SummarySubcellular events of Erysiphe cichoracearum infections of epidermal cells were visualized in living tissues of Arabidopsis plants carrying various green fluorescent protein (GFP)-tagged organelles via laser scanning confocal microscopy. Early in the infection sequence, cytoplasm and organelles moved towards penetration sites and accumulated near penetration pegs. Peroxisomes appeared to accumulate preferentially relative to the cytoplasm at penetration sites. Another early event, which preceded haustorium formation, was the aggregation of some GFP-tagged plasma membrane marker proteins into rings around penetration sites, which extended across cell-wall boundaries into neighboring cells. This feature localized to sites where papillae were deposited. The extrahaustorial membrane (EHM) encases the fungal feeding structure, the haustorium, separating it from the host cytoplasm. Eight plasma membrane markers were excluded from the EHM and remained in a collar-like formation around the haustorial neck. These observations support the suggestions that the EHM is a unique, specialized membrane and is different from the plasma membrane. Our results suggested two possibilities for the origin of the EHM: invagination of the plasma membrane coupled with membrane differentiation; or de novo synthesis of the EHM by targeted vesicle trafficking.

Chitin is a major component of fungal walls and insect exoskeletons. Plants produce chitinases upon pathogen attack and chito-oligomers induce defense responses in plants, though the exact mechanism behind this response is unknown. Using the ATH1 Affymetrix microarrays consisting of about 23,000 genes, we examined the response of Arabidopsis (Arabidopsis thaliana) seedlings to chito-octamers and hydrolyzed chitin after 30 min of treatment. The expression patterns elicited by the chitooctamer and hydrolyzed chitin were similar. Microarray expression profiles for several genes were verified via northern analysis or quantitative reverse transcription-PCR. We characterized T-DNA insertion mutants for nine chito-oligomer responsive genes. Three of the mutants were more susceptible to the fungal pathogen, powdery mildew, than wild type as measured by conidiophore production. These three mutants included mutants of genes for two disease resistance-like proteins and a putative E3 ligase. The isolation of loss-of-function mutants with enhanced disease susceptibility provides direct evidence that the chito-octamer is an important oligosaccharide elicitor of plant defenses. Also, this study demonstrates the value of microarray data for identifying new components of uncharacterized signaling pathways.Plants in the environment are constantly under siege by a multitude of disease-causing organisms including bacteria, fungi, viruses, and nematodes. Plants may resist pathogen attack using both preformed defenses (e.g. antimicrobial compounds) and inducible defense responses (for review, see HammondKosack and Jones, 2000;Heath, 2000). Inducible defenses can be activated upon recognition of general elicitors such as bacterial flagellin (Gomez-Gomez and Boller, 2002), the polypeptide systemin (Ryan and Pearce, 1998), and multiple host or pathogen cell wall fragments released during pathogen attack (Nü rnberger et al., 2004). Recently, oligosaccharide elicitors such as chito-oligomers and oligogalacturonides have received renewed attention as important signals in plant defense responses. The activation of defense genes by these elicitors is thought to be receptor-mediated though little is known about the initial perception and consequent signaling pathways involved in plant cells.Chito-oligosaccharides can be generated from the cell walls of pathogenic fungi by the action of endochitinases and were shown to elicit strong defense responses in many plant species (Stacey and Shibuya, 1997;Shibuya and Minami, 2001). In a previous study, we showed that transcript levels for 71 expressed sequence tags, representing 61 genes, were altered more than 3-fold in chito-oligomer treated Arabidopsis (Arabidopsis thaliana) seedlings, demonstrating the usefulness of Arabidopsis as a model for studying chitin signaling in plants . Further experimentation by Zhang et al. (2002) showed that this response was not mediated by the well-characterized salicylic acid, jasmonic acid, or ethylene-responsive plant defense pathways. More recently, Wan et al. (...

T-DNA-tagged rice plants were screened under cold- or salt-stress conditions to determine the genes involved in the molecular mechanism for their abiotic-stress response. Line 0-165-65 was identified as a salt-responsive line. The gene responsible for this GUS-positive phenotype was revealed by inverse PCR as OsGSK1 (Oryza sativa glycogen synthase kinase3-like gene 1), a member of the plant GSK3/SHAGGY-like protein kinase genes and an orthologue of the Arabidopsis brassinosteroid insensitive 2 (BIN2), AtSK21. Northern blot analysis showed that OsGSK1 was most highly detected in the developing panicles, suggesting that its expression is developmental stage specific. Knockout (KO) mutants of OsGSK1 showed enhanced tolerance to cold, heat, salt, and drought stresses when compared with non-transgenic segregants (NT). Overexpression of the full-length OsGSK1 led to a stunted growth phenotype similar to the one observed with the gain-of-function BIN/AtSK21 mutant. This suggests that OsGSK1 might be a functional rice orthologue that serves as a negative regulator of brassinosteroid (BR)-signaling. Therefore, we propose that stress-responsive OsGSK1 may have physiological roles in stress signal-transduction pathways and floral developmental processes.