Monitoring dissociation of chimerism through real-time PCR and scanning electron microscopy following in planta transformation of rough lemon (Citrus jambhiri Lush.)

Abstract

Citrus spp. are recalcitrant to in vitro shoot regeneration and we report an improved in planta protocol for genetic transformation of rough lemon that bypasses shoot regeneration in tissue culture. The features of the protocol were the use of an Agrobacterium suspension with an OD_{600 nm} = 0.6–1.0 supplemented with 100 μg acetosyringone, gentle shaking of embryo axes pricked at shoot apical meristems (from 2-day-old germinating seeds) at 70 rpm during agro-infection, followed by growth and development of plantlets at 30 °C. PCR screening of 2-month-old T₀ plants revealed the presence of an amplicon corresponding to the β-1,3-glucanase gene in the primary branches of 25 plants with a transformation efficiency of 7.74%. PCR analysis of the secondary branches of these plants after 18 months showed chimerism, i.e., the coexistence of transformed and untransformed branches in all 25 plants. Quantification of β-1,3-glucanase expression in the transformed secondary branches by qRT-PCR showed that plant number 32 had maximum (3.71-fold) relative transgene expression. The qRT-PCR analysis of all four tertiary branches arising from the transformed secondary branch of plant number 32 showed no significant differences in expression among themselves and from the transformed secondary branch, suggesting restoration of the transformed branches with uniform expression and dissociation of chimerism. Scanning electron microscopy examination of leaves from secondary and tertiary branches that uniformly expressed the transgene showed a smooth, waxy surface with non-significant variation in stomata, which had a narrow opening and a mean pore length of 4.22 ± 0.25–5.09 ± 0.36 µm. In contrast, the leaves of untransformed branch had a rough surface and a significantly large stomatal opening with a mean pore length of 7.82 ± 0.67 µm. The micro-morphological characteristics of the leaves confirmed the dissociation of chimerism in the transformed tertiary branches of plant number 32. The study demonstrates identification of chimerism after in planta transformation using PCR technique, and the novelty relates to monitoring dissociation of chimerism in transformed tertiary branches of T₀ generation using qRT-PCR analysis and its corroboration by electron microscopy. The protocol for genetic transformation in plants described in the present study can be used for trait improvement by transgenesis.