Nitrate Application Induces Adventitious Root Growth by Regulating Gene Expression Patterns in Apple Rootstocks

Adventitious root (AR) formation is an essential step in the vegetative propagation of apple rootstocks. Nitrate serves as an essential signaling molecule for regulating root architecture by inducing the expression of auxin-related genes. However, the underlying mechanisms of nitrate-mediated ARs remain to be explored in apple. In this study, stem cuttings of B9 apple rootstocks were treated with different nitrate treatments: T1 (9.4 mM/L), T2 (28.1 mM/L), and T3 (46.9 mM/L). The root morphological parameters indicated that T2 was the optimum nitrate level for AR formation and development in B9 apple rootstocks. Therefore, to identify the underlying molecular mechanism by which nitrate promotes AR formation, stem cuttings of B9 were grown on T2 and T3. Furthermore, morphological and anatomical observations of stem cuttings also revealed that the nitrate treatment (T2) promoted AR formation. The results indicated that nitrate perceptibly upregulated the relative expression of genes related to nitrate (MdNRT1.1, MdNRT2.1, MdNIA1, and MdANR1) and auxin biosynthesis (MdIAA14 and MdIAA23) in T2 cuttings compared with T3 cuttings. This resulted in enhanced expression of AR development-related genes (MdWOX11, MdARRO1, and MdSHR), collectively resulting in elevated expression of the cell cycle-related genes (MdCYCD1;1, MdCYCD3;1, and MdCYCP4;1). Overall, this study established a foundation for applied research work and shed light on nitrate-mediated AR formation in B9 apple rootstock and other fruit rootstock cuttings.

Automated summary

Nitrate treatment at 28.1 mM/L was found to be the optimal concentration for adventitious root (AR) formation and development in B9 apple rootstocks. This study demonstrated that nitrate promotes AR formation by upregulating genes related to nitrate and auxin biosynthesis, ultimately leading to increased expression of AR development-related genes and cell cycle-related genes. The findings provide insights into the molecular mechanisms underlying nitrate-mediated AR formation in apple rootstocks.