The Melampsora lini AvrL567 Avirulence Genes Are Expressed in Haustoria and Their Products Are Recognized inside Plant Cells

Abstract

The Linum usitatissimum (flax) L gene alleles, which encode nucleotide binding site-Leu rich repeat class intracellular receptor proteins, confer resistance against the Melampsora lini (flax rust) fungus. At least 11 different L resistance specificities are known, and the corresponding avirulence genes in M. lini map to eight independent loci, some of which are complex and encode multiple specificities. We identified an M. lini cDNA marker that cosegregates in an F2 rust family with a complex locus determining avirulence on the L5, L6, and L7 resistance genes. Two related avirulence gene candidates, designated AvrL567-A and AvrL567-B, were identified in a genomic DNA contig from the avirulence allele, whereas the corresponding virulence allele contained a single copy of a related gene, AvrL567-C. Agrobacterium tumefaciens-mediated transient expression of the mature AvrL567-A or AvrL567-B (but not AvrL567-C) proteins as intracellular products in L. usitatissimum and Nicotiana tabacum (tobacco) induced a hypersensitive response-like necrosis that was dependent on coexpression of the L5, L6, or L7 resistance gene. An F1 seedling lethal or stunted growth phenotype also was observed when transgenic L. usitatissimum plants expressing AvrL567-A or AvrL567-B (but not AvrL567-C) were crossed to resistant lines containing L5, L6, or L7. The AvrL567 genes are expressed in rust haustoria and encode 127 amino acid secreted proteins. Intracellular recognition of these rust avirulence proteins implies that they are delivered into host cells across the plant membrane. Differences in the three AvrL567 protein sequences result from diversifying selection, which is consistent with a coevolutionary arms race.

Guarding the Goods. New Insights into the Central Alarm System of Plants

“…Most recently, similar experiments with Avr proteins from Flor's flax rust pathogen have revealed that these are also detected from inside plant cells (Dodds et al, 2004), indicating that at least some fungal pathogens also translocate Avr/virulence proteins across host cell membranes. This latter work is particularly noteworthy, because coexpression of the flax NB-LRR protein L6 and the fungal Avr protein AvrL567 in tobacco (Nicotiana tabacum) induced an HR-like response, while expression of either protein alone did not.…”

Predicted Effector Molecules in the Salivary Secretome of the Pea Aphid (Acyrthosiphon pisum): A Dual Transcriptomic/Proteomic Approach

“…On the basis of their obligatory and specific association with host plants, aphids would be expected to have a large number of effectors with subtle effects. 46 It is possible, however, that some salivary proteins could be produced in other organs. Hemolymph is constantly pumped in the salivary glands and it is likely that other biological molecules including proteins produced in other tissues make their way into the Figure 5.…”

“…Different pea aphid clones elicit variable plant wound responses and R-gene resistance is known to be clone-specific 13,45,46 and perhaps linked to clone-specific salivary elicitors. An additional problem with combining transcriptomic and proteomic data sets relates to the wellcharacterized differences that exist in the expression levels between the two systems.…”

“…Without information on the structures of these proteins, we are unable to determine whether these sites under positive selection are located in surface regions that could be interacting directly with resistance proteins in their host-plant. 46 For all these candidate effectors associated with multiple gene copies, several additional paralogs also exist in the genome with no evidence yet that links them with the saliva. Many of the paralogs are truncated versions of the candidate salivary genes.…”

Identification of Plasmodiophora brassicae effectors — A challenging goal

“…Among these well-characterized biotrophic pathogens are: ( i ) Puccinia monoica [23] which, similar to aster yellows phytoplasma [24], can induce floral mimicry in order to promote its own sexual reproduction; ( ii ) Melampsora lini [25], which produces effector proteins in haustoria that are recognized inside the plant cell [26]; and ( iii ) Cladosporium fulvum , the tomato pathogen which, during infection, secretes the chitin-binding virulence factor Avr4 that is thought to protect the fungal chitin cell wall from hydrolysis by plant produced chitinases [27]. Extensive studies with P. graminis f. sp.…”