Shauna Somerville - Non Host Resistance

NON-HOST RESISTANCE IN ARABIDOPSIS

Non-host resistance is an operational definition that describes the resistance observed when all members of a plant species exhibit resistance to all members of a given pathogen species (Heath 1991, Thordal-Christensen 2003). This form of resistance is rarely studied by comparison with the more familiar race-cultivar specific resistance employed by plant breeders in developing disease resistant cultivars. A number of race-cultivar specific resistance genes have been cloned and found to encode proteins that have the hallmarks of receptors, features which are consistent with long-standing models of the function of host resistance proteins. In contrast, non-host resistance is thought to comprise a variety of distinct mechanisms that may include the production of pre-formed toxins or barriers, or a lack of essential metabolites or signaling molecules required by the pathogen. Thus, non-host resistance is thought to be multi-genic and the inactivation of any one component may not be sufficient to render a plant susceptible. A further component of the model developed by M. Heath (1991) is that basic compatibility is established in an incremental process over evolutionary time by the subversion of non-host defense mechanisms by a pathogen species.

Our group has characterized the non-host responses of Arabidopsis to the barley powdery mildew pathogen, Blumeria graminis f. sp. hordei. A majority of fungal propagules were arrested at the penetration stage and never formed haustoria (Yun et al. 2003, Zimmerli et al. 2004). However, a small percentage (~5%) were able to form haustoria and in some cases elongating secondary hyphae (a measure that the haustoria are functional). No conidiophores or asexual conidia developed. After surveying a variety of host responses to inoculation with B. g. hordei (e.g., papilla formation, callose deposition, H2O2 staining, dead cells), the most dramatic difference between responses to the barley and Arabidopsis powdery mildews was the callose response. Both fungi elicited papilla formation at sites of attempted penetration; however, the barley powdery mildew also elicited wide-spread deposition of callose in mesophyll cells subtending infected epidermal cells (Zimmerli et al. 2004).

A screen for Arabidopsis mutants with an altered callose response to B. g. hordei was initiated. The screen was designed to recover both callose-deficient and callose-enhanced mutants. To date, mutants in four pen (penetration resistance) complementation groups have been recovered. Two of the mutant classes, pen1 and pen2, were previously identified in other screens for defects in non-host resistance (Collins et al. 2003; Lipka et al. 2005). PEN1 encodes a syntaxin (SYP122) and PEN2 a glycosyl hydrolase. PEN3 (=PDR8) encodes an ABC transporter of the pleiotropic drug resistance class (Stein et al. 2006). Although the specific roles of each of these components in penetration resistance is unknown, our working hypothesis is that PEN2 participates in the synthesis of a toxic secondary metabolite and that PEN1 and PEN3 export materials to the cell wall space to block pathogen ingress. These defenses substances may be toxic compounds or materials to reinforce the cell wall (Stein et al. 2006).

Fig. 1. Penetration and formation of elongating secondary hyphae by the barley powdery mildew pathogen on three pen mutants. Left panel (1): % of germinated conidia that penetrate the host cell wall, Right panel (2): % of germinated conidia that form elongating secondary hyphae, a measure of successful establishment of haustoria (fungal feeding structure). Red, wild-type; Yellow, pen1 (48M3); green, pen2 (136N4); blue, pen3 (114N4). From Stein et al. 2006.

References:

Collins, N., H. Thordal-Christensen, V. Lipka, S. Bau, E. Kombrink, M. Stein, R. Hückelhoven, S. Somerville and P. Schulze-Lefert. 2003. Conserved SNARE secretion machinery components mediate cell wall penetration resistance against powdery mildew plant pathogens. Nature 425, 973-977.

Heath, M.C. 1991. The role of gene-for-gene interactions in the determination of host species specificity. Phytopathology 81: 127-130.

Lipka, V., J. Dittgen, P. Bednarek, R. Bhat, M. Wiemer, M. Stein, J. Landtag, W. Brandt, S. Rosahl, D. Scheel, F. Llorente, A. Molina, J. Parker, S. Somerville and P. Schulze-Lefert. 2005. Pre- and post-invasion defenses both contribute to nonhost resistance in Arabidopsis. Science 301, 1180-1183.

Stein, M., J. Dittgen, C. Sánchez-Rodriguez, B.-H. Hou, A. Molina, P. Schulze-Lefert, V. Lipka and S. Somerville. 2006. Arabidopsis PEN3/PDR8, an ATP binding cassette transporter, contributes to nonhost resistance to inappropriate pathogens that enter by direct penetration. The Plant Cell 18, 731-746.

Thordal-Christensen, H. 2003. Fresh insights into processes of nonhost resistance. Current Opinion in Plant Biology 6: 351-357.

