DEFENDING THE ARABIDOSOLANACEAE

¹ The Sainsbury Laboratory, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK

² Bioengineering Dept, UC Berkeley, California, USA

³ HRI, Wellesbourne, UK

Tomato Cf-genes confer race-specific resistance to Cladosporium fulvum. Resistance is mediated through recognition of specific products of fungal avirulence (Avr) genes. Cf-genes encode plasma membrane receptor-like proteins (RLPs), with an LRR domain that is involved in recognition, and no apparent signalling domain. These proteins are difficult to investigate purely by biochemical approaches. To understand any biological mechanism, it is crucial to choose the right biological system. Our studies on Cf-genes have revealed the existence of an RLP-based mechanism, the full understanding of which now requires deployment of a better system for forward and reverse genetics, namely Arabidopsis. The Arabidopsis Col-0 set of 57 RLPs has been defined and subjected to computational analysis. From analysis of genetic variation for Peronospora resistance in Arabidopsis, the RPP27 gene was defined (in Landsberg), and it also encodes an RLP. This observation opens up the possibility of forward genetics approaches to reveal RLP mechanisms. Reverse genetics approaches are also being undertaken. In Cf-9-carrying tobacco cell cultures, provision of Avr9 results in activation of a set of defence responses, including production of active oxygen, gene induction and cell death within 3–4 hrs. This cell death is prevented by proteasome inhibitors. Expression profiling of elicited tobacco cells revealed a set of Avr9, Cf-9 Rapidly Elicited (ACRE) genes. Virus-induced gene silencing (VIGS) was used to show that several E3 ubiquitin ligase genes, and a protein kinase, are required for cell death. In Arabidopsis cell cultures challenged with flagellin or a conserved flagellin peptide (flg22), similar defense responses are elicited. Arabidopsis orthologs of ACRE (AtACRE) genes are being investigated to assess their role in RPP27-dependent and other disease resistance.