Chrysanthemum MADS-box transcription factor CmANR1 modulates lateral root development via homo-/heterodimerization to influence auxin accumulation in Arabidopsis

Root system architecture is an important agronomic trait by which plants both acquire water and nutrients from the soil and adapt to survive in a complex environment. The adaptation of plant root systems to environmental constraints largely depends on the growth and development of lateral roots (LRs). MADS-box transcription factors (TFs) are important known regulators of plant growth, development, and response to environmental stimuli. However, the potential mechanisms by which they regulate LRs development remain poorly understood. Here, we identified a MADS-box chrysanthemum gene CmANR1, homologous to the Arabidopsis gene AtANR1, which plays a key role in the regulation of LR development. qRT-PCR assays indicated that CmANR1 was primarily expressed in chrysanthemum roots and was rapidly induced by exposure to high nitrate concentrations. Ectopic expression of CmANR1 in Arabidopsis significantly increased the number and length of emerged LRs compared to the wild-type (col) control, but had no obvious affect on primary root (PR) development. We also found that CmANR1 positively influenced auxin accumulation in LRs at least partly by improving auxin biosynthesis and transport, thereby promoting LR development. Furthermore, we found that ANR1 formed homo- and heterodimers through interactions with itself and AGL21 at its C-terminal domain. Overall, our findings provide considerable new information about the mechanisms by which the chrysanthemum MADS-box TF CmANR1 mediates LR development by directly altering auxin accumulation.