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Abstract Phytohormone levels are regulated through specialized enzymes, participating not only in their biosynthesis but also in post-signaling processes for signal inactivation and cue depletion.Arabidopsis thaliana(At) carboxylesterase 15 (CXE15) and carboxylesterase 20 (CXE20) have been shown to deplete strigolactones (SLs) that coordinate various growth and developmental processes and function as signaling molecules in the rhizosphere. Here, we elucidate the X-ray crystal structures of AtCXE15 (both apo and SL intermediate bound) and AtCXE20, revealing insights into the mechanisms of SL binding and catabolism. The N-terminal regions of CXE15 and CXE20 exhibit distinct secondary structures, with CXE15 characterized by an alpha helix and CXE20 by an alpha/beta fold. These structural differences play pivotal roles in regulating variable SL hydrolysis rates. Our findings, both in vitro and in planta, indicate that a transition of the N-terminal helix domain of CXE15 between open and closed forms facilitates robust SL hydrolysis. The results not only illuminate the distinctive process of phytohormone breakdown but also uncover a molecular architecture and mode of plasticity within a specific class of carboxylesterases. 

Summary Chloroplast Unusual Positioning 1 (CHUP1) plays an important role in the chloroplast avoidance and accumulation responses in mesophyll cells. In epidermal cells, prior research showed silencingCHUP1‐induced chloroplast stromules and amplified effector‐triggered immunity (ETI); however, the underlying mechanisms remain largely unknown.CHUP1 has a dual function in anchoring chloroplasts and recruiting chloroplast‐associated actin (cp‐actin) filaments for blue light‐induced movement. To determine which function is critical for ETI, we developed an approach to quantify chloroplast anchoring and movement in epidermal cells. Our data show that silencingNbCHUP1inNicotiana benthamianaplants increased epidermal chloroplast de‐anchoring and basal movement but did not fully disrupt blue light‐induced chloroplast movement.SilencingNbCHUP1auto‐activated epidermal chloroplast defense (ECD) responses including stromule formation, perinuclear chloroplast clustering, the epidermal chloroplast response (ECR), and the chloroplast reactive oxygen species (ROS), hydrogen peroxide (H₂O₂). These findings show chloroplast anchoring restricts a multifaceted ECD response.Our results also show that the accumulated chloroplastic H₂O₂inNbCHUP1‐silenced plants was not required for the increased basal epidermal chloroplast movement but was essential for increased stromules and enhanced ETI. This finding indicates that chloroplast de‐anchoring and H₂O₂play separate but essential roles during ETI.