Vascular wilt caused by Fusarium oxysporum f. sp. melonis (FOM) is one of the most important diseases in melon. Four pathogenic races named: 0, 1, 2, and 1.2 have been identified in population of this fungus based on the pathogenicity pattern on differential varieties. The gene Fom2 has been reported as a key gene for conferring resistance to the fungus race 1. The majority of Iranian melon cultivars are susceptible to race 1. Clarifying the occurrence pattern of Fom2 in the fungus race 1 in sensitive and resistant Iranian melon cultivars in comparison with two standard melon lines could be helpful for the investigation on the role of Fom2 in the cultivars reaction. Since, the full sequence of Fom2 from Melon and I2 from Tomato resistance sources were cloned; it has been shown that the LRR and NB-ARC domains in the coded proteins play critical roles in detection of Avr proteins and R proteins activation as well. Considering to the importance of NB-ARC domain and the lack of functional information in this region of the gene Fom2, this domain was selected for sequencing, identifying possible mutations and to study their possible relationship of mutations with the type of cultivars reaction against fungal races. Six melon cultivars including four Iranian local melon cultivars (Mashhadi, Shahde Shiraz, Khatooni and haghani) and two standard melon cultivars (Charentais Fom1 and Charentais Fom2) were used in this study. Two specific primer pairs (PSh20-F/R and PSh20.2-F/R) were designed and used for the tracking of partial and full-length sequence of Fom2. More investigation was performed on the diversity occurrence pattern among cultivars and its conformity with cultivars resistive pattern against the fungus race 1. Our results showed that the partial and the full length sequence of Fom2 had this potential to be tracked in sensitive as well as resistant cultivars. On the other hand, the cultivars reactions were not predictable based on the gene occurrence pattern. However, some studies have already been reported that the linked markers of Fom2 were not useful for the prediction of reaction pattern in Iranian melon cultivars. The partial segments of Fom2 amplified by PSh20-F/R primers were bi-directionally sequenced and the results were submitted in gene bank. Sequence analysis of NB-ARC domain in the six varieties led to detection of 16 polymorphic nucleotide positions, which nine of them appeared in protein levels. Six out of the nine mutations placed in AAA-ATPase subdomain. Although the polymorphic pattern of mutations in nucleotide and amino acid positions were not in accordance with the cultivars resistive pattern but the cultivars could be discriminated based on some other mutations. Due to the switching function of AAA-ATPase sub domain in ATP hydrolysis and activation of R proteins, variation in this region between the resistant and susceptible cultivars could be considerable. It seems the presence of identified mutations at AAA-ATPase subdomain of Fom2 could be considered in cultivars resistive pattern unconformity and it needs to be followed in future studies to clarify the action of etected mutations in this subdomain.

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