During sexual reproduction in flowering plants such as Arabidopsis, a tip-growing pollen tube (PT) is guided to the synergid cells of the female gametophyte, where it bursts and releases the two sperm. Here we show that PT reception and powdery mildew (PM) infection, which involves communication between a tip-growing hypha and a plant epidermal cell, share molecular components. NORTIA (NTA), a member of the MLO family originally discovered in the context of PM resistance, and FERONIA (FER), a receptor-like kinase, both control PT reception in synergids. Homozygous fer mutants also display PM resistance, revealing a new function for FER and suggesting that conserved components, such as FER and distinct MLO proteins, are involved in both PT reception and PM infection.

The endomembrane system is a complex and dynamic intracellular trafficking network. It is very challenging to track individual vesicles and their cargos in real time; however, affinity purification allows vesicles to be isolated in their natural state so that their constituent proteins can be identified. Pioneering this approach in plants, we isolated the SYP61 trans-Golgi network compartment and carried out a comprehensive proteomic analysis of its contents with only minimal interference from other organelles. The proteome of SYP61 revealed the association of proteins of unknown function that have previously not been ascribed to this compartment. We identified a complete SYP61 SNARE complex, including regulatory proteins and validated the proteome data by showing that several of these proteins associated with SYP61 in planta. We further identified the SYP121-complex and cellulose synthases, suggesting that SYP61 plays a role in the exocytic trafficking and the transport of cell wall components to the plasma membrane. The presence of proteins of unknown function in the SYP61 proteome including ECHIDNA offers the opportunity to identify novel trafficking components and cargos. The affinity purification of plant vesicles in their natural state provides a basis for further analysis and dissection of complex endomembrane networks. The approach is widely applicable and can afford the study of several vesicle populations in plants, which can be compared with the SYP61 vesicle proteome.

Plasma membrane compartmentalization spatiotemporally regulates cell-autonomous immune signaling in animal cells. To elucidate immediate early protein dynamics at the plant plasma membrane in response to the bacterial pathogen-associated molecular pattern (PAMP) flagellin (flg22) we employed quantitative mass spectrometric analysis on detergent-resistant membranes (DRMs) of Arabidopsis thaliana suspension cells. This approach revealed rapid and profound changes in DRM protein composition following PAMP treatment, prominently affecting proton ATPases and receptor-like kinases, including the flagellin receptor FLS2. We employed reverse genetics to address a potential contribution of a subset of these proteins in flg22-triggered cellular responses. Mutants of three candidates (DET3, AHA1, FER) exhibited a conspicuous defect in the PAMP-triggered accumulation of reactive oxygen species. In addition, these mutants showed altered mitogen-activated protein kinase (MAPK) activation, a defect in PAMP-triggered stomatal closure as well as altered bacterial infection phenotypes, which revealed three novel players in elicitor-dependent oxidative burst control and innate immunity. Our data provide evidence for dynamic elicitor-induced changes in the membrane compartmentalization of PAMP signaling components.To cope with the great number of potential pathogens, plants evolved specialized pattern recognition receptors (PRRs) 5 through which they detect pathogen-associated molecular patterns (PAMPs) at the cell surface (1). Within seconds to minutes after PAMP perception manifold intracellular responses occur, including ion fluxes across the plasma membrane (PM), increase of cytosolic Ca 2ϩ levels, production of reactive oxygen species (ROS) and protein phosphorylation. At later time points profound transcriptional changes, stomatal closure as well as local cell wall reinforcement take place (2).The best characterized plant PAMP perception system is the recognition of bacterial flagellin and its elicitor-active epitope, flg22, by the Arabidopsis PRR FLS2 (flagellin sensitive 2; (2)). FLS2 undergoes flg22-induced complex formation with BRl1-associated receptor kinase 1 (BAK1), which precedes and is required for FLS2 endocytosis (2, 3). Indeed, ligand-induced reduction in lateral membrane mobility of FLS2 has been observed in protoplasts (4), which could be explained by either ligand-dependent interactions of FLS2 with e.g. BAK1, the confinement of FLS2 to less mobile membrane compartments, or a combination of both. To ensure adequate perception of PAMPs and tightly regulated downstream signaling, the PM must be spatially highly organized and dynamic. In this context, the recruitment of FLS2 to specialized membrane domains seems crucial to enable ligand-induced endocytosis (5).During the past years, lateral compartmentalization has become a well-recognized topic in plant membrane research (6). The membrane raft hypothesis provides a plausible explanation for the spatial and temporal organization of biological membranes based on t...

The brassinosteroid (BR) signaling module is a central regulator of plant morphogenesis, as indicated by the large number of BRresponsive cell wall-related genes and the severe growth defects of BR mutants. Despite a detailed knowledge of the signaling components, the logic of this auto-/paracrine signaling module in growth control remains poorly understood. Recently, extensive cross-talk with other signaling pathways has been shown, suggesting that the outputs of BR signaling, such as gene-expression changes, are subject to complex control mechanisms. We previously provided evidence for a role of BR signaling in a feedback loop controlling the integrity of the cell wall. Here, we identify the first dedicated component of this feedback loop: a receptor-like protein (RLP44), which is essential for the compensatory triggering of BR signaling upon inhibition of pectin de-methylesterification in the cell wall. RLP44 is required for normal growth and stress responses and connects with the BR signaling pathway, presumably through a direct interaction with the regulatory receptor-like kinase BAK1. These findings corroborate a role for BR in controlling the sensitivity of a feedback signaling module involved in maintaining the physicochemical homeostasis of the cell wall during cell expansion.brassinosteroids | cell wall integrity | pectin

Intracellular Ca2+ transients are an integral part of the signaling cascade during pathogen-associated molecular pattern (PAMP)-triggered immunity in plants. Yet, our knowledge about the spatial distribution of PAMP-induced Ca2+ signals is limited. Investigation of cell- and tissue-specific properties of Ca2+-dependent signaling processes requires versatile Ca2+ reporters that are able to extract spatial information from cellular and subcellular structures, as well as from whole tissues over time periods from seconds to hours. Fluorescence-based reporters cover both a broad spatial and temporal range, which makes them ideally suited to study Ca2+ signaling in living cells. In this study, we compared two fluorescence-based Ca2+ sensors: the Förster resonance energy transfer (FRET)-based reporter yellow cameleon NES-YC3.6 and the intensity-based sensor R-GECO1. We demonstrate that R-GECO1 exhibits a significantly increased signal change compared with ratiometric NES-YC3.6 in response to several stimuli. Due to its superior sensitivity, R-GECO1 is able to report flg22- and chitin-induced Ca2+ signals on a cellular scale, which allowed identification of defined [Ca2+]cyt oscillations in epidermal and guard cells in response to the fungal elicitor chitin. Moreover, we discovered that flg22- and chitin-induced Ca2+ signals in the root initiate from the elongation zone.