Serine/threonine-protein kinase (PubMed:30212650). Regulates organ size in coordination with MOB1A by modulating cell proliferation and cell expansion, possibly by facilitating cell cycle exit (PubMed:26685188). Positive regulator of the pathogen-associated molecular pattern (PAMP, e.g. flg22)-triggered immunity (PTI) signaling by stabilizing BIK1 and activating RBOHD by phosphorylation to promote the extracellular reactive oxygen species (ROS) burst involved in defense responses to bacterial infection (PubMed:30212650). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with MOB1A and MOB1B via its N-terminal region at the plasma membrane and in the nucleus (PubMed:26685188). Binds to BIK1 to phosphorylate and stabilize it (PubMed:30212650). Interacts with and phosphorylates RBOHD upon flagellin perception to activate it (PubMed:30212650). Observed at the plasma membrane and in the nucleus when associated with MOB1A. Mostly expressed in mature tissues of roots, shoots, hypocotyls, cotyledons, stems, leaves and flowers, as well as in the shoot apical meristem (SAM). In seedlings, observed in the hypocotyls of both etiolated and light-grown plants (PubMed:26685188). Also expressed in the quiescent center and the maturation zone of primary roots (PubMed:26685188). Later present in cotyledons and the first pair of true leaves (PubMed:26685188). Accumulates strongly in mature rosette leaves (PubMed:26685188). Detected in the pollen of opened flowers (PubMed:26685188). Lower levels in dividing tissues (PubMed:26685188). Present in developed vascular tissues, stipules of true leaves, mature trichomes and guard cells (PubMed:26685188). Autophosphorylates. Dwarf plants with reduced cell numbers, endoreduplication and cell expansion leading to a slow growth (PubMed:26685188, PubMed:30212650). Altered pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) signaling characterized by a compromised ability to elicit a reactive oxygen species (ROS) burst in response to microbial features (e.g. flg22) (PubMed:30212650). Enhanced resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 (Pst) due to high levels of salicylic acid (SA) and subsequent constitutive expression of pathogenesis-related genes (e.g. PR1) and associated with reduced jasmonic acid (JA) levels (PubMed:30212650). The double mutant sik1 mob1a is arrested at the seedling stage (PubMed:26685188). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily. Truncated N-terminus.