From signaling to catabolism: terminal tails in plant hormone regulation

Plant hormone signaling is shaped not only by biosynthesis and perception but also by the controlled removal of bioactive molecules. Although catabolic pathways were long viewed as passive sinks, emerging evidence indicates that hormone inactivation can determine the amplitude and duration of signaling outputs. This review examines recent advances in phytohormone homeostasis, with emphasis on strigolactone (SL) and salicylic acid (SA), two systems in which the enzymatic basis of hormone deactivation has been resolved at mechanistic and structural levels. In SL biology, the receptor DWARF14 couples perception to hydrolysis but is not optimized for bulk hormone clearance. The identification of carboxylesterases such as CXE15 establishes a dedicated catabolic route in which activity is governed by conformational gating mediated by an N-terminal helix. In parallel, SA catabolism is mediated by 2-oxoglutarate-dependent oxygenases, including DOWNY MILDEW RESISTANT 6 and DMR6-LIKE OXYGENASE 1, which act in feedback control of immunity and are further regulated through ubiquitin-mediated degradation. Together, these examples show that hormone catabolism can operate as an active and tunable layer of plant signaling networks, with terminal protein regions contributing to the control of enzyme activity, stability, and signal duration.