Toward Isolation of a Gene Conferring Astringency Loss in Persimmon Fruit

Persimmon (Diospyros kaki ; 2n = 6x = 90) is one of the most economically important fruit crops in East Asia. Astringent taste caused by proanthocyanidins (PAs) in mature fruit is one of the unique characteristics of persimmon. Due to the importance of this character, persimmon cultivars are classified based on the astringency at maturity and seed formation. Some cultivars lose astringency only with seed formation (pollination-variant and non-astringent; PVNA), while others do independent of seed formation (pollination-constant and non-astringent; PCNA). Because fruit of PCNA cultivars stops accumulating PA in the early stage of fruit development and becomes edible without any artificial de-astringency treatment, breeding and improvement of PCNA cultivars is one of the main modern persimmon breeding goals. Although PCNA trait is known to be conferred by a recessive allele at a single locus called ASTRINGENCY (AST), the determinant gene remains to be identified partly because of complicated segregation of the alleles in hexaploid persimmon. To characterize the AST locus, we performed chromosome walking and comparative genomics using BAC and F₁ siblings of D. lotus (2n = 2x = 30), a close relative to D. kaki. Although the astringency phenotype and marker genotypes were consistent with each other in all of 296 D. kaki F₁ individuals tested, genetic and physical maps of D. lotus constructed at AST locus suggested that this locus is probably located at euchromatic region. This could indicate the comparative genomics approach using D. lotus would possibly work. We also conducted differential expression, clustering, and co-expression network analyses using fruit transcriptomes obtained from segregating individuals in D. kaki. It appeared that DkMYB4 and/or DkMYC1, which have been reported as PA regulators, have a central role in down-regulation of genes encoding PA biosynthetic enzymes in PCNA fruit. Furthermore, ABA-related genes appeared to be possibly involved in the difference between PCNA and non-PCNA. Currently we are conducting genomic analysis at AST locus and also integrating the functional cues obtained from the trancriptomics into the genomics.