Efficient heat-shock removal of the selectable marker gene in genetically modified grapevine

Cisgenesis is one of the new plant breeding technologies emerging as a promising tool for the future, more publicly accepted than the traditional transgenic approach. One of the requirements for a cisgenic plant is the absence of selectable marker genes in the genome. In this study, a system for marker gene removal after selection of transgenic plants has been tested in grapevine. This is based on a binary vector containing a heat-shock-inducible promoter which, upon induction, activates a recombinase to produce the excision of a FRT-flanked box. After the removal of this cassette hosting both the marker gene, nptII, and the recombinase itself, the reporter gene gus may be expressed. Gene transfer experiments on grapevine embryogenic callus were carried out via Agrobacterium tumefaciens. Different heat-shock treatments with variable temperatures and heat incubations times were tested on a selected line and the optimal conditions for a complete removal of nptII with the subsequent gus transcription were found. Plants were analysed by means of qPCR on genomic DNA, to quantify nptII removal, and by a fluorimetric assay to measure gus activity. Our study is conceived as a proof-of-concept to investigate the feasibility of this method in grapevine in view of developing an efficient cisgenic approach in this valuable fruit crop.