Walnut Rootstock Transformation and Regeneration from Vegetative Tissue

The primary goal of this proposal is to develop novel regeneration technologies needed to engineer walnut rootstocks for pest and disease resistance. The productivity of the walnut industry of California is dependent the availability of superior rootstocks to combat soil disease and pest problems. Individual selections displaying resistance to either Phytophthora crown and root rots or to root lesion nematode, Pratylenchus vulnus, have been identified and can be clonally propagated but these selections are susceptible to crown gall. Rootstocks with multiple resistances are clearly desirable. Walnut transformation depends on the use of somatic embryos. Somatic embryos are readily obtained from immature, and therefore uncharacterized, seed but not from vegetative or other mature tissues. This precludes inserting any genes into a rootstock exhibiting another superior trait if the trait was identified in a seedling or mature tree. Walnuts have a long generation time and the most promising rootstocks, hybrids of English walnut x black walnut, are sterile or nearly sterile, factors that severely impede developing multiple resistances by breeding. The recent discoveries of genes that stimulate regeneration provide a unique avenue to develop cultivar independent transformation for walnut. Oncogene silencing gene constructs previously developed by us will be introduced into walnut rootstocks that are resistant to Phytophthora and/or root lesion nematode. The crown gall resistant lines will be propagated and introduced into the field in two locations in California for further testing of horticultural traits and disease and pest resistance with the goal of developing a new rootstock variety that will be resistant to major soil based disease and pests of significance for the California walnut industry.