Genome-wide analysis of the rice and arabidopsis non-specific lipid transfer protein (nsLtp) gene families and identification of wheat nsLtp genes by EST data mining

<p>Abstract</p> <p>Background</p> <p>Plant non-specific lipid transfer proteins (nsLTPs) are encoded by multigene families and possess physiological functions that remain unclear. Our objective was to characterize the complete <it>nsLtp </it>gene family in rice and arabidopsis and to perform wheat EST database mining for <it>nsLtp </it>gene discovery.</p> <p>Results</p> <p>In this study, we carried out a genome-wide analysis of <it>nsLtp </it>gene families in <it>Oryza sativa </it>and <it>Arabidopsis thaliana </it>and identified 52 rice <it>nsLtp </it>genes and 49 arabidopsis <it>nsLtp </it>genes. Here we present a complete overview of the genes and deduced protein features. Tandem duplication repeats, which represent 26 out of the 52 rice <it>nsLtp </it>genes and 18 out of the 49 arabidopsis <it>nsLtp </it>genes identified, support the complexity of the <it>nsLtp </it>gene families in these species. Phylogenetic analysis revealed that rice and arabidopsis nsLTPs are clustered in nine different clades. In addition, we performed comparative analysis of rice <it>nsLtp </it>genes and wheat (<it>Triticum aestivum</it>) EST sequences indexed in the UniGene database. We identified 156 putative wheat <it>nsLtp </it>genes, among which 91 were found in the 'Chinese Spring' cultivar. The 122 wheat non-redundant nsLTPs were organized in eight types and 33 subfamilies. Based on the observation that seven of these clades were present in arabidopsis, rice and wheat, we conclude that the major functional diversification within the nsLTP family predated the monocot/dicot divergence. In contrast, there is no type VII nsLTPs in arabidopsis and type IX nsLTPs were only identified in arabidopsis. The reason for the larger number of <it>nsLtp </it>genes in wheat may simply be due to the hexaploid state of wheat but may also reflect extensive duplication of gene clusters as observed on rice chromosomes 11 and 12 and arabidopsis chromosome 5.</p> <p>Conclusion</p> <p>Our current study provides fundamental information on the organization of the rice, arabidopsis and wheat <it>nsLtp </it>gene families. The multiplicity of nsLTP types provide new insights on arabidopsis, rice and wheat <it>nsLtp </it>gene families and will strongly support further transcript profiling or functional analyses of <it>nsLtp </it>genes. Until such time as specific physiological functions are defined, it seems relevant to categorize plant nsLTPs on the basis of sequence similarity and/or phylogenetic clustering.</p

Aspartate Oxidase Plays an Important Role in Arabidopsis Stomatal Immunity

Perception of pathogen-associated molecular patterns (PAMPs), such as bacterial flagellin (or the peptide flg22), by surface-localized receptors activates defense responses and subsequent immunity. In a previous forward-genetic screen aimed at the identification of Arabidopsis (Arabidopsis thaliana) flagellin-insensitive (fin) mutants, we isolated fin4, which is severely affected in flg22-triggered reactive oxygen species (ROS) bursts. Here, we report that FIN4 encodes the chloroplastic enzyme ASPARTATE OXIDASE (AO), which catalyzes the first irreversible step in the de novo biosynthesis of NAD. Genetic studies on the role of NAD have been hindered so far by the lethality of null mutants in NAD biosynthetic enzymes. Using newly identified knockdown fin alleles, we found that AO is required for the ROS burst mediated by the NADPH oxidase RBOHD triggered by the perception of several unrelated PAMPs. AO is also required for RBOHD-dependent stomatal closure. However, full AO activity is not required for flg22-induced responses that are RBOHD independent. Interestingly, although the fin4 mutation dramatically affects RBOHD function, it does not affect functions carried out by other members of the RBOH family, such as RBOHC and RBOHF. Finally, we determined that AO is required for stomatal immunity against the bacterium Pseudomonas syringae. Altogether, our work reveals a novel specific requirement for AO activity in PAMP-triggered RBOHD-dependent ROS burst and stomatal immunity. In addition, the availability of viable mutants for the chloroplastic enzyme AO will enable future detailed studies on the role of NAD metabolism in different cellular processes, including immunity, in Arabidopsis

