggest that mossd1 mutant appressoria could penetrate into HvCEBiP-silenced plants but subsequent growth of the infection hyphae became restricted by host defense responses. However, at the few inoculation sites showing severe lesions (Type III), infection hyphae were seen to develop from appressoria without visible host cell death (Figure 5C). Taken together, these results suggest that HvCEBiP contributes to host defense responses expressed after invasion of epidermal cells by M. oryzae infection hyphae.
Expression profiling of defense-related genes in HvCEBiP-silenced plants
Barley expresses two layers of basal defense in response to infection by Magnaporthe oryzae
In our previous study, we generated an ssd1 mutant of M. oryzae, in which the infection of rice plants was restricted by a defense response involving death of the initially infected epidermal cell . This cell death reaction expressed by rice in response to compatible isolates of M. oryzae has been termed 'whole-plant specific resistance' (WPSR), and is independent of R-gene mediated resistance in rice [23, 24]. In the present study, infection assays revealed that the mossd1 mutant also showed attenuated pathogenicity on barley. However, the host defense responses expressed in barley to appressorial penetration by the mossd1 mutant took the form of papilla deposition at attempted fungal entry sites rather than host cell death. The phenomenon of papilla formation during M. oryzae infection of barley has also been reported by other authors . In rice, papilla-like wall appositions were also observed beneath appressoria of M. oryzae, although these appeared small and thin with electron microscopy . Therefore, the formation of papillae appears to be a general form of basal defense against attempted appressorial penetration by M. oryzae in barley. However, the efficiency of papillae in restricting appressorial penetration seems to be weak because the wild-type strain could successfully penetrate into plant cells with high frequency, as shown in Figure 2C. Apart from papilla formation, a localized cell death reaction was also observed in the initially penetrated host cells in which infection hyphae had developed. This cell death reaction was observed in the leaves of BSMV:HvCEBiP-treated barley plants after infection by both the ssd1 mutant and the wild-type strain of M. oryzae. The cell death reaction was associated with inhibition of fungal growth because infection hyphae had not developed beyond the first infected epidermal, even after 7 days. The barley cell death reaction resembles WPSR in rice  and conceivably it represents a basal defense response triggered after successful penetration by M. oryzae appressoria. It therefore appears that barley deploys two distinct layers of basal defenses against appressorium-mediated infection by M. oryzae, namely papilla formation and localized cell death. Two similar layers of plant defense were also shown to operate during non-host resistance of Arabidopsis to powdery mildew fungi .
HvCEBiP is involved in basal resistance to appressorial penetration by M. oryzae
In our recent work, we used the C. orbiculare ssd1 mutant to show that a specific MAPK pathway in N. benthamiana plays a critical role in host basal defense but genes required for R-gene mediated resistance (RAR1, SGT1 and HSP90) do not . Here, we used the M. oryzae ssd1 mutant to examine the role in basal defense of genes required for penetration resistance and R-gene mediated resistance. Ror1 and Ror2 were identified as genes required for mlo-specific resistance against the barley powdery mildew fungus Blumeria graminis f. sp. hordei and Ror2 shows functional homology to syntaxin AtSYP121 in Arabidopsis . Rar1 was originally shown to be required for race-specific resistance triggered by resistance gene Mla12 against B. graminis f. sp. hordei expressing the avirulence gene AvrMla12 [28, 29]. Rom1 was identified as a restoration of Mla12-specified resistance (rom1) mutant that restores disease resistance to B. graminis f. sp. hordei carrying the avirulence gene AvrMla12 . However, infectivity of the mossd1 mutant was not significantly enhanced on any of these barley mutants compared to wild-type plants, suggesting that genes required for R-gene mediated resistance do not play a role in basal defense against M. oryzae, consistent with findings from the C. orbiculare-N. benthamiana interaction .
In addition to the increased frequency of brown necrotic fleck symptoms induced by the mossd1 mutant on BSMV:HvCEBiP-treated plants, a few inoculation sites also showed formation of severe blast lesions (Type III symptom) as shown in Figure 5A. Lesion formation was not associated with localized cell death reactions and infection hyphae developed extensively, colonizing many host cells. This suggests that in some cases the mossd1 mutant was able to infect HvCEBiP-silenced plants without triggering cell death-associated defense responses. This raises the possibility that HvCEBiP might be involved in mediating the localized cell death response of barley epidermal cells to invasion by M. oryzae infection hyphae. Thus, HvCEBiP might contribute not only to papilla-based defenses but also to the hypersensitive cell death response to cell invasion. HvCEBiP does not appear to play a central role in nonhost resistance because the non-adapted pathogen C. graminicola produced no symptoms on silenced plants. In contrast, the LysM domain receptor kinase CERK1 was reported to contribute weakly to the resistance of Arabidopsis thaliana against the incompatible pathogen Alternaria brassicicola .
Is HvCEBiP a specific receptor for components of the mossd1mutant?
In the interaction between cucumber anthracnose pathogen C. orbiculare and N. benthamiana, we reported previously that the altered fungal cell wall composition conferred by ssd1 gene disruption triggers plant basal resistance through the activation of a specific plant MAPK cascade . We hypothesized that activation of the MAPK pathway might result from recognition of fungal PAMP(s) by corresponding plant receptor protein(s). In this study, we attempted to determine whether HvCEBiP is a specific receptor for PAMPs expressed uniquely by the mossd1 mutant, in which case pathogenicity of the wild-type strain should not be affected by the silencing of HvCEBiP. However, the wild-type strain Hoku-1 showed a slight increase in pathogenicity on HvCEBiP-silenced plants, suggesting that HvCEBiP is a receptor for component(s) shared by both the wild-type M. oryzae and mossd1 mutant.
