Morphological and molecular identification of Fusarium wilt resistance in tomato cultivars mediated by defense gene response

Tomato is a major vegetable crop in Quetta, Balochistan, Pakistan, but its production is severely affected by Fusarium wilt caused by Fusarium oxysporium f. sp. lycopersici. The region has a continental semi-arid Mediterranean climate, which exacerbates the impact of this disease. This study aimed to identify resistance in tomato cultivars against Fusarium wilt and also understand the underlying mechanisms of host defense gene response. Sources of resistance against F. oxysporium f. sp. lycopersici in diverse tomato genotypes were identified, and expression profiling of defense genes strengthened these findings. For a comprehensive understanding of the host-pathogen interaction, the pathogen was isolated using a standard tissue isolation method. Morphological characterization revealed oval-shaped conidia of the pathogen by microscopic examination, similar to F. oxysporium f. sp. lycopersici. Molecular identification via PCR amplification of the Internal Transcribed Spacer (ITS) and Translation Elongation Factor (TEF1-α) regions supported these morphological observations and the amplified product sequence of Tom-BAC1-5 showed 99–100% similarity with the F. oxysporium f. sp. lycopersici species complex. Pathogenicity assays demonstrated that 3 cultivars (Tom/cr/pr06, Tom/cr/pr05, and Tom/cr/pr04) were highly susceptible to FOL and two cultivars (Tom/cr/pr07 and Rio Grande) were moderately resistant. The 3,3′-Diaminobenzidine DAB staining of infected tissues visually showed that highly susceptible cultivars exhibited darker staining and significant vascular damage. In contrast, moderately resistant cultivars exhibited pale staining and reduced vascular damage. Gene expression profiling further showed that these two cultivars (Tom/cr/pr07 and Rio Grande) had significantly elevated transcript levels of PR1a, WRKY33, and PAL, suggesting activation of salicylic acid signaling and phenylpropanoid biosynthesis pathways. Conversely, susceptible cultivars showed delayed or reduced expression of these defense-related genes, underscoring the importance of early transcriptional responses for effective resistance. Overall, this study highlights WRKY33 and PR1a as potential molecular markers for resistance screening and provides novel insights into the defense signaling mechanisms against F. oxysporium f. sp. lycopersici. These findings contribute to the sustainable management of Fusarium wilt and enlighten future tomato breeding programs to enhance disease resistance.