A broad exploration of a transgenic population of citrus:: stability of gene expression and phenotype

A collection of 70 transgenic citrus plants for the uidA and nptII genes have been maintained under screenhouse conditions over a period of 4-5 years. A detailed scanning of the plants allowed us to detect four phenotypic off-type plants and a large variation of transgene integration and expression patterns among the population. Off-type plants were analysed and characterised as nucellar tetraploids, probably originating from tetraploid starting tissues rather than from somaclonal variation events. Transgene integration and expression analyses revealed that: (I) a significant negative correlation was found between copy number and GUS activity; (2) rearrangements of the T-DNA inserts did not imply low expression levels; and (3) stability of integration and expression of the transgenes was confirmed for all the transformants grown under natural environmental conditions. These combined features validate transformation as a tool for the genetic improvement of citrus.