Subcellular localization and differential expression provide insights into the putative function of the nematode resistance gene Hs4

Plant-parasitic nematodes are economically important threats to global crop production. The beet cyst nematode (Heterodera schachtii) is a crucial pest in sugar beet (Beta vulgaris ssp. vulgaris). While all species of the genus Beta are highly susceptible, the three species of the beet wild relative genus Patellifolia are entirely resistant. Recently, we cloned the Hs4 gene from P. procumbens, which confers complete resistance. In this study, we aimed to determine whether putative Hs4 orthologs exist in Beta and Patellifolia species. The Hs4 gene consisted of 4999 bp, with six exons and five introns. Patellifolia species contain highly similar Hs4 homologs. Single nucleotide polymorphisms and insertions/deletions between accessions and species could be detected. We found an exonic integration of three bases, resulting in the addition of one amino acid. Interestingly, this variant was present in single accessions of all three Patellifolia species. Beta vulgaris ssp. vulgaris contains an Hs4 homolog (BvHs4) with 60 % protein identity to Hs4. BvHs4 homologs were present in all Beta species analyzed. Further, we examined the expression patterns of Hs4 and BvHs4 homologs. While Hs4 homologs from Patellifolia species are strongly expressed in roots, BvHs4 homologs are expressed mainly in leaves. When the spatio-temporal expression of Hs4 was examined, no response to nematode inoculation was observed. These results are highly relevant for searching for functional Hs4 alleles and breeding nematode-resistant varieties.