Root growth angle: An important trait that influences the deep rooting of apple rootstocks

•We examined the phenotypic variation in root growth of apple rootstocks.•Shallow-rooted M9 had a smaller root growth angle (RGA) than deep-rooted Baleng Crab.•RGA and root length are important for deep rooting traits in Malus domestica.•Both RGA and root length showed transgressive segregation in hybrid progeny.•MdDRO1 and MdPIN11 are important genes that contribute to determine root growth angle.

Root growth angle (RGA) is one of the major morphological traits that determine whether a plant develops a shallow or deep root system. To date, however, no genetic information has been available regarding RGA in apple (Malus domestica Borkh.). In this study, phenotypic variation in RGA was investigated using deep-rooted Baleng Crab (BC), shallow-rooted M9, and 115 intraspecific hybrid progenies derived from a BC × M9 cross. Auxin (IAA) content and the expression of genes that control RGA and root gravitropism, MdDROs and MdPIN11, were also analyzed. The results revealed that M9 had a smaller RGA than Baleng Crab. The IAA level and the expression of both MdDRO1 and MdPIN11 were significantly lower in the roots of M9 than in those of Baleng Crab, indicating the potential importance of auxin signaling and these two genes in the control of RGA. Furthermore, fewer amyloplasts were detected in columella cells in the root cap of M9 in comparison with Baleng Crab. Both RGA and root length showed a normal distribution and transgressive segregation in hybrid progeny, suggesting that both these traits are quantitative in nature and controlled by polygenes. We conclude that both RGA and root length are important traits contributing to deep rooting in M. domestica and that these might be influenced by MdDRO1 and MdPIN11.