Advances in the Molecular Genetic Analysis of the Flax-Flax Rust Interaction

Abstract

The L6 and M rust-resistance genes, representing two of the five rust-resistance gene loci in flax (Linum usitatissimum), have been cloned. The molecular data are fully consistent with earlier genetic data: the L locus is a single gene with multiple alleles expressing different rust resistance specificities, and the M locus is complex, containing an array of about 15 similar genes. Thus, while L6 and M resistance genes have 86% nucleotide identity, their locus structure is very different. These genes encode products belonging to the nucleotide binding site-leucine-rich repeat class of disease-resistance proteins. Analysis of alleles from the L locus and chimeric genes is providing evidence suggesting that important specificity determinants occur in the C-terminal half of the proteins, the region containing the leucine-rich repeats. The isolation and characterization of the rust (Melampsora lini) avirulence genes that correspond to the cloned rust-resistance genes is one of the major challenges remaining to the understanding of this system.

Intragenic Recombination and Diversifying Selection Contribute to the Evolution of Downy Mildew Resistance at the RPP8 Locus of Arabidopsis

“…Class 1 MHC genes are thought to undergo turnover through cycles of birth and death as inactive or obsolete genes are supplanted by more efficient copies arising from duplication and divergence (Nei and Hughes, 1992). This process also may operate in plant disease resistance loci, which typically contain duplicated genes with unknown functions (Martin et al, 1994;Anderson et al, 1997;Wang et al, 1998). A significant fraction of these genes could be "molecular fossils" arising from gene turnover during the host-pathogen arms race.…”

“…The TIR-NBS-LRR subclass is defined by an N-terminal region that resembles the cytoplasmic signaling domain of the Toll and interleukin1 transmembrane receptors (Parker et al, 1997). This subclass includes genes that specify resistance to a virus ( N in tobacco) (Whitham et al, 1994), fungi ( L6 and M in flax) (Lawrence et al, 1995;Anderson et al, 1997), and oomycetes ( RPP5 , RPP1A , RPP1B , and RPP1C in Arabidopsis, where RPP signifies resistance to Peronospora parasitica ) (Parker et al, 1997;Botella et al, 1998). The second subclass (LZ-NBS-LRR) contains a leucine zipper-like motif in place of the TIR domain and is represented by the genes RPM1 (Grant et al, 1995), RPS2 (Bent et al, 1994;Mindrinos et al, 1994), and Prf (Salmeron et al, 1996).…”

“…The Plant Cell repeated sequences in R gene clusters is a critical mechanism in R gene evolution (reviewed in Ellis et al, 1997;Hulbert, 1997), and the chimeric structure of the rpp8-Col allele adds to a growing body of molecular data that supports this proposal. Intra-allelic recombinants have been discovered in mutational screens at the M and RPP5 loci (Anderson et al, 1997;Parker et al, 1997).…”

“…Intra-allelic recombinants have been discovered in mutational screens at the M and RPP5 loci (Anderson et al, 1997;Parker et al, 1997). These recombinant alleles arose from ectopic recombination between LRR-encoding modules that caused expansions or contractions in LRR copy number, thereby inactivating the gene.…”

Two modes of pathogen recognition by plants

“…In a remarkable arc of history, they do so by using cloned alleles from flax and flax rust loci that were defined genetically five decades ago by Flor (2,3,5,6). Combined with other recent breakthroughs, this work suggests that plant R proteins can function either by directly detecting the corresponding Avr protein (the ''receptor-ligand'' model) or by perceiving alterations in plant machines that are targets of Avr protein action in the promotion of pathogen virulence [the ''guard hypothesis'' (7-9)].…”