Genotypic and transcriptomic characterization of a SAD gene cluster in European hazelnut (Corylus avellana L.)

European hazelnut (Corylus avellana L.) is a nut crop whose kernels are rich in unsaturated fatty acids, a feature that has so far been only marginally studied from a genetic perspective in this species. In the current work, a preliminary genome-wide association study (GWAS) for fatty acid content in hazelnut kernels was conducted on a diversity panel of 89 individuals, using 40,113 single nucleotide polymorphisms (SNPs). The analysis reported significant marker-trait association for five and six SNPs with arachidic acid (C20:0) and linoleic acid (C18:2Δ9,12), respectively. A strong association with arachidic acid (p value = 2.86 x 10 11) was observed for a SNP on chromosome 4 in proximity of a cluster including seven gene models putatively encoding for stearoyl- [acyl-carrier-protein] (ACP) Δ9-desaturase (SAD), the enzyme that introduces a first double bound in C18 fatty acids in plants. As a parallel approach, a genome-wide characterization of fatty acid desaturases (FADs) was performed identifying 27 members belonging to this gene family in the reference genome of this species. Comparative genomics analysis between European hazelnut and other Fagales highlighted the presence of a SAD gene cluster in the other species analysed except Persian walnut (Juglans regia L.). Finally, the characterization of FAD gene expression profiles in vegetative organs like leaves, petioles, nodes, internodes and roots of both plantlets grown in pot (in vivo) and in tissue culture (in vitro) was carried out. Differences among the organs in the expression patterns of paralogues of FAD genes were observed, with a general conservation of the transcriptional profiles between the two growth systems. Among the SAD genes arranged in cluster, CaSAD.9 (LOC132177089) showed high expression levels [log2(TPM+1) ≥ 6.83] in all the organs of plantlets from both growth systems.