Mutation of a bHLH transcription factor allowed almond domestication.

Wild almond species accumulate the bitter and toxic cyanogenic diglucoside amygdalin. Almond domestication was enabled by the selection of genotypes harboring sweet kernels. We report the completion of the almond reference genome. Map-based cloning using an F1 population segregating for kernel taste led to the identification of a 46-kilobase gene cluster encoding five basic helix-loop-helix transcription factors, bHLH1 to bHLH5. Functional characterization demonstrated that bHLH2 controls transcription of the P450 monooxygenase-encoding genes PdCYP79D16 and PdCYP71AN24, which are involved in the amygdalin biosynthetic pathway. A nonsynonymous point mutation (Leu to Phe) in the dimerization domain of bHLH2 prevents transcription of the two cytochrome P450 genes, resulting in the sweet kernel trait.

A qNMR approach for bitterness phenotyping and QTL identification in an F1 apricot progeny.

In apricot the bitter flavor of seeds is determined by the amount of amygdalin, a cyanogenic glucoside whose cleavage by endogenous enzymes, upon seed crushing, releases toxic hydrogen cyanide. The presence of such a poisonous compound is an obstacle to the use and commercialization of apricot seeds for human or animal nutrition. To investigate the genetic loci involved in the determination of the bitter phenotype a combined genetic and biochemical approach was used, involving a candidate gene analysis and a fine phenotyping via quantitative nuclear magnetic resonance, on an F1 apricot progeny. Seven functional markers were developed and positioned on the genetic maps of the parental lines Lito and BO81604311 and seven putative QTLs for the bitterness level were determined. In conclusion, this analysis has revealed some loci involved in the shaping of the bitterness degree; has proven the complexity of the bitter trait in apricot, reporting an high variance of the QTLs found over the years; has showed the critical importance of the phenotyping step, whose precision and accuracy is a pre-requisite when studying such a multifactorial character.

Genomic variation in Streptococcus mutans: deletions affecting the multiple pathways of beta-glucoside metabolism.

The genome of Streptococcus mutans UA159 contains two phospho-beta-glucosidase genes, bglA and celA, which occur in operon-like arrangements along with genes for components of phosphotransferase transport systems and a third phospho-beta-glucosidase encoded by the arb gene, which does not have its own associated transport system but relies on uptake by the bgl or cel systems. Targeted inactivation of each of the phospho-beta-glucosidase genes revealed that bglA is involved in aesculin hydrolysis, celA is essential for utilisation of cellobiose, amygdalin, gentobiose and salicin, and arb is required for utilisation of arbutin. Inactivation of genes for the phosphotransferase systems revealed an overlap of specificity for transport of beta-glucosides and also indicated that further, unidentified transport systems exist. The cel and arb genes are subject to catabolite repression by glucose, but the regM gene is not essential for catabolite repression. Screening a collection of isolates of S. mutans revealed strains with deletions affecting the msm, bgl and/or cel operons. The phenotypes of these strains could largely be explained on the basis of the results obtained from the knockout mutants of S. mutans UA159 but also indicated the existence of other pathways apparently absent from UA159. The extensive genetic and phenotypic variation found in beta-glucoside metabolism indicates that there may be extensive heterogeneity in the species.