Genetic Stability
When a gene is transferred or induced to change by physical or chemical agents
in long-lived perennials such as fruit trees, it is essential that stable patterns of
gene expression are maintained for long periods of time and, although fruit trees
are normally vegetatively propagated, the T-DNA should also be heritable in the
progeny. Several studies have been carried out on genetic stability and
inheritability with marker genes (James et al. 1996) or with important genes for
agronomic performance, such as rolABC in transgenic kiwi plants of both cvs
staminate GTH and pistillate Hayward (Rugini et al. 1997; 2000a). After 12
years the staminate rolABC plants still maintain the same morphology and the
offspring (transgenic staminate X normal pistillate) was transgenic in 50% of
plants. The cherry rootstock Colt, transgenic for RiT-DNA which seems able to
modify the scion vigour (Rugini and Gutierrez-Pesce 1999), showed the same
stability after four years in the field (Rugini, pers. com.). Transgenic apricot for
virus coat protein still maintains its tolerance to viruses after several years in the
field (Laimer, pers. com.). A lot of work has been done in the USA by Scorza
and co-workers on transgenic plants of Prunus domestica carrying plum pox
virus coat protein (PPV-CP), gus and nptII genes. The expression has been
stable in the greenhouse for over five years and the progeny produced from
hybridisation of transgenic plants carrying plum pox virus coat protein inherited
the transgenes and expressed it (Ravelonandro et al. 1997). One should note that
in some cases the transgenic plants may require different agronomic
management in the field to optimise the performance of the plants, that is,
vigorous growth observed in kiwi rolABC plants may require less N2 fertilisation
to avoid pathogen attacks (Balestra et al. 2001) and maybe less water.
