Identification of the HAK gene family reveals their critical response to potassium regulation during adventitious root formation in apple rootstock

Adventitious root formation is a bottleneck during vegetative proliferation. Potassium (K+) is an essential macronutrient for plants. K+ accumulation from the soil and its distribution to the different plant organs is mediated by K+ transporters named K+ transporter (KT), K+ uptake (KUP), or high-affinity K+ (HAK). This study aimed to identify members of the HAK gene family in apples and to characterize the effects of K+ supply on adventitious root formation and on the expression of HAK genes and the genes that putatively control auxin transport, signaling, and cell fate during adventitious root formation. In this study, 34 HAK genes (MdHAKs) were identified in the apple (Malus × domestica ‘Golden Delicious’) genome. A phylogenetic analysis divided MdHAKs into four clusters (I, II, III, and IV), comprising 16, 1, 4, and 13 genes, respectively. The syntenic relationships revealed that 62.5% of the total MdHAK genes arise from genomic duplication events. Chromosome location, domain structure, motif analysis, and physico-chemical characteristics were subsequently investigated. Furthermore, the application of K+ indicated the emergence of adventitious roots at 8 d and produced more adventitious roots at 16 d than the K+-free control (CK) treatment. In addition, various MdHAKs showed root-specific expression in B9 apple rootstock stem cuttings and enhanced expression during the initiation and emergence stages of adventitious root formation in response to K+ treatment. Additionally, K+ treatment enhanced the expression levels of MdPIN1, MdPIN2, and MdAUX1. Further data indicated that a higher expression of MdWOX11, MdLBD16, and MdLBD29 and of cell cycle-related genes contributed to the auxin-stimulated adventitious root formation in response to K+.