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Phenotyping tools and genetic knowledge to facilitate breeding of dhurrin content and cyanogenic potential in sorghum

Abstract

Cyanogenic glucosides are important secondary compounds found in plants serving roles such as plant defense, pollinator attraction, nitrogen (N) sources, and drought tolerance. Sorghum (S. bicolor [L.] Moench), an important grain crop predominantly grown in drought-prone environments, contains a cyanogenic glucoside known as dhurrin where it functions as a source of hydrogen cyanide (HCN) after the leaf tissue is disrupted. Dhurrin has been hypothesized to serve as an osmoprotectant, N turnover source, and sorghum aphid resistance mechanism. In addition, dhurrin concentrations can vary due to growth stage, environment, and genotype, and this variability can cause limitations for effective dhurrin phenotyping. To facilitate the breeding of dhurrin and HCNp, we developed a semi-quantitative phenotyping method to detect HCNp and investigated the genetics of dhurrin and HCN variation in global sorghum germplasm. In the first study, we developed a simple, semi-quantitative, high-throughput phenotyping method to detect HCNp in sorghum leaf tissue. Biochemical methods have been used to determine dhurrin content quantitatively, however these methods are laborious and costly. As a result, we developed a semi-quantitative phenotypic assay using commercial test strip paper to measure HCNp utilizing a F13 Stg Recombinant Inbred Line (RIL) population with previously reported dhurrin concentrations. We found that later sampling time improved the detection of HCNp variation with broad-sense heritability (H2) values highest at flowering. In addition, we found that other covariates such as leaf number may play a role in effective phenotyping. Altogether this assay can be used to screen a sorghum breeding population in both a greenhouse and field setting for smallholder breeding programs looking to advance their breeding generations more efficiently. In the second study we sought to understand the genetics underlying HCN and dhurrin variability, as well as investigate the relationship between drought and dhurrin using diverse sorghum landraces. We found no direct correlation between dhurrin and drought, but the slight positive correlation could suggest other environmental factors, such as pest pressures, are driving HCN and dhurrin variation. To further understand the biological relationship between dhurrin and HCN, we conducted a genome-wide association study (GWAS) for HCNp and dhurrin. We identified several significant associations between HCNp and known dhurrin biosynthetic and catabolic genetic markers, but major biosynthesis loci were not all significantly associated with HCNp. In addition, we performed a GWAS on dhurrin and found peaks associated with the dhurrin biosynthetic gene cluster, as well as other unknown loci that could contribute to dhurrin variation. This suggests that genetic variation for genes in the dhurrin biosynthesis, catabolism, and recycling pathway contributes to HCNp variability, and they are not direct proxies for each other. As a result, breeders should de-couple phenotyping methods for dhurrin and HCNp depending on the trait of interest.

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2023 Summer.
Includes bibliographical references.

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