GENETIC ENGINEERING OF WALNUT (JUGLANS REGIA L.)
Gene transfer using a somatic embryo system has become relatively routine in walnut. Somatic embryo cultures are initiated from developing zygotic embryos, and through repetitive embryogenesis, cultures can be maintained indefinitely. Genes are transferred into single cells of an embryogenic embryo using modified Agrobacterium tumefaciens strains as vectors. These typically contain the gene of interest along with selectable and scorable marker genes. Over several generations of embryogenesis on a selection medium, embryos that originated from cells containing transgenes survive and reproduce more vigorously than untransformed embryos. Selection efficiency is improved by also using a scorable marker. Confirmed transgenic embryos can be germinated and grown into plants or grafted onto seedling rootstock.
Notable in this system is that the original cultures are derived from developing seed and do not have the same genotype as the parent cultivar. As a result, the probability that the first generation of transgenic trees will be commercially acceptable is limited. Instead, they will have to be used as germplasm in a breeding program. To that end, we have started embryo cultures from highly precocious parents with the expectation that the performance of the novel genes can be evaluated and that the trees can be used in controlled crosses at a very young age. We expect that commercial cultivars derived from these controlled crosses will have the transgenes stably incorporated because they will have undergone the screening process of meiosis.
Traits that we are trying to change by genetic engineering and the gene sources include: resistance to codling moth from Bacillus thuringiensis; rootability from A. rhizogenes; nematode resistance from the snow drop plant; and, fungal resistance from tobacco, pear, barley and nettle. These projects are in different stages of development, with the codling moth resistance being most advanced. Plants containing the selected gene have been shown in laboratory trials to inhibit development of the codling moth larva and are being planted in field trials this year.