Requirement of a Homolog of Glucosidase Ⅱ β-Subunit for EFR-Mediated Defense Signaling in Arabidopsis thaliana

EFR is a plasma-membrane resident receptor responsible for recognition of microbial elongation factorTu （EF-Tu） and thus triggering plant innate immunity to fend off phytopathogens. Functional EFR must be subject to the endoplasmic reticulum quality control （ERQC） machinery for the correct folding and proper assembly in order to reach its final destination. Genetic studies have demonstrated that ERD2b, a counterpart of the yeast or mammalian HDEL receptor ERD2 for retaining proteins in the endoplasmic reticulum （ER） lumen, is required for EFR function in plants （Li et al., 2009）. In this study, we characterized the Arabidopsis glucosidase Ⅱ β--subunit via the H DEL motif against the non-redundant protein database. Data mining also revealed that the glucosidase Ⅱ β--subunit gene has a highly similar expression pattern to ERD2b and the other known ERQC components involved in EFR biogenesis. Importantly, the T-DNA insertion lines of the glucosidase Ⅱ β-subunit gene showed that EFR-controlled responses were substantially reduced or completely blocked in these mutants. The responses include seedling growth inhibition, induction of marker genes, MAP kinase activation, and callose deposition, trigged by peptide elf18, a full mimic of E F-Tu. Taken together, ourdata indicate a requirement of the glucosidase Ⅱ β-subunitfor EFR function.

The tumor suppressor kinase LKB1:lessons from mouse models

Mutations in the tumor suppressor gene LKB1 are important in hereditary Peutz–Jeghers syndrome,as well as in sporadic cancers including lung and cervical cancer.LKB1 is a kinase-activating kinase,and a number of LKB1-dependent phosphorylation cascades regulate fundamental cellular and organismal processes in at least metabolism,polarity,cytoskeleton organization,and proliferation.Conditional targeting approaches are beginning to demonstrate the relevance and specificity of these signaling pathways in development and homeostasis of multiple organs.More than one of the pathways also appear to contribute to tumor growth following Lkb1 deficiencies based on a number of mouse tumor models.Lkb1-dependent activation of AMPK and subsequent inactivation of mammalian target of rapamycin signaling are implicated in several of the models,and other less well characterized pathways are also involved.Conditional targeting studies of Lkb1 also point an important role of LKB1 in epithelial–mesenchymal interactions,significantly expanding knowledge on the relevance of LKB1 in human disease.

Kunitz Trypsin Inhibitor： An Antagonist of Cell Death Triggered by Phytopathogens and Fumonisin B1 in Arabidopsis

Programmed cell death （PCD） is a central regulatory process in both plant development and in plant responses to pathogens. PCD requires a coordinate activation of pro-apoptotic factors such as proteases and suppressors inhibiting and modulating these processes. In plants, various caspase-like cysteine proteases as well as serine proteases have been implicated in PCD. Here, we show that a serine protease （Kunitz trypsin） inhibitor （KTI1） of Arabidopsis acts as a functional KTI when produced in bacteria and in planta. Expression of AtKTI1 is induced late in response to bacterial and fungal elicitors and to salicylic acid. RNAi silencing of the AtKTI1 gene results in enhanced lesion development after infiltration of leaf tissue with the PCD-eliciting fungal toxin fumonisin B1 （FB1） or the avirulent bacterial pathogen Pseudomonas syringae pv tomato DC3000 carrying avrB （Pst avrB）. Overexpression of AtKTI1 results in reduced lesion development after Pst avrB and FB1 infiltration. Interestingly, RNAi silencing of AtKTI1 leads to enhanced resistance to the virulent pathogen Erwinia carotovora subsp, carotovora SCC1, while overexpression of AtKTI1 leads to higher susceptibility towards this pathogen. Together, these data indicate that AtKTI1 is involved in modulating PCD in plant-pathogen interactions.