The transcriptional regulator BZR1 mediates trade-off between plant innate immunity and growth

The molecular mechanisms underlying the trade-off between plant innate immunity and steroid-mediated growth are controversial. Here, we report that activation of the transcription factor BZR1 is required and sufficient for suppression of immune signaling by brassinosteroids (BR). BZR1 induces the expression of several WRKY transcription factors that negatively control early immune responses. In addition, BZR1 associates with WRKY40 to mediate the antagonism between BR and immune signaling. We reveal that BZR1-mediated inhibition of immunity is particularly relevant when plant fast growth is required, such as during etiolation. Thus, BZR1 acts as an important regulator mediating the trade-off between growth and immunity upon integration of environmental cues

Cellulose-derived oligomers act as damage-associated molecular patterns and trigger defense-like responses

The plant cell wall, often the site of initial encounters between plants and their microbial pathogens, is composed of a complex mixture of cellulose, hemicellulose, and pectin polysaccharides as well as proteins. The concept of damage-associated molecular patterns (DAMPs) was proposed to describe plant elicitors like oligogalacturonides (OGs), which can be derived by the breakdown of the pectin homogalacturon by pectinases. OGs act via many of the same signaling steps as pathogen- or microbe-associated molecular patterns (PAMPs) to elicit defenses and provide protection against pathogens. Given both the complexity of the plant cell wall and the fact that many pathogens secrete a wide range of cell wall-degrading enzymes, we reasoned that the breakdown products of other cell wall polymers may be similarly biologically active as elicitors and may help to reinforce the perception of danger by plant cells. Our results indicate that oligomers derived from cellulose are perceived as signal molecules in Arabidopsis (Arabidopsis thaliana), triggering a signaling cascade that shares some similarities to responses to well-known elicitors such as chitooligomers and OGs. However, in contrast to other known PAMPs/DAMPs, cellobiose stimulates neither detectable reactive oxygen species production nor callose deposition. Confirming our idea that both PAMPs and DAMPs are likely to cooccur at infection sites, cotreatments of cellobiose with flg22 or chitooligomers led to synergistic increases in gene expression. Thus, the perception of cellulose-derived oligomers may participate in cell wall integrity surveillance and represents an additional layer of signaling following plant cell wall breakdown during cell wall remodeling or pathogen attack

\u3ci\u3ePseudomonas\u3c/i\u3e HopU1 modulates plant immune receptor levels by blocking the interaction of their mRNAs with GRP7

Pathogens target important components of host immunity to cause disease. The Pseudomonas syringae type III-secreted effector HopU1 is a mono-ADP-ribosyltransferase required for full virulence on Arabidopsis thaliana. HopU1 targets several RNA-binding proteins including GRP7, whose role in immunity is still unclear. Here, we show that GRP7 associates with translational components, as well as with the pattern recognition receptors FLS2 and EFR. Moreover, GRP7 binds specifically FLS2 and EFR transcripts in vivo through its RNA recognition motif. HopU1 does not affect the protein–protein associations between GRP7, FLS2 and translational components. Instead, HopU1 blocks the interaction between GRP7 and FLS2 and EFR transcripts in vivo. This inhibition correlates with reduced FLS2 protein levels upon Pseudomonas infection in a HopU1- dependent manner. Our results reveal a novel virulence strategy used by a microbial effector to interfere with host immunity

Arabidopsis receptor-like protein30 and receptor-like kinase suppressor of BIR1-1/EVERSHED mediate innate immunity to necrotrophic fungi

Effective plant defense strategies rely in part on the perception of non-self determinants, so-called microbe-associated molecular patterns (MAMPs), by transmembrane pattern recognition receptors leading to MAMP-triggered immunity. Plant resistance against necrotrophic pathogens with a broad host range is complex and yet not well understood. Particularly, it is unclear if resistance to necrotrophs involves pattern recognition receptors. Here, we partially purified a novel proteinaceous elicitor called SCLEROTINIA CULTURE FILTRATE ELICITOR1 (SCFE1) from the necrotrophic fungal pathogen Sclerotinia sclerotiorum that induces typical MAMP-triggered immune responses in Arabidopsis thaliana. Analysis of natural genetic variation revealed five Arabidopsis accessions (Mt-0, Lov-1, Lov-5, Br-0, and Sq-1) that are fully insensitive to the SCFE1-containing fraction. We used a forward genetics approach and mapped the locus determining SCFE1 sensitivity to RECEPTOR-LIKE PROTEIN30 (RLP30). We also show that SCFE1-triggered immune responses engage a signaling pathway dependent on the regulatory receptor-like kinases BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE1 (BAK1) and SUPPRESSOR OF BIR1-1/EVERSHED (SOBIR1/EVR). Mutants of RLP30, BAK1, and SOBIR1 are more susceptible to S. sclerotiorum and the related fungus Botrytis cinerea. The presence of an elicitor in S. sclerotiorum evoking MAMP-triggered immune responses and sensed by RLP30/SOBIR1/BAK1 demonstrates the relevance of MAMP-triggered immunity in resistance to necrotrophic fungi

