Dihydrochalcone glycoside biosynthesis in Malus is regulated by two MYB‐like transcription factors and is required for seed development

SUMMARY Dihydrochalcones (DHCs) including phlorizin (phloretin 2′‐O‐glucoside) and its positional isomer trilobatin (phloretin 4′‐O‐glucoside) are the most abundant phenylpropanoids in apple (Malus spp.). Transcriptional regulation of DHC production is poorly understood despite their importance in insect‐ and pathogen‐plant interactions in human physiology research and in pharmaceuticals. In this study, segregation in hybrid populations and bulked segregant analysis showed that the synthesis of phlorizin and trilobatin in Malus leaves are both single‐gene‐controlled traits. Promoter sequences of PGT1 and PGT2, two glycosyltransferase genes involved in DHC glycoside synthesis, were shown to discriminate Malus with different DHC glycoside patterns. Differential PGT1 and PGT2 promoter activities determined DHC glycoside accumulation patterns between genotypes. Two transcription factors containing MYB‐like DNA‐binding domains were then shown to control DHC glycoside patterns in different tissues, with PRR2L mainly expressed in leaf, fruit, flower, stem, and seed while MYB8L mainly expressed in stem and root. Further hybridizations between specific genotypes demonstrated an absolute requirement for DHC glycoside production in Malus during seed development which explains why no Malus spp. with a null DHC chemotype have been reported. Significance Statement Dihydrochalcones (DHCs) are the characteristic phenylpropanoids found in all Malus species. Our results reveal the genetic basis for DHC glycoside production between different apple genotypes and in different apple tissues. We further demonstrate that DHC glycoside production is necessary for Malus seed development, which likely explains why no Malus plants have been found with a null DHC chemotype.