- Hop, Humulus lupulus L. is a dioecious perennial species. The female hop plant produces flowers that mature into ‘cones’ which are used as a raw material in the beer brewing process to impart bitterness, flavor, and aroma. The craft beer industry in the current global beer market continues to experience growth as consumers sustain the demand for high quality, full-flavored beers. The majority of the world’s hops are grown in the United States and Germany. Washington, Idaho, and Oregon are the leading hop growing states in the U.S.
The hop industry relies on a limited number of cultivars that are recognized for their brewing profile and consumer familiarity with the product. For these reasons, brewers are very reluctant to change hop cultivars in their recipes, particularly aroma hop cultivars. This limits the use of traditional crossing strategies when the breeding goal is to generate a replacement cultivar for an aroma type that enjoys wide industry acceptance. Hop cultivars are highly heterozygous, and progenies produced from traditional crossing techniques may express a large number of traits which are substantially different from the female parent. In contrast, induced mutations such as those caused by gamma radiation, change only one or a few specific traits of an elite cultivar, and can possibly improve agronomic performance without significantly changing the brewing profile. Available literature is lacking on the use of gamma radiation on hop. Therefore,
the objective of this research was to determine the lethal dose of gamma radiation to kill 50% (LD50) of micropropagated nodal sections of Cascade and Centennial hop cultivars, document the vigor of surviving genotypes and determine the percent DNA change or genetic distance of each cultivar within each treatment.
The experiment was arranged as a three-factor completely randomized design (CRD) the factors being cultivar (Cascade, Centennial), gamma radiation dosage (0,2,3,4 Gy), and replicates (three). A treatment combination consisted of a Petri plate containing 50 nodal sections of a cultivar assigned to a gamma dosage. Following irradiation, plants were evaluated in tissue culture and in the greenhouse.
The LD50 of surviving genotypes in tissue culture, growth (cm), presence of roots, and plant division were recorded. The LD50 of surviving genotypes after greenhouse establishment and plant growth was also recorded. Genotypes were randomly selected within each treatment combination for DNA extraction and sequencing to obtain high molecular weight DNA for PCR pyrosequencing with an Illumina HiSeq 3000 machine. After SNP calling, pairwise genetic distance estimates for each treatment/cultivar were calculated using Bray-Curtis dissimilarity estimates. Genetic distances between treatments samples versus control samples were used to quantify genetic changes.
Survival ratings of Cascade plantlets after irradiation suggest an increase in gamma dose appears to be associated with a decreased survival. An LD50 for Cascade plantlets is estimated to be 4 Gy, this suggested rate are consistent with the previous study of hop (Getty, 2015). The LD50 after greenhouse establishment is around 3.5 Gy for Cascade. Gamma dosage significantly affected shoot growth in Cascade. Plants subjected to 2 Gy of gamma radiation had the highest mean shoot growth in contrast to 4 Gy which had the shortest mean shoot growth. Gamma
dosage did not significantly affect root growth although, a numeric trend suggested the treatment irradiated with 2 Gy had the highest presence of roots and 4 Gy had the lowest. A strong correlation between survival of Cascade plantlets was observed with shoot and root growth. This suggests a dose with a higher presence of root emergence, shoot growth, and plantlet production will have a higher probability surviving in tissue culture and greenhouse establishment.
Survival ratings from Centennial plants did not suggest a trend of higher radiation dose resulting in decreased survival. However, a biotic contamination in the stock plants presumably increased the death rate of Centennial plants in all treatments as indicated by the low survival of non-treated plants. For this reason, drawing conclusions from the Centennial data was more difficult. An LD50 for Centennial plantlets is estimated to be around 3 Gy or 4 Gy after greenhouse establishment.
Higher radiation dosages resulted in a larger genetic distance from the control in Cascade and Centennial hop. Cascade and Centennial followed the trend of LD50 ratings, treatments irradiated with the highest level of radiation reached an LD50 and had the highest genetic distance compared to the control. Cascade explants irradiated with the highest level of radiation (4 Gy) had the highest genetic distance from the control, in contrast to the lowest dose (2 Gy) which has the lowest genetic distance from the control.
Mutation-based breeding has the potential to improve well-adapted hop cultivars by modifying relatively few traits to increase their productivity or quality while maintaining the unique brewing characteristics of the original cultivar. The study describes a method that can be used to obtain an effective dose to induce mutations and an LD50 for nodal sections of micropropagated Cascade and Centennial hop using gamma radiation.