Effect of darkness treatment on the morphology, hormone status and gene expression of developing adventitious root in apple rootstock

Adventitious root (AR) formation plays a critical role for the clonal propagation of horticultural crops. In this study, Malus prunifolia var. ringo micro-cuttings were treated with IBA and darkness treatments to investigate the mechanism of darkness effects AR formation. Morphological and anatomical observation revealed that darkness promotes more ARs formation. Root morphology, and hormones level were evaluated during AR formation. Results indicated that darkness promotes AR formation by increasing auxin level at stages of 1 d and 3 d, moreover, ZR, ABA, GA(3), BR and JA also changed. The number of ARs in darkness-treated group was significantly higher than that of control treatment; Additionally, genes related to light signal pathway-, hormones-, carbohydrate metabolism-, cell cycle-, and root development were determined by RT-qPCR. Auxin- and cytokinin-related genes, such as MdARF1, MdGH3;1, MdPAT1-1; MdPIN1, MdCRF2, MdCRF4, and MdRR10 were significantly induced by darkness treatment, which corresponded with higher hormones levels. Furthermore, light-signal-related genes (MdASP3, MdATPC1, MdBGLU13, MdCBA1, MdLHCB6, MdLHCA1, MdNPQ4, MdPTE1 and MdPSAL) were also induced by darkness-treatment, resulting changes in the expression profiles of cell cycle-related genes (MdCYCB2;1, MdCYCB2;3-1 and MdCYCB2;3-2) and also in the expression of root development-related genes (MdLRP1, MdRHS19, MdSGR1 and MdSHI). Collectively, darkness treatment mediated auxin-, cytokines-, light signal-, cell cycle-, carbohydrate metabolism-, and root development-related pathways to regulating AR formation. This work laid the foundation for applied research and could be useful in future crop improvement programs. Key message This study revealed that darkness treatment during AR induction phase promoted more AR formation in plant tissue culture, and suggest that natural conditions of darkness should be simulated to improve rooting system in future research.

Automated summary

Darkness treatment promotes the formation of adventitious roots in plant tissue culture by regulating pathways related to auxins, cytokinins, light signals, cell cycle, and root development.