Auxin Regulation and MdPIN Expression during Adventitious Root Initiation in Apple Cuttings

Adventitious root (AR) formation is critical for the efficient propagation of elite horticultural and forestry crops. Despite decades of research, the cellular processes and molecular mechanisms underlying AR induction in woody plants remains obscure. We examined the details of AR formation in the apple (Malus domestica) M.9 rootstock, the most widely used dwarf rootstock for intensive production, and investigated the role of polar auxin transport in post-embryonic organogenesis. AR formation begins with a series of founder cell divisions and elongation of interfascicular cambium adjacent to vascular tissues. This process was associated with a relatively high indole acetic acid (IAA) content and hydrolysis of starch grains. Exogenous auxin treatment promoted cell division, as well as the proliferation and reorganization of the endoplasmic reticulum and Golgi membrane. By contrast, treatment with the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) inhibited cell division in the basal region of the cutting and resulted in abnormal cell divisions during early AR formation. In addition, PIN-FORMED (PIN) transcripts were expressed differentially throughout the whole AR development process, with the up-regulation of MdPIN8 and MdPIN10 during induction, an up-regulation of MdPIN4, MdPIN5 and MdPIN8 during extension, and an up-regulation of all MdPINs during AR initiation. This research provides a deeper understanding of the cellular and molecular underpinnings of the AR process in woody plants.