Transient expression of the <i>Arabidopsis thaliana</i>callose synthase PMR4 increases penetration resistance to powdery mildew in barley

Localized cell wall thickenings, so called papillae, are a common plant defense response to fungal attack at sites of penetration of the plant cell. The major constituent of papillae is callose, a (1,3)-β-glucan polymer, which contributes to slowing or blocking the invading fungal hyphae. In the model plant Arabidopsis thaliana, we could recently show that the overexpression of PMR4(POWDERY MILDEW RESITANT 4), which encodes a stress induced callose synthase, results in complete powdery mildew resistance. To evaluate if these findings are also transferable to monocot crops, we transiently expressed PMR4 under control of the 35S promoter in leaves of barley (Hordeum vulgare) seedlings, which were subsequently inoculated with the virulent powdery mildew Blumeria graminis f. sp. hordei. Fusion of the green fluorescent protein (GFP) to PMR4 allowed the identification of successfully transformed barley cells, which showed an increased penetration resistance to B. graminis compared to control cells that express only GFP.PMR4-GFP localized in a similar pattern at the site of attempted fungal penetration as observed inA. thaliana, which suggests that similar transport mechanisms of the callose synthase might exist in dicot and monocot plants.

Role of arthropod communities in bioenergy crop litter decomposition

The extensive land use conversion expected to occur to meet demands for bioenergy feedstock production will likely have widespread impacts on agroecosystem biodiversity and ecosystem services, including carbon sequestration. Although arthropod detritivores are known to contribute to litter decomposition and thus energy flow and nutrient cycling in many plant communities, their importance in bioenergy feedstock communities has not yet been assessed. We undertook an experimental study quantifying rates of litter mass loss and nutrient cycling in the presence and absence of these organisms in three bioenergy feedstock crops—miscanthus （Miscanthus x giganteus）, switchgrass （Panicum virgatum）, and a planted prairie community. Overall arthropod abundance and litter decomposition rates were similar in all three communities. Despite effective reduction of arthropods in experimental plots via insecticide application, litter decomposition rates, inorganic nitrogen leaching, and carbon–nitrogen ratios did not differ significantly between control （with arthropods） and treatment （without arthropods） plots in any of the three community types. Our findings suggest that changes in arthropod faunal composition associated with widespread adoption of bioenergy feedstock crops may not be associated with profoundly altered arthropod-mediated litter decomposition and nutrient release.

RNA-Seq Analysis of Developing Nasturtium Seeds （Tropaeolum majus）： Identification and Characterization of an Additional Galactosyltransferase Involved in Xyloglucan Biosynthesis

A deep-sequencing approach was pursued utilizing 454 and Illumina sequencing methods to discover new genes involved in xyloglucan biosynthesis, cDNA sequences were generated from developing nasturtium （Tropaeolum majus） seeds, which produce large amounts of non-fucosylated xyloglucan as a seed storage polymer. In addition to known xyloglucan biosynthetic genes, a previously uncharacterized putative xyloglucan galactosyltransferase was iden- tified. Analysis of an Arabidopsis thaliana mutant line defective in the corresponding ortholog （AT5G62220） revealed that this gene shows no redundancy with the previously characterized xyloglucan galactosyltransferase, MUR3, but is required for galactosyl-substitution of xyloglucan at a different position. The gene was termed XLT2 for Xyloglucan L-side chain galactosylTransferase position 2. It represents an enzyme in the same subclade of glycosyltransferase family 47 as MUR3. A double mutant defective in both MUR3 （mur3.1） and XLT2 led to an Arabidopsis plant with xyloglucan that consists essentially of only xylosylated glucosyl units, with no further substitutions.

Arabidopsis Heterotrimeric G-protein Regulates Cell Wall Defense and Resistance to Necrotrophic Fungi

