| Plant roots are composed of primary root,lateral roots,and adventitious roots.The density,length and spatial distribution of lateral roots determine the size of the surrounding soil environment that plants can use,which is essential for the root system to anchor plants and absorb water and nutrients.Previous studies have shown that auxin is critical to lateral root growth.Classic TIR1/AFB-Aux/IAA-ARF auxin signal modules can regulate lateral root priming,initiation,primordium formation,and emergence.MPK cascades are highly conserved signalling modules in eukaryotes,which are critical to plant growth and stress responses.Extracellular signals can activate the substrate protein through the phosphorylation of MPK cascades and lead to intracellular responses.The MKK4/MKK5-MPK3/MPK6 module can regulate the lateral root primordia formation and lateral root emergence through the non-classical auxin signaling pathway and IDA polypeptide signaling pathway,respectively.However,how the downstream of MPK3/6 regulate lateral root development remains to be further elucidated.The results of yeast two-hybrid assays,BiFC assays and Co-IP assays indicated that MPK3/6 could interact with TIR1,IAA8 and IAA16 in canonical auxin signaling.Then,the in vitro phosphorylation assays showed that MPK3/6 can phosphorylate IAA8/16 but not TIR1 after being activated by MKK5DD.Therefore,IAA8/16 may be the substrate proteins that are directly phosphorylated by MPK3/6,and the significance of the interactions between TIR1 and MPK3/6 remains to be further studied.We found that the phosphorylation of IAA8 could not affect the intracellular localization and its interaction with ARFs,but promoted the degradation of IAA8 protein.The iaa8 and iaa16 mutants showed increased LR density in comparison to wild type.The lateral root phenotype assays of MPK3SR/iaa8/iaa16 is ongoing currently.In summary,we found that IAA8 and IAA16 could interact with MPK3/6 and be phosphorylated by MPK3/6.Phosphorylation may change the protein stability of IAA8 and IAA16 and thus regulate the development of lateral roots.This study has enriched our understanding of the regulation of lateral root development and provided new evidence on the lateral root development by the coregulation of auxin signaling pathway and MPK cascades pathway. |
