Different genes for different folks in tree crops: What works and what does not

Whereas disease, pest and herbicide resistance have served as valuable entry points for plant biotechnology in general, the emphasis with crop plants is clearly shifting towards modifying traits that enhance quality, nutritional content and productivity. In tree crops this would involve specifically perturbing the expression patterns of endogenous genes associated with the trait under investigation. New tools such as enhanced gene silencing technology, EST database development, and total genome sequencing will likely accelerate the potential impact of this kind of work. Established plant transformation methods will continue to play crucial roles in evaluating newly discovered endogenous genes and in characterizing their function in whole plants. Examples of some of the areas in which we are currently integrating these kinds of approaches include the manipulation of sugar and ethylene metabolism in apples (Malus ×domestica Borkh.), of oil/fat composition in walnut (Juglans regia L.) and of growth architecture in citrus. In almonds [Prunus dulcis (Mill.) D.A. Webb] we are studying self-incompatibility, a trait with potentially significant implications regarding productivity.