Optimisation of tomato Micro-tom regeneration and selection on glufosinate/Basta and dependency of gene silencing on transgene copy number.

KEY MESSAGE: An efficient protocol of transformation and selection of transgenic lines of Micro-tom, a widespread model cultivar for tomato, is reported. RNA interference silencing efficiency and stability have been investigated and correlated with the number of insertions. Given its small size and ease of cultivation, the tomato (Solanum lycopersicon) cultivar Micro-tom is of widespread use as a model tomato plant. To create and screen transgenic plants, different selectable markers are commonly used. The bar marker carrying the resistance to the herbicide glufosinate/Basta, has many advantages, but it has been little utilised and with low efficiency for identification of tomato transgenic plants. Here we describe a procedure for accurate selection of transgenic Micro-tom both in vitro and in soil. Immunoblot, Southern blot and phenotypic analyses showed that 100 % of herbicide-resistant plants were transgenic. In addition, regeneration improvement has been obtained by using 2 mg/l Gibberellic acid in the shoot elongation medium; rooting optimisation on medium containing 1 mg/l IAA allowed up to 97 % of shoots developing strong and very healthy roots after only 10 days. Stable transformation frequency by infection of leaf explants with Agrobacterium reached 12 %. Shoots have been induced by combination of 1 mg/l zeatin-trans and 0.1 mg/l IAA. Somatic embryogenesis of cotyledon on medium containing 1 mg/l zeatin + 2 mg/l IAA is described in Micro-tom. The photosynthetic psbS gene has been used as reporter gene for RNA silencing studies. The efficiency of gene silencing has been found equivalent using three different target gene fragments of 519, 398 and 328 bp. Interestingly, silencing efficiency decreased from T0 to the T3 generation in plants containing multiple copies of the inserted T-DNA, while it was stable in plants containing a single insertion.