The Cf-9 disease resistance protein is present in an approximately 420-kilodalton heteromultimeric membrane-associated complex at one molecule per complex

Physical interaction between RRS1-R, a protein conferring resistance to bacterial wilt, and PopP2, a type III effector targeted to the plant nucleus

RRS1-R confers broad-spectrum resistance to several strains of the causal agent of bacterial wilt, Ralstonia solanacearum. Although genetically defined as recessive, this R gene encodes a protein whose structure combines the TIR-NBS-LRR domains found in several R proteins and a WRKY motif characteristic of some plant transcriptional factors and behaves as a dominant gene in transgenic susceptible plants. Here we show that PopP2, a R. solanacearum type III effector, which belongs to the YopJ/AvrRxv protein family, is the avirulence protein recognized by RRS1-R. Furthermore, an interaction between PopP2 and both RRS1-R and RRS1-S, present in the resistant Nd-1 and susceptible Col-5 Arabidopsis thaliana ecotypes, respectively, was detected by using the yeast split-ubiquitin two-hybrid system. This interaction, which required the full-length R protein, was not observed between the RRS1 proteins and PopP1, another member of the YopJ/AvrRxv family present in strain GMI1000 and that confers avirulence in Petunia. We further demonstrate that both the Avr protein and the RRS1 proteins colocalize in the nucleus and that the nuclear localization of the RRS1 proteins are dependent on the presence of PopP2.

Dominant-negative receptor uncovers redundancy in the Arabidopsis ERECTA Leucine-rich repeat receptor-like kinase signaling pathway that regulates organ shape

Arabidopsis ERECTA, a Leu-rich repeat receptor-like Ser/Thr kinase (LRR-RLK), regulates organ shape and inflorescence architecture. Here, we show that a truncated ERECTA protein that lacks the cytoplasmic kinase domain (DeltaKinase) confers dominant-negative effects when expressed under the control of the native ERECTA promoter and terminator. Transgenic plants expressing DeltaKinase displayed phenotypes, including compact inflorescence and short, blunt siliques, that are characteristic of loss-of-function erecta mutant plants. The DeltaKinase fragment migrated as a stable approximately 400-kD protein complex in the complete absence of the endogenous ERECTA protein and significantly exaggerated the growth defects of the null erecta plants. A functional LRR domain of DeltaKinase was required for dominant-negative effects. Accumulation of DeltaKinase did not interfere with another LRR-RLK signaling pathway (CLAVATA1), which operates in the same cells as ERECTA but has a distinct biological function. Both the erecta mutation and DeltaKinase expression conferred a lesser number of large, disorganized, and expanded cortex cells, which are associated with an increased level of somatic endoploidy. These findings suggest that functionally redundant RLK signaling pathways, including ERECTA, are required to fine-tune the proliferation and growth of cells in the same tissue type during Arabidopsis organogenesis.

Deciphering plant-pathogen communication: fresh perspectives for molecular resistance breeding

Activation of local and systemic plant defences in response to pathogen attack involves dramatic cellular reprogramming. Over the past 10 years many novel genes, proteins and molecules have been discovered as a result of investigating plant-pathogen interactions. Most attempts to harness this knowledge to engineer improved disease resistance in crops have failed. Although gene efficacy in transgenic plants has often been good, commercial exploitation has not been possible because of the detrimental effects on plant growth, development and crop yield. Biotechnology approaches have now shifted emphasis towards marker-assisted breeding and the construction of vectors containing highly regulated transgenes that confer resistance in several distinct ways.

