Field Performance of Transgenic ‘M.26' Apples Overexpessing a Peach CBF Gene

The ability to cold acclimate and undergo a period of dormancy is essential to temperate woody plants in order to survive freezing winter temperatures. CBF genes have been shown to regulate a large number of cold-regulated (COR) genes whose products are thought to contribute to freezing tolerance.  The role of CBF genes in cold response and acclimation has been well documented in both herbaceous and woody plants.  Our previous research has demonstrated that overexpression of a peach CBF gene in ‘M.26’ apple increased freezing tolerance of non-acclimated plants, initially reduced growth, and surprisingly, resulted in short-day induced dormancy (Wisniewski, et al.  2010. Planta 233: 971–983).  The current study reports on the field performance of transgenic ‘M.26’ apple overexpressing a peach CBF gene (T166) and transgenic ‘M.26’ apple in which the expression of a native CBF gene was silenced (T186). Performance of these lines was compared to untransformed (wt) ‘M.26’ trees. Self-rooted trees were planted in the field on October 7, 2010 and various phenotypic characteristics have been monitored since then.  In Fall 2010, the T166 line exhibited an immediate response to cool temperatures and short photoperiod after planting.  T166 trees exhibited a large increase in anthocyanins in their leaves followed by a rapid senescence. By November 4, 2010, T166 trees had lost all their leaves while wt, and T186 trees still had green, intact leaves.  In spring of 2011, the T186 (CBF-silencing line) was the first to break bud and began to leaf out prior to the wt ‘M.26’ trees.  Last to break bud were the T166 trees.  The mean date of leaf emergence varied by over a week between the three lines. Current year shoot growth, stem diameter, and the number of laterals were all reduced in the T166 line overexpressing the peach CBF gene. The pattern of leaf senescence and drop in the fall was similar to that observed in the previous year. Thus it appears that overexpression of a peach CBF gene in apple has significant, long-term effects on several phenological events in apple. Observations for budbreak and other growth characteristics for 2012 will be presented.