Advances in Persian Walnut (Juglans regia L.) Breeding Strategies

Abstract

Walnut (Juglans regia L.) is one of the oldest trees with harvestable products known to humans and has a history dating to 7000 BC in Persia. Walnut breeding programs aim to release productive scion cultivars with disease resistance and high-quality nuts, along with rootstocks resistant to biotic and abiotic stresses. Genetic improvement of walnut began with the selection of superior trees in their main centers of origin, primarily from the Persian plateau. The first selection and grafting of superior walnut genotypes began in France. The first organized walnut-breeding program employing targeted hybridization began in the USA in 1948, primarily using introduced French cultivars and selected local genotypes derived from seed imported from centers of origin (Iran, Afghanistan, China). Currently, both conventional hybridization with phenotypic evaluation and molecular breeding approaches are used in the USA programs as well as those in France, China, Iran, Spain and Italy. Recent advances in biotechnology and genomics show potential to accelerate cultivar development. In addition, the exploration, description, and preservation of biodiverse germplasm can provide a gene bank of desirable traits and enable biotechnologists to conduct breeding more accurately and rapidly in the future. Recent advancements have opened up new avenues to enhance the efficiency of walnut breeding to release new scions and rootstocks. These include next-generation sequencing (NGS) techniques, bioinformatics tools, high-throughput genotyping platforms and genomics-based approaches such as genome wide association studies (GWAS), marker-assisted selection (MAS), genomic selection (GS) and genome editing with the CRISPR-Cas9 system. In this chapter, we describe the background and development of conventional walnut breeding programs in the leading walnut producing countries of the USA, France, China, Iran and Turkey, and finally focus on the current use and status of molecular breeding and biotechnology in walnut breeding.