Function In Disease Resistance Research Articles

Arabidopsis Extra Large G-Protein 2 (XLG2) Interacts with the Gβ Subunit of Heterotrimeric G Protein and Functions in Disease Resistance

Heterotrimeric GTP-binding proteins, which consist of Galpha, Gbeta, and Ggamma subunits, play important roles in transducing extracellular signals perceived by cell surface receptors into intracellular physiological responses. In addition to a single prototypical Galpha protein (GPA1), Arabidopsis has three unique Galpha-like proteins, known as XLG1, XLG2, and XLG3, that have been found to be localized in nuclei, although their functions and mode of action remain largely unknown. Through a transcriptomic analysis, we found that XLG2 and XLG3 were rapidly induced by infection with the bacterial pathogen Pseudomonas syringae, whereas the XLG1 transcript level was not affected by pathogen infection. A reverse genetic screen revealed that the xlg2 loss-of-function mutation causes enhanced susceptibility to P. syringae. Transcriptome profiling revealed that the xlg2 mutation affects pathogen-triggered induction of a small set of defense-related genes. However, xlg1 and xlg3 mutants showed no difference from wild-type plants in resistance to P. syringae. In addition, the xlg2 xlg3 double mutant and the xlg1 xlg2 xlg3 triple mutant were not significantly different from the xlg2 single mutant in the disease resistance phenotype, suggesting that the roles of XLG1 and XLG3 in defense, if any, are less significant than for XLG2. Constitutive overexpression of XLG2 leads to the accumulation of abnormal transcripts from multiple defense-related genes. Through co-immunoprecipitation assays, XLG2 was found to interact with AGB1, the sole Gbeta subunit in Arabidopsis, which has previously been found to be a positive regulator in resistance to necrotrophic fungal pathogens. However, no significant difference was found between three xlg single mutants, the xlg2 xlg3 double mutant, the xlg triple mutant, and wild-type plants in resistance to the necrotrophic fungal pathogens Botrytis cinerea or Alternaria brassicicola. These results suggest that XLG2 and AGB1 are components of a G-protein complex different from the prototypical heterotrimeric G-protein and may have distinct functions in modulating defense responses.

Pathogen-Induced Expressional Loss of Function is the Key Factor in Race-Specific Bacterial Resistance Conferred by a Recessive R Gene xa13 in Rice

The fully recessive disease resistance (R) gene xa13, which mediates race-specific resistance to Xanthomonas oryzae pv. oryzae (Xoo), encodes a plasma membrane protein that differs by one amino acid from that encoded by its dominant (susceptible) allele Xa13. The molecular mechanism of xa13-mediated resistance is largely unknown. Here we show that, compared with its dominant allele, expressional non-reaction of xa13 to Xoo infection, not its protein composition, is the key factor for xa13-mediated resistance. We used the promoter (P(Xa13)) of the dominant Xa13, which was induced by only the incompatible Xoo strain for xa13, to regulate xa13 and xa13(Leu49) (a natural recessive allele of xa13) in the rice line IRBB13 carrying xa13. The transgenic plants showed the same level of susceptibility and bacterial growth rate as those of the rice line carrying dominant Xa13, accompanied by the induced accumulation of xa13 or xa13(Leu49) proteins. Constitutive expression of dominant XA13 or different xa13 proteins (xa13, xa13(Leu49), xa13(Ala85) or xa13(Val184)) in IRBB13 had no effect on Xoo infection in the transgenic plants. These results suggest that race-specific pathogen-induced Xa13 expression is critical for infection. Thus, xa13 stands out from other R genes in that its functions in disease resistance are due to only the loss of pathogen-induced transcriptional motivation caused by natural selection.

Co-option of a default secretory pathway for plant immune responses

Cell-autonomous immunity is widespread in plant-fungus interactions and terminates fungal pathogenesis either at the cell surface or after pathogen entry. Although post-invasive resistance responses typically coincide with a self-contained cell death of plant cells undergoing attack by parasites, these cells survive pre-invasive defence. Mutational analysis in Arabidopsis identified PEN1 syntaxin as one component of two pre-invasive resistance pathways against ascomycete powdery mildew fungi. Here we show that plasma-membrane-resident PEN1 promiscuously forms SDS-resistant soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) complexes together with the SNAP33 adaptor and a subset of vesicle-associated membrane proteins (VAMPs). PEN1-dependent disease resistance acts in vivo mainly through two functionally redundant VAMP72 subfamily members, VAMP721 and VAMP722. Unexpectedly, the same two VAMP proteins also operate redundantly in a default secretory pathway, suggesting dual functions in separate biological processes owing to evolutionary co-option of the default pathway for plant immunity. The disease resistance function of the secretory PEN1-SNAP33-VAMP721/722 complex and the pathogen-induced subcellular dynamics of its components are mechanistically reminiscent of immunological synapse formation in vertebrates, enabling execution of immune responses through focal secretion.

