Nitrate application affects root morphology by altering hormonal status and gene expression patterns in B9 apple rootstock nursery plants

Abstract

Lateral roots (LRs) are critical for absorbing water, minerals, and nutrients. Nitrate is a vital signaling molecule for regulating LRs growth. However, understanding how nitrate interacts with endogenous hormones and genes to coordinate LR development is a bottleneck. In this study, B9 apple rootstock nursery plants were cultured hydroponically and treated with different nitrate treatments (Control; T1, 2.25 mM L⁻¹; T2, 9 mM L⁻¹; T3, 18 mM L⁻¹; T4, 36 mM L⁻¹; and T5, 72 mM L⁻¹), where T5 outperformed with the other treatments. Thereupon, nursery plants were treated with control and T5 to explore the underlying physiological and molecular mechanisms by which nitrate promoted LR growth. Plant height, stem diameter, root morphological parameters, endogenous hormones, and gene expression levels were measured. Results showed that T5 promoted LR growth by increasing the endogenous contents of IAA and GA, and by decreasing the contents of ABA, JA, and ZR at most time points. Moreover, the relative expression levels of nitrate transporter genes were upregulated in the control group, and nitrate assimilation genes were upregulated in T5 nursery plants at various time points. In addition, the transcript abundances of auxin-related genes were higher in T5 nursery plants, which increased auxin contents. Resultantly, <i>MdSHR</i>, <i>MdGATA1</i>, and <i>MdSCR1</i> expressions were induced, hence increasing LRs growth. In contrast, higher auxin contents raised <i>MdWOX11</i> expression, which enhanced the <i>MdLBD16</i> and <i>MdLBD29</i> expression levels, thus prompting the transcripts of cell cycle-related genes, <i>MdCYCD1;1</i> and <i>MdCYCP4;1</i>. Overall, the changes in hormonal contents and gene expression levels followed superior LR growth by T5.

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