Almonds (Prunus dulcis) have been grown since antiquity. They are bitter and sweet. Sweet are used in food and have a pleasant taste and characteristic aroma. Bitter also go to the production of, for example, almond oil. To find out why sweet almonds are sweet, scientists analyzed the sweet almond genome and found a mutation in the bHLH2 protein, which is involved in the synthesis of amygdalin, a toxic substance formed in the nuts of wild almond and giving it a bitter taste. Mutation “broke” protein in sweet almonds and amygdalin in it ceased to form.

Nuts of wild almond, in contrast to the cultured, contain the substance amygdalin, which gives them a bitter taste and makes poisonous. In the human stomach, amygdalin disintegrates to form toxic substances, hydrocyanic acid and its salts. Therefore, the selection of plants with sweet kernel has become the key for the domestication of almonds. Genetic studies have shown that sweet almonds are the result of a dominant mutation in the locus, which is called the “Sweet kernel” (Sweet kernel, Sk). It was unclear, until recently, what genes are in its composition

Analysis of the bHLH genes showed that in a sweet almond in one of them, bHLH2, a mutation appeared that led to the replacement of the amino acid leucine, in the 346 position of the polypeptide chain, with phenylalanine. Unlike leucine, phenylalanine has a benzene ring, a bulky structure that can disrupt protein. Scientists from the University of Murcia have suggested that the replacement either interferes with the formation of the dimer bHLH2 (the transcription factors bHLH work “in a pair”), or it is formed, but does not work.

To find out which of the hypotheses is correct, scientists identified the bHLH2 proteins, derived from sweet and bitter almonds, using electrophoresis to divide them by size. It turned out that both proteins form dimers. On the other hand, the sweet almond dimer was unable to bind to DNA (and, accordingly, regulate transcription). Therefore, the researchers concluded that in the sweet almond bHLH2 dimer does not work and prevents the biosynthesis of poisonous amygdalin.