An enhanced transient expression system in plants based on suppression of gene silencing by the p19 protein of tomato bushy stunt virus

Transient gene expression is a fast, flexible and reproducible approach to high-level expression of useful proteins. In plants, recombinant strains of Agrobacterium tumefaciens can be used for transient expression of genes that have been inserted into the T-DNA region of the bacterial Ti plasmid. A bacterial culture is vacuum-infiltrated into leaves, and upon T-DNA transfer, there is ectopic expression of the gene of interest in the plant cells. However, the utility of the system is limited because the ectopic protein expression ceases after 2-3 days. Here, we show that post-transcriptional gene silencing (PTGS) is a major cause for this lack of efficiency. We describe a system based on co-expression of a viral-encoded suppressor of gene silencing, the p19 protein of tomato bushy stunt virus (TBSV), that prevents the onset of PTGS in the infiltrated tissues and allows high level of transient expression. Expression of a range of proteins was enhanced 50-folds or more in the presence of p19 so that protein purification could be achieved from as little as 100 mg of infiltrated leaf material. The effect of p19 was not saturated in cells that had received up to four individual T-DNAs and persisted until leaf senescence. Because of its simplicity and rapidity, we anticipate that the p19-enhanced expression system will have value in industrial production as well as a research tool for isolation and biochemical characterisation of a broad range of proteins without the need for the time-consuming regeneration of stably transformed plants.

Recognition specificity and RAR1/SGT1 dependence in barley Mla disease resistance genes to the powdery mildew fungus

A large number of resistance specificities to the powdery mildew fungus Blumeria graminis f. sp. hordei map to the barley Mla locus. This complex locus harbors multiple members of three distantly related gene families that encode proteins that contain an N-terminal coiled-coil (CC) structure, a central nucleotide binding (NB) site, a Leu-rich repeat (LRR) region, and a C-terminal non-LRR (CT) region. We identified Mla12, which encodes a CC-NB-LRR-CT protein that shares 89 and 92% identical residues with the known proteins MLA1 and MLA6. Slow Mla12-triggered resistance was altered dramatically to a rapid response by overexpression of Mla12. A series of reciprocal domains swaps between MLA1 and MLA6 identified in each protein recognition domain for cognate powdery mildew fungus avirulence genes (AvrMla1 and AvrMla6). These domains were within different but overlapping LRR regions and the CT part. Unexpectedly, MLA chimeras that confer AvrMla6 recognition exhibited markedly different dependence on Rar1, a gene required for the function of some but not all Mla resistance specificities. Furthermore, uncoupling of MLA6-specific function from RAR1 also uncoupled the response from SGT1, a protein known to associate physically with RAR1. Our findings suggest that differences in the degree of RAR1 dependence of different MLA immunity responses are determined by intrinsic properties of MLA variants and place RAR1/SGT1 activity downstream of and/or coincident with the action of resistance protein-containing recognition complexes.

Understanding the functions of plant disease resistance proteins

Many disease resistance (R) proteins of plants detect the presence of disease-causing bacteria, viruses, or fungi by recognizing specific pathogen effector molecules that are produced during the infection process. Effectors are often pathogen proteins that probably evolved to subvert various host processes for promotion of the pathogen life cycle. Five classes of effector-specific R proteins are known, and their sequences suggest roles in both effector recognition and signal transduction. Although some R proteins may act as primary receptors of pathogen effector proteins, most appear to play indirect roles in this process. The functions of various R proteins require phosphorylation, protein degradation, or specific localization within the host cell. Some signaling components are shared by many R gene pathways whereas others appear to be pathway specific. New technologies arising from the genomics and proteomics revolution will greatly expand our ability to investigate the role of R proteins in plant disease resistance.

Establishment of biotrophy by parasitic fungi and reprogramming of host cells for disease resistance

Parasitic biotrophs such as mildews and rusts evolved specific mechanisms that keep host cells alive during infection. These fungi appear to absorb nutrients mainly by proton-symport-driven transporter proteins that reside in specialized feeding structures. Accumulating evidence suggests that biotrophic fungi both suppress induction of plant defense responses in physical proximity to infection sites and induce specific host genes for the establishment of biotrophy. The peculiarities of biotrophic pathogenesis likely reflect diverse types of plant disease-resistance responses. The cloning of race-specific resistance genes to powdery mildew infection and of genes required for their function provides first insights into molecular mechanisms enabling the host to recognize mildew effector components and suggests candidate mechanisms of resistance signaling. Resistance to powdery mildew fungi that result from mutations in host genes promises to shed light on mechanisms that are required for the establishment of disease susceptibility.