The Arabidopsis heterotrimeric G-protein controls defense responses to necrotrophic and vascular fungi. The agbl mutant impaired in the Gβ subunit displays enhanced susceptibility to these pathogens. Gβ/AGB1 forms an obligate dimer with either one of the Arabidopsis Gγsubunits （γ1/AGG1 and γ2/AGG2）. Accordingly, we now demonstrate that the aggl agg2 double mutant is as susceptible as agbl plants to the necrotrophic fungus Plectosphaerella cucumerina. To elucidate the molecular basis of heterotrimeric G-protein-mediated resistance, we performed a comparative transcriptomic analysis of agbl-1 mutant and wild-type plants upon inoculation with P cucumerina. This analysis, together with metab- olomic studies, demonstrated that G-protein-mediated resistance was independent of defensive pathways required for resistance to necrotrophic fungi, such as the salicylic acid, jasmonic acid, ethylene, abscisic acid, and tryptophan-derived metabolites signaling, as these pathways were not impaired in agbl and aggl agg2 mutants. Notably, many mis-reguiated genes in agbl plants were related with cell wall functions, which was also the case in aggl agg2 mutant. Biochemical analyses and Fourier Transform InfraRed （FTIR） spectroscopy of cell walls from G-protein mutants revealed that the xylose content was lower in agbl and aggl agg2 mutants than in wild-type plants, and that mutant walls had similar FTIR spec-tratypes, which differed from that of wild-type plants. The data presented here suggest a canonical functionality of the Gβ and Gγ1/γ2 subunits in the control of Arabidopsis immune responses and the regulation of cell wall composition.

RIP1 （ROP Interactive Partner 1）/ICR1 Marks Pollen Germination Sites and May Act in the ROP1 Pathway in the Control of Polarized Pollen Growth

Rho family small GTPases are universal signaling switches in the control of cell polarity in eukaryotic cells. Their polar distribution to the cell cortex is critical for the execution of their functions, yet the mechanism for this distribution is poorly understood. Using a yeast two-hybrid method, we identified RIP1 （ROP interactive partner 1）, which belongs to a family of five members of novel proteins that share a C-terminal region that interacts with ROP. When expressed in Arabidopsis pollen, green fluorescence protein GFP-tagged RIP1 was localized to the nucleus of mature pollen. When pollen grains were hydrated in germination medium, GFP-RIP1 switched from the nucleus to the cell cortex at the future pollen germination site and was maintained in the apical cortex of germinating pollen and growing pollen tubes. RIP1 was found to interact with ROP1 in pollen tubes, and the cortical RIP1 localization was influenced by the activity of ROP1. Overexpression of RIP1 induced growth depolarization in pollen tubes, a phenotype similar to that induced by ROP1 overexpression. Interestingly, RIP1 overexpression enhanced GFP-ROP1 recruitment to the plasma membrane （PM） of pollen tubes. Based on these observations, we hypothesize that RIP1 is involved in the positive feedback regulation of ROP1 localization to the PM, leading to the establishment of a polar site for pollen germination and pollen tube growth.

An Arabidopsis Lipid Flippase Is Required for Timely Recruitment of Defenses to the Host- Pathogen Interface at the Plant Cell Surface

Deposition of cell wall-reinforcing papillae is an integral component of the plant immune response. TheArabidopsis PENETRATION 3 （PEN3） ATP binding cassette （ABC） transporter plays a role in defense against numerous pathogens and is recruited to sites of pathogen detection where it accumulates within papillae. However, the trafficking pathways and regulatory mechanisms contributing to recruitment of PEN3 and other defenses to the host-pathogen interface are poorly understood. Here, we report a confocal microscopy-based screen to identify mutants with altered localization of PEN3-GFP after inoculation with powdery mildew fungi. We identified a mutant, aberrant localization of PEN3 3 （alp3）, displaying accumulation of the normally plasma membrane （PM）-Iocalized PEN3-GFP in endomembrane compartments. The mutant was found to be disrupted in the P4-ATPase AMINOPHOSPHOLIPID ATPASE 3 （ALA3）, a lipid flippase that plays a critical role in vesicle formation. We provide evidence that PEN3 undergoes continuous endocytic cycling from the PM to the trans-Golgi network （TGN）. In alp3, PEN3 accumulates in the TGN, causing delays in recruitment to the host-pathogen interface. Our results indicate that PEN3 and other defense proteins continuously cycle through the TGN and that timely exit of these proteins from the TGN is critical for effective pre-invasive immune responses against powdery mildews.

Arabinosylation of a Yariv-Precipitable Cell Wall Polymer Impacts Plant Growth as Exemplified by the Arabidopsis Glycosyltransferase Mutant ray1

Dear Editor, Plant cells are encased by cell walls--rigid yet highly dynamic composite structures, which consist of cellulose, hemicelluloses, pectic polysaccharides, glycoproteins, and the polyphenol lignin （Somerville et al., 2004）. Biosynthesis of the wall polysaccharides is facilitated by donor- and acceptor- substrate-specific glycosyltransferases catalyzing the transfer of the sugar moiety from a nucleotide sugar to form a specific glycosidic linkage. Glycosyltransferases have been grouped into families based on primary structure, one of which is fam- ily GT77 （www.cazy.org/GT77_all.html）.