Transcription factor MdWRKY50 correlates with spur-type formation in apple by modulating endogenous gibberellin biosynthesis

The spur-type trait is an important breeding goal for apple (Malus × domestica Borkh.) cultivar, due to its advantages in yield and orchard mechanical management. MdWRKY50 encoding a transcription factor were identified to have dramatically lower expression in spur type. RNA interference of it leads to significantly shortened internodes in transgenic apple plants, while its overexpression resulted in opposite phenotype. The shortened internode of transgenic apple with RNA interfered MdWRKY50 was in good correlation with the bioactive gibberellin (GA) level. Furthermore, MdWRKY50 was proved to directly binds to the promoter of MdGA3ox, encoding a rate-limiting biosynthetase in GA biosynthesis, as confirmed by Chromatin immunoprecipitation followed by quantitative PCR (ChIP-qPCR) and Electrophoretic Mobility Shift Assay (EMSA). This binding up-regulated the expression of MdGA3ox. The low expression of MdWRKY50-MdGA3ox regulatory pathway is conserved across six spur-type apple cultivars in contrast to the high expressions across six standard-type apple cultivars. Our findings established a MdWRKY50-MdGA3ox module regulating GA-mediated internode elongation and coorelating with spur-type formation, which might contribute to the molecular breeding of spur-type cultivars adapted to high-density, mechanized apple orchards.