Rice CEBiP is a receptor-like protein containing two LysM domains, which was originally identified in enzymes that degrade the bacterial cell wall component peptidoglycan . Recent biochemical analysis showed that the LysM domain can also mediate binding to chitin oligosaccharides . The genome of Arabidopsis contains five LysM domain-containing receptor-like kinases , among which CERK1 (At3g21630) was identified as a receptor-like protein required for chitin signaling in Arabidopsis . Although the function of the other LysM domain-containing receptor-like kinases is unknown, it is tempting to speculate that plants possess multiple receptor proteins for the perception of particular classes of pathogen-derived oligosaccharides. It is likely that other PAMP receptors, in addition to HvCEBiP, are conserved in barley and contribute to basal resistance to M. oryzae.
Rice CEBiP recognizes chitin oligosaccharides derived from fungal cells leading to the expression of plant disease resistance against fungal infection. We evaluated the involvement of putative chitin receptor gene HvCEBiP in barley basal resistance using the mossd1 mutant of Magnaporthe oryzae, which enhances host basal defense responses. The mossd1 mutant showed attenuated pathogenicity on barley and appressorial penetration was restricted by the formation of papillae at attempted entry sites. On HvCEBiP-silenced plants, the mutant produced small brown necrotic flecks or blast lesions accompanied by appressorium-mediated penetration into plant epidermal cells. Wild-type M. oryzae also produced slightly more severe symptoms on the leaves of HvCEBiP-silenced plants. These results indicated that HvCEBiP is involved in basal resistance against appressorium-mediated infection and that basal resistance could be triggered by the recognition of chitin oligosaccharides derived from M. oryzae.
Plant growth conditions and fungal strains
Hordeum vulgare wild-type cultivars Fiber-snow, Ingrid and Sultan5, and genetic mutants mlo5, mlo5ror1, mlo5ror2, rar1 and rom1 were grown in a controlled environment chamber (16 h photoperiod, 24°C). Magnaporthe oryzae Hoku-1 was used as the wild-type strain in this study. The mossd1 mutants K1 and K4 were generated as reported previously . These fungal cultures were maintained at 24°C on oatmeal agar medium (6.0 g powder oatmeal, 1.25 g agar per 100 ml distilled water) under continuous light. Colletotrichum graminicola isolate MAFF236902 was described previously .
Pathogen inoculation and cytological assays
To induce conidiation, two week-old cultures of M. oryzae were washed with sterile water to remove aerial hyphae and then incubated for a further 3 days. For inoculation, conidial suspension was sprayed (5 ml; 1 × 106 conidia/ml) or spotted (10 μl; 1 × 105 conidia/ml) onto the third leaves of H. vulgare and incubated in a humid plastic box at 24°C. For evaluation of invasive growth ability, the surface of barley leaves was scratched with a sterile plastic pipette tip and droplets of conidial suspensions were placed directly onto the wound sites. Cytological observations and the detection of papillae were performed as follows. Inoculated leaves were cut to 1 cm × 1 cm size and decolorized with a 3:1 mixture of ethanol:chloroform and mounted under a coverslip in lactophenol solution. Autofluorescent papillae formed beneath appressoria were visualized by epifluoresence. The accumulation of H2O2 in host cells was detected by staining with 3,3'-diaminobenzidine .
Total RNA was extracted from barley leaves using TRIzol Reagent (Invitrogen) following the manufacturer's protocol. RT-PCR was performed using ReverTra Dash RT-PCR kit (Toyobo) following the manufacturer's protocol. The primers used for RT-PCR are listed in Additional file 1: Table S1. The sequence data of HvPAL, HvPR-1, HvPR-2a, HvPR-5, HvRBOHA and HvEF1α can be found in GeneBank with accession numbers Z49147, Z21494, AY612193, AF355455, AJ871131 and Z50789, respectively.
A 298 bp partial fragment of HvCEBiP was amplified by primer pairs HvCBP1-S1 (5'-CCAAAGACCCTCAAGAAGGA-3') and HvCBP1-AS1 (5'-AGCCGTTGGAATAACCACTG-3') from cDNA of H. vulgare and subcloned into the pGEM-T easy vector (Promega). The resulting construct was digested by NotI and a fragment containing the amplified sequence of HvCEBiP was introduced into the NotI site of pSL038-1 in the antisense orientation. This construct was designated as pγ:HvCEBiPas.
Virus-induced gene silencing
BSMV genomic RNAs were transcribed in vitro as previously described with some modifications . The reaction was performed at 37°C for 60 min in 50 μl of reaction buffer containing 1 μg of linearized plasmids, 1 μl of T7 RNA polymerase (Takara), 10 μl of 50 mM DTT, 6 μl of 10 mM NTPs (rATP, rCTP, rUTP), 0.4 μl of 10 mM rGTP and 5 μl of 5 mM m7G(ppp)G RNA cap structure analog (New England Biolabs). After the reaction, 1.62 μl of 10 mM rGTP and 1 μl of T7 RNA polymerase were added to the reaction mixture, and further incubated at 37°C for 60 min. Transcribed α, β, γ genomic RNAs were mixed in a 1:1:1 ratio with 20 μl FES and inoculated onto the first-developed leaves of H. vulgare plants with gentle rubbing. The third-developed leaves were used for evaluating fungal infections.
We are grateful to Dr. Kazuyuki Mise (Kyoto University) for technical advice about in vitro transcription. We are grateful to Dr. Steve Scofield (USDA-ARS, West Lafayette) for providing BSMV vectors and Professor Paul Schulze-Lefert (Max Planck Institute for Plant Breeding Research) for providing barley mutant seeds. This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (No.19380029 and 21380031) and JSPS Fellowships from the Ministry of Education, Culture, Sports, Science and Technology (No. 19380024).
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