Fusarium wilt is caused by pathogenic fungi in the genus Fusarium, which infect the roots of susceptible plants and are able to spread throughout the vascular system, stimulating the production of gums and gels that block metabolite and water transport (Beckman, 1987). In resistant plants, infection is restricted to vascular tissue in the lower part of the root system, coincident with the production of antifungal metabolites and the formation of structural barriers to infection (Beckman,1987). The I-2 resistance gene has been introgressed from Lypersicon pimpinellifolium to cultivated tomato (L. esculentum) and protects plants from race 2 (Alexander,1945) of Fusarium osporum f.sp. lycopersici (Stall and Walter,1965; Cirulli and Alexander, 1966). I-2 as been isolated by map-based cloning and encodes a cytosolic protein with an N-terminal leucine zipper, a

putative nucleotide binding site, and seventeen C-terminal 17 leucine-rich repeats (SImons et al.1998). A cytological study involving tomato plants containing an I-2 promoter-gusA construct showed that the pattern of I-2 expression was coincident with the site of pathogen containment in resistant roots (Mes et al., 2000).

Fusarium oxysporum. sp. vasinfectum is found in most countries where cotton (Gossypium spp. ) is grown. The pathogenicity of seven Australian isolates of this fungus has been analysed and no significant resistance was seen in 11 G. hirsutum cultivars, although G. aboreum cv. Roseum was resistant (Davis et al. , 1996). These results are consistent with the observation that most commercial cotton cultivars show relatively poor resistance to Australian races of F.oxysporum f. sp. vasinfectum.

Although Fusarium oxysporum, in is probably a species complex with different vegetative incompatibility groups (Fernandez et al. 1994; Davis et al. 1996) and polyphyletic origins (Skovgaard et al. 2001), it is possible that elicitors may be shared between different formae speciales. Hence, the avirulence factor of F. oxysporum f. sp. Iycopersici race 2 that is recognised by I-2 in tomato might also be present in races of F. oxysporum f. sp. vasinfectum. The availability of genetic, molecular and

cytological data relating to I-2-mediated resistance make I-2 a good candidate (indeed the only candidate) for expression in plants lacking genetic resistance to Fusarium. Many resistance genes function in plants from the same taxonomic family e. g. pepper BS-2 functions in other Solanaceaous plants such as tomato (Tai et al. 1999), and some are also able to recognise their cognate elicitors and trigger a defence response in plants from other taxonomic families e. g. Cf-4 in lettuce (van der Hoorn et al. 2000) and Cf-9 in oilseed rape (Hennin et al. 2001, 2002) or initiate a Iigand

independent response e. g. RPW8 in native tobacco (Peart et al. 2002). Hence, it is possible that I-2 could trigger defence responses in plants outside the Solanaceaous such as cotton. The overall aim of the project was therefore to test the tomato I-2 gene for its ability to confer Fusarium resistance in cotton (Gossypium hirsutum).