Expression of disease reaction in transgenic tomato overexpressing Trichoderma harzianum chitinase gene as influenced by plant age

Early blight caused by Alternaria solani is a major production constraint in the cultivation of tomato. Success through conventional breeding has been elusive for the development of resistant varieties from wild sources due to transfer of several unacceptable quality and yield attributes from them. Therefore, transformation of tomato with gene encoding pathogen cell wall degrading enzyme chitinase isolated from Trichoderma harzianum was carried out and transgenic plants were generated. Two homozygous lines N3–5 and N8–4 with 2.5× and 4.0× higher chitinase expression, respectively, were shortlisted and were assessed at fully grown (mature) stage in net house bioassays with Alternaria solani through in vitro detached leaf as well as challenging with fungal inoculum and were compared with disease reaction at the seedling stage. Screening at the seedling stage in T2 generation in the earlier study revealed that one of the transgenic lines N3–5 exhibited moderately resistant reaction with significant reduction in lesion size as compared to the untransformed control under in vitro detached leaf bioassay and challenge inoculation assays. Screening at fully grown stage through challenge inoculation in T3 and T4 generation recorded an intermediate disease reaction (between susceptible and resistant) with significantly lower disease incidence of early blight as compared to susceptible reaction of control plants. In the in vitro detached leaf assay, these lines exhibited significantly reduced lesion size compared to control at 65 days but this difference was eliminated at 74 and 85 days signifying thereby the role of plant age.