Transgressive segregation reveals mechanisms of Arabidopsis immunity to Brassica-infecting races of white rust (Albugo candida)

Arabidopsis thaliana accessions are universally resistant at the adult leaf stage to white rust (Albugo candida) races that infect the crop species Brassica juncea and Brassica oleracea. We used transgressive segregation in recombinant inbred lines to test if this apparent species-wide (nonhost) resistance in A. thaliana is due to natural pyramiding of multiple Resistance (R) genes. We screened 593 inbred lines from an Arabidopsis multiparent advanced generation intercross (MAGIC) mapping population, derived from 19 resistant parental accessions, and identified two transgressive segregants that are susceptible to the pathogen. These were crossed to each MAGIC parent, and analysis of resulting F2 progeny followed by positional cloning showed that resistance to an isolate of A. candida race 2 (Ac2V) can be explained in each accession by at least one of four genes encoding nucleotide-binding, leucine-rich repeat (NLR) immune receptors. An additional gene was identified that confers resistance to an isolate of A. candida race 9 (AcBoT) that infects B. oleracea. Thus, effector-triggered immunity conferred by distinct NLR-encoding genes in multiple A. thaliana accessions provides species-wide resistance to these crop pathogens

Transgressive segregation reveals mechanisms of Arabidopsis immunity to Brassica-infecting races of white rust (Albugo candida)

Arabidopsis thaliana accessions are universally resistant at the adult leaf stage to white rust (Albugo candida) races that infect the crop species Brassica juncea and Brassica oleracea. We used transgressive segregation in recombinant inbred lines to test if this apparent species-wide (nonhost) resistance in A. thaliana is due to natural pyramiding of multiple Resistance (R) genes. We screened 593 inbred lines from an Arabidopsis multiparent advanced generation intercross (MAGIC) mapping population, derived from 19 resistant parental accessions, and identified two transgressive segregants that are susceptible to the pathogen. These were crossed to each MAGIC parent, and analysis of resulting F 2 progeny followed by positional cloning showed that resistance to an isolate of A. candida race 2 (Ac2V) can be explained in each accession by at least one of four genes encoding nucleotide-binding, leucine-rich repeat (NLR) immune receptors. An additional gene was identified that confers resistance to an isolate of A. candida race 9 (AcBoT) that infects B. oleracea. Thus, effector-triggered immunity conferred by distinct NLR-encoding genes in multiple A. thaliana accessions provides species-wide resistance to these crop pathogens

\u3ci\u3ePseudomonas\u3c/i\u3e HopU1 modulates plant immune receptor levels by blocking the interaction of their mRNAs with GRP7

Pathogens target important components of host immunity to cause disease. The Pseudomonas syringae type III-secreted effector HopU1 is a mono-ADP-ribosyltransferase required for full virulence on Arabidopsis thaliana. HopU1 targets several RNA-binding proteins including GRP7, whose role in immunity is still unclear. Here, we show that GRP7 associates with translational components, as well as with the pattern recognition receptors FLS2 and EFR. Moreover, GRP7 binds specifically FLS2 and EFR transcripts in vivo through its RNA recognition motif. HopU1 does not affect the protein–protein associations between GRP7, FLS2 and translational components. Instead, HopU1 blocks the interaction between GRP7 and FLS2 and EFR transcripts in vivo. This inhibition correlates with reduced FLS2 protein levels upon Pseudomonas infection in a HopU1- dependent manner. Our results reveal a novel virulence strategy used by a microbial effector to interfere with host immunity