Serine protease that degrades the two-component response regulator ARR4 (PubMed:27274065, PubMed:29180814). Regulates ARR4 stability by targeting ARR4 in the nucleus for degradation. Acts upstream of ARR4 and regulates the activity of ARR4 in cytokinin and light-signaling pathways. ARR4 mediates the cross-talk between light and cytokinin signaling through modulation of the activity of phytochrome B (PubMed:27274065). Homooctamer formed by 4 homodimers (PubMed:29180814). Interacts with ARR4 (PubMed:27274065). Expressed in roots, cotyledons, rosette leaves, cauline leaves, stems, flowers and siliques. Induced by trans-zeatin (at protein level). Belongs to the peptidase S1C family.