Plant and animal cells use receptor kinases that span the plasma membrane and function to transfer extracellular signals to the interior of the cell.In plants,binding of the ligand to the receptors extracellular domains thought to induce interaction with a co-receptor kinase.Juxtaposition of the two cytoplasmic protein kinase domains then results in their activation via auto-and/or trans-phosphorylation,and a signal transduction cascade that alters gene expression in the nucleus.