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Stem rust, caused by Puccinia graminis f. sp. tritici, is a devastating disease on wheat and barley. A single barley gene, Rpg1, has provided durable resistance since its commercial introduction in the 1940s. The cloned Rpg1 gene encodes a protein with two tandem protein kinase domains, one an active kinase (pK2) and one a pseudokinase (pK1). Function of both domains is required for resistance. The gene is constitutively expressed in all tissues with elevated levels in the epidermis. It is mostly cytoplasmic with small, but significant amount associated with the cell membrane. We have been studying this gene and protein to try to understand how it works and why it has been so durable. Here we report our most recent results showing that RPG1 is phosphorylated within 5 min after urediniospores from avirulent, but not virulent, races land on the leaf surface. Two effector proteins were isolated from the ungerminated spores and shown to work cooperatively to induce RPG1 phosphorylation and eventual degradation. The proteins were identified as a hypothetical protein (PGTG10537.2) with a fibronectin type III and BRCA1 C-terminal domains and vacuolar protein sortingassociated protein 9 (PGTG_16791). The rapidity of the effector function and the nature of the two protein effectors indicate that a unique mechanism for effector entry and signaling in the host cell is involved. This hypothetical mechanism may be similar to what is observed in animal cells where fibronectin proteins with an RGD-binding domain act to mediate communications between the extracellular matrix and plasma membrane.
Full nonhost resistance can be defined as immunity, displayed by an entire plant species against all genotypes of a plant pathogen. The genetic basis of (non)host-status of plants is hard to study, since identification of the responsible genes would require interspecific crosses that suffer from sterility and abnormal segregation. There are some plant/potential pathogen combinations where only 10% or less of the accessions are at most moderately susceptible. These may be regarded as marginal host or near-nonhost, and can provide insights into the genes that determine whether a plant species is a host or a nonhost to a would-be pathogen. Barley (Hordeum vulgare L.) is a near-nonhost to several rust pathogens (Puccinia) of cereals and grasses. By crossing and selection we developed an experimental line, SusPtrit, with high susceptibility to at least nine different heterologous rust taxa such as the wheat and Agropyron leaf rusts (caused by P. triticina and P. persistens, respectively). On the basis of SusPtrit and several regular, fully resistant barley accessions, we developed mapping populations. We established that the near-nonhost resistance to heterologous rusts inherits polygenically (QTLs). The QTLs have different and overlapping specificities. In addition, an occasional R-gene is involved. In each population, different sets of loci were implicated in resistance. Very few resistance genes were common between the populations, suggesting a high redundancy in barley for resistance factors. Selected QTLs have been introduced into near-isogenic lines to be fine-mapped. Our results show that the barley- Puccinia system is ideal to investigate the genetics of host-status to specialized plant pathogens.