May be involved in an oxidative stress-mediated signaling cascade that phosphorylates downstream MAP kinases MPK3 and MPK6. May suppress auxin signaling that promotes cell cycle. Functionally redundant to ANP2 and ANP3 in the positive regulation of cytokinesis. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Expressed in roots, inflorescence stems, flower buds and flowers. Low amount in rosette and cauline leaves. The protein kinase activity of isoform 1S is higher than that of isoform 1L. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.