Yun, B.W., H.A. Atkinson, C. Gaborit, A. Greenland, N.D. Read, J.A. Pallas and G.J. Loake. 2003. Loss of actin cytoskeletal function and EDS1 activity, in combination, severely compromises non-host resistance in Arabidopsis against wheat powdery mildew. Plant Journal 34: 768-777.

Zimmerli, L., M. Stein, V. Lipka, P. Schulze-Lefert and S. Somerville. 2004. Host and nonhost pathogens elicit different jasmonate/ethylene response in Arabidopsis. The Plant Journal 40, 633-646.

Gene Expression Profiling of Nonhost Resistance

A comparison of the expression profiles of Arabidopsis plants infected with the barley powdery mildew (incompatible, non-host interaction) to those infected with the Arabidopsis powdery mildew, Golovinomyces cichoracearum (compatible interaction) showed that a majority of transcripts were either induced or repressed by both types of pathogens, even though the outcomes of the infections were markedly different (Zimmerli et al. 2004). Among the transcripts that were repressed by infection with either pathogen are a number of photosynthesis and carbon metabolism-related transcripts. This repression is thought to represent a reallocation of resources to defense responses. A number of common defense-related transcripts were up-regulated in response to infections by either pathogen and may represent defense genes associated with basal resistance, a form of resistance with limited effectiveness against the Arabidopsis powdery mildew. A relatively small number of genes were differentially expressed or expressed more rapidly following infections with B. graminis f.sp. hordei.

Fig. 2. Comparison of the gene expression profiles of Arabidopsis plants inoculated with a host pathogen or a nonhost pathogen. Three time points were monitored: 8, 18 and 24 hours post-inoculation. H: plants inoculated with the Arabidopsis powdery mildew, G. cichoracearum; NH: plants inoculated with the barley powdery mildew, B. g. hordei; UN: uninoculated control plants. From Zimmerli et al. 2004. From Zimmerli et al. 2004.

Reference
Zimmerli, L., M. Stein, V. Lipka, P. Schulze-Lefert and S. Somerville. 2004. Host and nonhost pathogens elicit different jasmonate/ethylene responses in Arabidopsis. (in revision).

Chito-oligomer-Induced Changes in Gene Expression and their Role in Plant-Pathogen Interactions

Chitin is a common component of fungal and insect cell walls and is likely to be one of the signals exchanged between host and pathogen that impact the outcome of an infection. Chito-oligomers are known to elicit defense responses, such as the production of reactive oxygen species and the induction of defense gene expression. Because of the ubiquity of chitin in pathogenic and non-pathogenic fungi and insects, chito-oligomers are likely to be non-specific elicitors that may play a role in triggering non-host defenses or basal defenses. In collaboration with Gary Stacey (University of Missouri, Columbia, MO), we have generated a profile of the Arabidopsis genes induced or repressed by treatment with chito-oligomers (Ramonell et al. 2002, Ramonell et al. 2004, Zhang et al. 2002). A number of transcripts were up-regulated at the earliest time point, 10 min, with the largest number of changes in transcript levels occurring at 30 min after chito-oligomer treatment. Comparisons with the transcript profiles of salicyclic acid-, ethylene- or jasmonate-treated plants suggested that chito-oligomers do not act via these well-known signaling pathways.

Fig. 3. Number of differentially expressed genes at various times following chito-oligomer treatment. Dark boxes, down-regulated genes. White boxes, up-regulated genes. From: Ramonell et al. 2002.

Insertion mutants of three chito-oligomer responsive genes, two disease resistance gene-like genes and an E3 ligase gene, were shown to be mildly compromised in their resistance to the Arabidopsis powdery mildew pathogen (Ramonell et al. 2004). These results confirm that chito-oligomer-based signaling does contribute to resistance to powdery mildew.

References:

Ramonell, K.M., B. Zhang, R.M. Ewing, Y. Chen, D.Xu, G. Stacey and S. Somerville. 2002. Microarray analysis of chitin elicitation in Arabidopsis thaliana. Molecular Plant Pathology 3: 301-311.

Ramonell, K., M. Berrocal-Lobo, S. Koh, J. Wan, H. Edwards, G. Stacey and S. Somerville. 2005. Loss-of-function mutations in chitin responsive genes show increased susceptibility to the powdery mildew pathogen, Erysiphe cichoracearum. Plant Physiology 138, 1027-1036.

Zhang, B., K. Ramonell, S. Somerville and G. Stacey. 2002. Characterization of early, chitin-induced gene expression in Arabidopsis. Molecular Plant-Microbe Interactions 15: 963-970.