The use of agroinfiltration for transient expression of plant resistance and fungal effector proteins in Nicotiana benthamiana leaves

Agroinfiltration is a versatile, rapid and simple technique that is widely used for transient gene expression in plants. In this chapter we focus on its use in molecular plant pathology, and especially for the expression of plant resistance (R) and fungal avirulence (Avr) (effector) genes in leaves of Nicotiana benthamiana. Co-expression of an R gene with the corresponding Avr gene triggers host-defence responses that often culminate in a hypersensitive response (HR). This HR is visible as a necrotic sector in the infiltrated leaf area. Staining of the infiltrated leaves with trypan blue allows visual scoring of the HR. Furthermore, fusion of a fluorescent tag to the recombinant protein facilitates determination of its sub-cellular localization by confocal microscopy. The matching gene pair I-2 and Avr2, respectively from tomato and the fungal root-pathogen Fusarium oxysporum f. sp. lycopersici, is presented as a typical example.

Coiled-coil domain-dependent homodimerization of intracellular barley immune receptors defines a minimal functional module for triggering cell death

Plants and animals have evolved structurally related innate immune sensors, designated NLRs, to detect intracellular nonself molecules. NLRs are modular, consisting of N-terminal coiled-coil (CC) or TOLL/interleukin-1 receptor (TIR) domains, a central nucleotide-binding (NB) domain, and C-terminal leucine-rich repeats (LRRs). The polymorphic barley mildew A (MLA) locus encodes CC-containing allelic immune receptors recognizing effectors of the pathogenic powdery mildew fungus. We report the crystal structure of an MLA receptor's invariant CC domain, which reveals a rod-shaped homodimer. MLA receptors also self-associate in vivo, but self-association appears to be independent of effector-triggered receptor activation. MLA CC mutants that fail to self-interact impair in planta cell death activity triggered by the CC domain alone and by an autoactive full-length MLA receptor that mimics its ATP-bound state. Thus, CC domain-dependent dimerization of the immune sensor defines a minimal functional unit and implies a role for the dimeric CC module in downstream immune signaling.

The small heat shock protein 20 RSI2 interacts with and is required for stability and function of tomato resistance protein I-2

Race-specific disease resistance in plants depends on the presence of resistance (R) genes. Most R genes encode NB-ARC-LRR proteins that carry a C-terminal leucine-rich repeat (LRR). Of the few proteins found to interact with the LRR domain, most have proposed (co)chaperone activity. Here, we report the identification of RSI2 (Required for Stability of I-2) as a protein that interacts with the LRR domain of the tomato R protein I-2. RSI2 belongs to the family of small heat shock proteins (sHSPs or HSP20s). HSP20s are ATP-independent chaperones that form oligomeric complexes with client proteins to prevent unfolding and subsequent aggregation. Silencing of RSI2-related HSP20s in Nicotiana benthamiana compromised the hypersensitive response that is normally induced by auto-active variants of I-2 and Mi-1, a second tomato R protein. As many HSP20s have chaperone properties, the involvement of RSI2 and other R protein (co)chaperones in I-2 and Mi-1 protein stability was examined. RSI2 silencing compromised the accumulation of full-length I-2 in planta, but did not affect Mi-1 levels. Silencing of heat shock protein 90 (HSP90) and SGT1 led to an almost complete loss of full-length I-2 accumulation and a reduction in Mi-1 protein levels. In contrast to SGT1 and HSP90, RSI2 silencing led to accumulation of I-2 breakdown products. This difference suggests that RSI2 and HSP90/SGT1 chaperone the I-2 protein using different molecular mechanisms. We conclude that I-2 protein function requires RSI2, either through direct interaction with, and stabilization of I-2 protein or by affecting signalling components involved in initiation of the hypersensitive response.

STANDing strong, resistance proteins instigators of plant defence

Resistance (R) proteins are involved in specific pathogen recognition and subsequent initiation of host defence. Most R proteins are nucleotide binding - leucine rich repeat (NB-LRR) proteins, which form a subgroup within the STAND (signal transduction ATPases with numerous domains) family. Activity of these multi-domain proteins depends on their ability to bind and hydrolyse nucleotides. Since R protein activation often triggers cell-death tight regulation of activation is essential. Autoinhibition, which seems to be accomplished by intramolecular interactions between the various domains, is important to retain R proteins inactive. This review summarizes recent data on intra- and intermolecular interactions that support a model in which pathogen perception triggers a series of conformational changes, allowing the newly exposed NB domain to interact with downstream signalling partners and activate defence signalling.

Electrocardiographic studies in female carriers of Duchenne muscular dystrophy

The incidence of electrocardiographic features "typical" for the Duchenne progressive muscular dystrophy (PMD) was investigated in 191 carriers: 136 mothers and 55 daughers. The typical PMD pattern was seen in 6.6% of the mothers, and a pattern similar to that observed in PMD in 2.9%. Among daughters of carriers the percentages were 16.4 and 1.8 respectively. The daughters obviously belong to a much lower age bracket (average 13.3 years) and therefore must be followed for the eventual disappearance of the "juvenile" ECG features, which, in this group, may reduce the incidence of the typical PMD electrocardiograms. The need for more thorough clinical appraisal of the myocardium in carriers is emphasized, especially in those with the "typical" PMD electrocardiograms, with the help of hemodynamic investigations.