Field evaluation of apple overexpressing a peach CBF gene confirms its effect on cold hardiness, dormancy, and growth

- Unique field study of apple trees transgenic for abiotic stress resistance covering three growing seasons.
- Overexpression of a peach CBF gene in apple leads to reduced growth over three growing seasons.
- Overexpression of a peach CBF gene in apple leads to delayed bud break and early entry into dormancy over three growing seasons.
- Enhanced leaf freezing tolerance is maintained in the field by overexpression of a peach CBF gene in apple.

In recent years, the scientific literature has become replete with examples of the improvement of abiotic stress tolerance by overexpression of specific genes. Few studies, however, have evaluated transgenic plants under field conditions or the impact of overexpression on non-target traits. We previously reported that constitutive overexpression of a peach (Prunus persica) CBF (C-Repeat Binding Factor)/DREB1 (Dehydration Response Element Binding) gene in an apple (Malus × domestica) rootstock variety (M.26) resulted in a modest increase in cold hardiness of acclimated and non-acclimated leaves but also unexpectedly resulted in apple plants that were subject to short-day-induced dormancy. In the present study, a transgenic apple line constitutively overexpressing a peach CBF gene was evaluated under field conditions for three years to determine the impact of CBF overexpression on growth, phenology, and cold hardiness compared to untransformed apple. One-year-old seedling trees were planted in an orchard in autumn, 2010 and data recorded over three growing seasons. Results indicated significantly reduced growth, early leaf senescence, delayed budbreak, temporal changes in pigment accumulation and enhanced freezing tolerance in the transgenic line (T166) compared to un-transformed M.26. These data highlight the importance of conducting long-term field studies on transgenic plants modified to increase abiotic stress tolerance.