The Forest Health Initiative, American Chestnut (Castanea dentata) as a Model for Forest Tree Restoration: Biological Research Program

The Forest Health Initiative (FHI) was developed and implemented to test the hypothesis that a coordinated effort in biotechnology research could lead to resistant trees capable of restoring a species in a relevant time frame. As a test case, the American chestnut (Castanea dentata) was chosen for study as it is an iconic forest tree species in the eastern United States and southeastern Canada that has been nearly extirpated by chestnut blight which is caused by an introduced fungal pathogen (Cryphonectria parasitica). In addition, the species has attracted research investments over many decades, leading to some promising possibilities for effecting restoration. The FHI, now completing its third year, has integrated genomics-based candidate gene discovery with robust clonal propagation and gene transformation systems capable of producing hundreds of independent events for dozens of genes per year. A promising early leaf assay for blight reaction has been developed that will be instrumental in screening the large amount of material in production through these systems. Regulatory permits for testing some of the earliest transgene events have been obtained as small-scale field testing is beginning. High density genetic maps and various mapping populations are being analyzed to gain additional insights into the specific genes found in quantitative trait loci (QTLs) for resistances to blight and Phytophthora root rot (caused by P. cinnamomi, also known as ink disease). Nearly complete genome sequences of three of the blight resistance QTLs have been determined. These sequences are greatly assisting these analyses, as have comparisons with peach and other completed plant genomes, demonstrating the power of comparative genomics. These results show great promise for meeting the Proc. V Int. Chestnut Symposium Eds.: M.L. Double and W.L. MacDonald Acta Hort. 1019, ISHS 2014 180 challenge proposed by the FHI hypothesis for American chestnut. In addition, other forest tree species under threat should benefit from similarly designed initiatives. INTRODUCTION A forest health crisis is now widely apparent in the U.S. and various places globally (Rapport et al., 1998; Pautasso et al., 2010). Much of the crisis relates directly to introduced pests and pathogens, where native tree species have little to no evolved defenses (Leibhold et al., 1995; Lovett et al., 2006). Other aspects such as climate change and forest fragmentation are also at play in advancing the crisis (Woods et al., 2010). At the same time, unprecedented biotechnological advancements are available for application in many species (Merkle et al., 2007; Wheeler and Sederoff, 2009). These include genome mapping and sequencing, genome-wide gene expression analysis, candidate gene discovery, gene transformation and genetic engineering, and markerassisted breeding. Clearly time is of the essence for developing and implementing systematic approaches to defining and solving forest health problems (Chornesky, 2005; Ellison, 2005; Hain, 2006). But what form would such a concerted effort take? One such approach, the Forest Health Initiative (FHI) initially developed in 2009, is currently being applied to American chestnut (Castanea dentata) and its two major diseases-chestnut blight (caused by Cryphonectria parasitica) and Phytophthora root rot (caused by P. cinnamomi). The FHI comprises a scientifically focused attempt to develop resistant trees under social (public acceptance, regulatory approval) engagement and a tight timeline. The social aspects can be gleaned from the www.foresthealthinitative.org as can the overall science plan and various research results. A summary of the key scientific approaches and current results is provided here.