Abstract
This study investigates the potential of Trichoderma harzianum to mitigate the effects of Beet curly top Iran virus (BCTIV) on tomato plants. Tomato seedlings at the four-leaf stage were treated with a T. harzianum suspension and subsequently agroinoculated with a BCTIV infectious clone. The experiment included four treatments: mock plants (C), BCTIV-inoculated plants (V), Trichoderma-treated plants (T), and plants both infected with BCTIV and treated with Trichoderma (TV). Three weeks post-inoculation, symptom development and virus accumulation were assessed. At 45 days post-inoculation, root colonization by T. harzianum was confirmed. The disease severity index indicated a significant reduction in TV plants compared to V plants. Virus accumulation was also significantly lower in TV plants. Real-time PCR analysis showed increased expression of defense-related genes (HSP90, AGO2a, PR1) in TV plants, suggesting enhanced plant defense responses. Additionally, TV plants exhibited the highest fresh and dry weight among all groups. The presence of T. harzianum spores in the roots of TV plants confirmed successful colonization. These findings demonstrate that T. harzianum enhances tomato resistance to BCTIV by activating plant defense mechanisms, reducing disease severity and viral replication, promoting healthier growth and greater biomass in the treated tomato plants.