Rpg1-mediated durable stem rust resistance: mechanisms of action
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.