Phosphorylation Status of B Beta Subunit Controls PP2A activity in Ethylene-mediated Root Growth Inhibition

The various combinations and regulations of different subunits of phosphatase PP2A holoenzymes underlie their functional complexity and importance. We found that phosphorylation status of Bβ of PP2A acts as a switch to regulate the activity of PP2A. In the absence of ethylene, phosphorylated Bβ leads to an inactivation of PP2A; the substrate EIR1 remains to be phosphorylated, preventing the EIR1 mediated auxin transport in epidermis, leading to normal root growth. Upon the ethylene treatment, the dephosphorylated Bβ mediates the formation of A2-C4-Bβ protein complex to activate PP2A, resulting in the dephosphorylation of EIR1 to promote auxin transport in epidermis of elongation zone, leading to root growth inhibition. Altogether, our research revealed a novel molecular mechanism by which the dephosphorylation of Bβ subunit switches on the PP2A activity to dephosphorylate EIR1 to establish EIR1 mediated auxin transport in epidermis in elongation zone for root growth inhibition in response to ethylene.Significance StatementRoot growth is critical to the establishment of planted seedlings. Ethylene plays an important role in the root growth. Yet, the molecular mechanisms that how ethylene regulates the root growth are largely unexplored. This research sheds light on the molecular mechanisms of the determination of the functional specificity and activity of PP2A in response to stimulus in cell type specific manner and reveals the novel molecular mechanism through which ethylene signaling regulates EIR1 dephosphorylation through Bβ dephosphorylation to active EIR1 mediated auxin transport in epidermis in elongation zone, leading to root growth inhibition.