High nitrate inhibited adventitious roots formation in apple rootstock by altering hormonal contents and miRNAs expression profiles

The adventitious roots (ARs) formation is a major setback for the mass proliferation of apple rootstocks. Nitrate is a vital signaling molecule for the regulation of root architecture. However, the inhibitory effect of nitrate at a high dose is missing. In this study, the physiological and molecular mechanism underlying ARs inhibition was tested. The B9 stem cuttings were grown on low and high nitrate concentrations, namely T1 and T2, respectively, and the T2 appeared as ARs inhibiting dose, where the rooting percentage was only 28% that was 130% less than T1 cuttings. Moreover, the average root numbers of T2 were only 31, which was 78% lower than T1 cuttings. At most time points, the endogenous hormones: JA, BR, ABA, CTK, GA, and AUX were exhibited at higher levels in response to T2, leading to hormonal imbalance in the plant and led to ARs inhibition. In addition, 205 known miRNAs belonging to 42 families and 438 novel miRNA were observed differentially expressed by the nitrate treatments. Where miR160a, miR390a, miR167, miR394, and miR169a were found to be related to auxin and ABA signaling, and miR171, miR166, miR156, miR319, and miR396 were related with cell fate transformation, proliferation, and enlargement. Our results propose that ARs formation is a complicated biological process, which was influenced by multiple hormone signaling pathways. This work is a pioneer giving knowledge on the inhibitory effects of high nitrate concentration on AR formation for future studies in woody plants.

Automated summary

This study revealed that high nitrate concentration inhibits the formation of adventitious roots in apple rootstocks by affecting multiple hormone signaling pathways, leading to hormonal imbalance and growth inhibition of ARs. Furthermore, differential expression of known and novel miRNAs was observed, suggesting their involvement in auxin and ABA signaling as well as cell fate transformation, proliferation, and enlargement.