Genome-wide Detection and Analysis of Alternative Splicing for Nucleotide Binding Site-Leucine-Rich Repeats Sequences in Rice

Alternative splicing is a major contributor to genomic complexity and proteome diversity, yet the analysis of alternative splicing for the sequence containing nucleotide binding site and leucine-rich repeats (NBS-LRR) domain has not been explored in rice ( Oryza sativa L.). Hidden Markov model (HMM) searches were performed for NBS-LRR domain. 875 NBS-LRR-encoding sequences were obtained from the Institute for Genomic Research (TIGR). All of them were used to blast Knowledge-based Oryza Molecular Biological Encyclopaedia (KOME), TIGR rice gene index (TGI), and Universal Protein Resource (UniProt) to obtain homologous full-length cDNAs (FL-cDNAs), tentative consensus sequences, and protein sequences. Alternative splicing events were detected from genomic alignment of FL-cDNAs, tentative consensus sequences, and protein sequences, which provide valuable information on splice variants of genes. These sequences were aligned to the corresponding BAC sequences using the Spidey and Sim4 programs and each of the proteins was aligned by tBLASTn. Of the 875 NBS-LRR sequences, 119 (13.6%) sequences had alternative splicing where multiple FL-cDNAs, TGI sequences and proteins corresponded to the same gene. 71 intron retention events, 20 exon skipping events, 16 alternative termination events, 25 alternative initiation events, 12 alternative 5′ splicing events, and 16 alternative 3′ splicing events were identified. Most of these alternative splices were supported by two or more transcripts. The data sets are available at http://www.bioinfor.org Furthermore, the bioinformatics analysis of splice boundaries showed that exon skipping and intron retention did not exhibit strong consensus. This implies a different regulation mechanism that guides the expression of splice isoforms. This article also presents the analysis of the effects of intron retention on proteins. The C-terminal regions of alternative proteins turned out to be more variable than the N-terminal regions. Finally, tissue distribution and protein localization of alternative splicing were explored. The largest categories of tissue distributions for alternative splicing were shoot and callus. More than one-thirds of protein localization for splice forms was plasma membrane and cytoplasm. All the NBS-LRR proteins for splice forms may have important function in disease resistance and activate downstream signaling pathways.

Secretome Analysis Reveals anArabidopsisLipase Involved in Defense againstAlternaria brassicicola

The Arabidopsis thaliana secretome was analyzed by the proteomic approach, which led to the identification of secreted proteins implicated in many aspects of cell biology. We then investigated the change in the Arabidopsis secretome in response to salicylic acid and identified several proteins involved in pathogen response. One of these, a secreted lipase with a GDSL-like motif designated GDSL LIPASE1 (GLIP1), was further characterized for its function in disease resistance. glip1 plants were markedly more susceptible to infection by the necrotrophic fungus Alternaria brassicicola compared with the parental wild-type plants. The recombinant GLIP1 protein possessed lipase and antimicrobial activities that directly disrupt fungal spore integrity. Furthermore, GLIP1 appeared to trigger systemic resistance signaling in plants when challenged with A. brassicicola, because pretreatment of the glip1 mutant with recombinant GLIP1 protein inhibited A. brassicicola-induced cell death in both peripheral and distal leaves. Moreover, glip1 showed altered expression of defense- and ethylene-related genes. GLIP1 transcription was increased by ethephon, the ethylene releaser, but not by salicylic acid or jasmonic acid. These results suggest that GLIP1, in association with ethylene signaling, may be a critical component in plant resistance to A. brassicicola.

Chloroisonicotinamide derivative induces a broad range of disease resistance in rice and tobacco.

Systemic acquired resistance (SAR) is a potent innate immunity system in plants that is effective against a broad range of pathogens. SAR in dicotyledonous plants such as tobacco and Arabidopsis has been partially elucidated and is mediated by salicylic acid (SA). However, the SAR mechanism of monocotyledonous rice plants remains to be clarified, although some similarities between SAR mechanisms in both types have been reported. Here we have characterized N-cyanomethyl-2-chloroisonicotinamide (NCI) as an effective SAR inducer in both plant species. Soil drench application of NCI induces a broad range of disease resistance in tobacco and rice and, more specifically, PR gene expression in tobacco. Both SA measurements in wild-type NCI-treated tobacco and pathogenic infection studies using NahG transgenic tobacco plants indicate that NCI-induced resistance enhancement does not require SA. Therefore, it is suggested that NCI induces SAR by triggering signaling at the same level as or downstream of SA accumulation as do both benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester and 2,6-dichloroisonicotinic acid. The fact that all of these chemicals are effective in rice and tobacco suggests that several common components function in disease resistance in both plant species.

Nonparametric Joint Estimators for Disease Resistance and Survival Functions in Survival/Sacrifice Experiments

The nonparametric estimators proposed in this paper simultaneously estimate the disease resistance function, S(t), for the distribution of time to occurrence of disease, and the survival function, P(t), for the distribution of time to death caused by the disease. The estimators are applicable to animal survival data when the disease of interest is irreversible and the cause of death of each animal can be determined. Diseases need not be classified as universally lethal or universally nonlethal, and time to occurrence of the disease of interest need not be observable directly. It is shown that the proposed estimators are maximum likelihood estimators under the condition that S(t)/P(t) is monotonically decreasing. Application of the derived estimators is illustrated with experimental survival/sacrifice data, and results of a simulation study of the estimators' performance are reported.