Biotic and Abiotic Stress Responses in Tomato Breeding Lines Resistant and Susceptible to Tomato Yellow Leaf Curl Virus

In the eyes of a tomato grower, resistance to TYLCV, as opposed to susceptibility, is defined by the absence of, or mild, disease symptoms, and acceptable yield. In resistant cultivars and breeding lines, the amount of virus that can be detected with molecular tools is usually smaller than that in the susceptible plants, especially during the first 4 weeks after inoculation. Genetic studies have indicated that several genes, expressed as quantative trait loci (QTL), are involved in providing the resistance phenotype described above. Several QTLs have been localized to tomato chromosomes using polymorphic DNA markers (see Part V, Chapter 2). However, the molecular basis of resistance to TYLCV remains totally unknown. Moreover, the physiological state of susceptible vs. resistant plants, before and after inoculation, has never been compared. To provide some clues on what makes a plant resistant to TYLCV and another susceptible, we have considered the virus as a particular case of stress, among many that a tomato plant may face, and resistance as a particular case of successful response to stress. The response of plants to biotic and abiotic stresses has been studied intensively. A stress response is initiated when plants recognize stress at the cellular level, activating signal transduction pathways that transmit information within the individual cell and throughout the plant, leading to the changes in the expressing of many gene networks. Hence plants respond to biotic and abiotic stresses by activation of R-gene mediated and “signal transduction” defense response pathways (Figure 1).