AtbHLH112Is Involved In Regulation Of Lateral Root Development And Salt Stress Response In Arabidopsis

The root systems provide the sites of water and nutrient uptake from the soil by plants, and anchor plants in the ground. Modulating lateral root (LR) development through changing LR number, spacing, initiation and outgrowth is a very important survival strategy for plants to reach mineral nutrients and water, and to avoid damage in unfavorable conditions. To figure out how plant shapes the root system architecture, is of widespread interest, and of great significance to the utilization of land resources and crop improvement. In compared with our knowledge on lateral root initiation, the mechanisms of regulation of lateral root emergence are largely unknown, except the importance of auxin signaling in this process.In our studies, we characterized a novel bHLH (basic helix-loop-helix) transcription factor bHLH112in Arabidopsis that may respond to stresses and function in regulating LR development, and detailed results are briefly present below:1. The expression patterns of bHLH112were analyzed. Our results indicated that while bHLH112expressed in all tested organs, highly abundant transcripts of this gene were detected in rosette leaves, stems and cauline leaves. And pbHLH112::GUS expressed mainly in the mature vascular tissues and trichomes. Quantitative real-time PCR (qRT-PCR) showed that the expression of bHLH112responds to salicylic acid, abscisic acid and high salt, indicating that bHLH112may be involved the processes triggered by these factors. The bHLH112-GFP protein localized to the nuclei.2. The bHLH112-overexpressing lines showed reduced visible LRs. The detailed examinations under microscope revealed that abnormal LRP with flat head and more than four cell layers in endodermal layer were accounted as high as over45%of total number of LRP and LRs in bHLH112-overexpressing lines, suggesting that their emergences were prevented by cortical layer in the transgenic lines.3. The relationship between the LR development deficiency in bHLHll2-overexpressing lines and auxin was evidenced. DR5::GUS staining showed that DR5activity decreased fast during the LR emergence, and disappeared from the cortical and epidermal cells surrounding LRP in bHLH112-overexpressing plants. Exogenous application of indole-3-acetic acid (IAA) can rescue the decreased LR densities and DR5activity of bHLH112-overexpressing lines compared with wild-type plants. Furthermore, expressions of genes encoding the cell wall remodeling enzymes that function to facilitate cell separation for LRP emergence were down-regulated bHLH112-overexpressing lines.4. The bHLHl12-overexpressing lines exhibited increased sensitivity to low concentration of NaCl in comparison with wild-type plants, including inhibition of cotyledon greening and reduction of LR number. And under NaCl treatment, expressions of the salt response genes was induced to higher levels in bHLH172-overexpressing lines than in wild-type plants, demonstrating that bHLH112positively regulates their expressions.Our results evidenced that overexpression of bHLH112suppressed lateral root emergence possibly by decreasing localized auxin activity, resulting in reduced expressions of cell wall remodeling enzyme genes and abnormal cortical cell separation. And bHLH112positively regulates the responses of Arabidopsis to salt stress.Our studies first described the functions of member of bHLH transcription factor family subgroup X in Arabidopsis, and demonstrated the patterns of regulation of LR emergence by auxin. Furthermore, results present here would help to reveal how plant shapes the root system architecture in unfavorable conditions.