The Road to Resistance in Forest Trees

Abstract

In recent years, forests have been exposed to an unprecedented rise in pests and pathogens. This, coupled with the added challenge of climate change, renders forest plantation stock vulnerable to attack and severely limits productivity. Genotypes resistant to such biotic challenges are desired in plantation forestry to reduce losses. Conventional breeding has been a main avenue to obtain resistant genotypes. More recently, genetic engineering has become a viable approach to develop resistance against pests and pathogens in forest trees. Tree genomic resources have contributed to advancements in both these approaches. Genome-wide association studies and genomic selection in tree populations have accelerated breeding tools while integration of various levels of omics information facilitates the selection of candidate genes for genetic engineering. Furthermore, tree associations with non-pathogenic endophytic and subterranean microbes play a critical role in plant health and may be engineered in forest trees to improve resistance in the future. We look at recent studies in forest trees describing defense mechanisms using such approaches and propose the way forward to developing superior genotypes with enhanced resistance against biotic stress.

Keywords: breeding; candidate genes; endophytes; genetic engineering; genomics.

Figure 1

Two avenues to achieving resistance in forest trees are depicted. In tree breeding approaches, (A) resistant phenotypes are identified in field and tested under controlled conditions. Such resistant genotypes (green trees) are selected as parents in breeding programs to introduce resistance into a susceptible background (orange trees). Marker-assisted selection has supported plant breeding programs and new genomic tools could enhance selection of resistant trees, e.g. GWAS and GS. In the genetic engineering approach (B), structured breeding programs provide the information about individual resistant or susceptible genotypes that are investigated using a multitude of omic approaches to identify defense mechanisms governing resistance and the power lies in the integration of such studies to a systems level. In addition, summation of such studies into a user-friendly database can assist with comparisons to identify genes and pathways especially important for resistance against specific challenges. A network analysis across species provides support for key genes or pathways to manipulate. The next steps are to test the function of such genes in model systems, and this information is updated in the database. Successful candidates conferring a degree of resistance can be introduced into the forest tree of interest with intense laboratory and field testing (blue trees). The transgenic tree conferring resistance may be bred into various clones selected through breeding programs to pyramid resistance.