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SUMMARY Cell differentiation and morphogenesis are crucial for the establishment of diverse cell types and organs in multicellular organisms. Trichome cells offer an excellent paradigm for dissecting the regulatory mechanisms of plant cell differentiation and morphogenesis due to their unique growth characteristics. Here, we report the isolation of an Arabidopsis mutant,aberrantlybranchedtrichome 3–1(abt3‐1), with a reduced trichome branching phenotype. Positional cloning and molecular complementation experiments confirmed thatabt3‐1is a new mutant allele ofAuxin resistant 1(AXR1), which encodes the N‐terminal half of ubiquitin‐activating enzyme E1 and functions in auxin signaling pathway. Meanwhile, we found that transgenic plants expressing constitutively active version ofROP2(CA‐ROP2) caused a reduction of trichome branches, resembling that ofabt3‐1. ROP2 is a member of Rho GTPase of plants (ROP) family, serving as versatile signaling switches involved in a range of cellular and developmental processes. Our genetic and biochemical analyses showedAXR1genetically interacted withROP2and mediated ROP2 protein stability. The loss ofAXR1aggravated the trichome defects ofCA‐ROP2and induced the accumulation of steady‐state ROP2. Consistently, elevatedAXR1expression levels suppressedROP2expression and partially rescued trichome branching defects inCA‐ROP2plants. Together, our results presented a new mutant allele ofAXR1, uncovered the effects ofAXR1andROP2during trichome development, and revealed a pathway ofROP2‐mediated regulation of plant cell morphogenesis in Arabidopsis. 

Abstract The duality of nitrate is nowhere better exemplified than in human physiology—a detrimental pollutant but also a protective nutrient—particularly as connected to nitric oxide. Aside from limited insights into nitrate uptake and storage, foundational nitrate biology has lagged. Genetically encoded fluorescent biosensors can address this gap with real‐time imaging, but such technologies for mammalian cell applications remain rare. Here, we designed and engineered a biosensor fusing the green fluorescent protein EGFP and the nitrate recognition domain NreA fromStaphylococcus carnosus. Seven rounds of directed evolution and 15 mutations resulted in NitrOFF. NitrOFF has a high degree of allosteric communication between the domains reflected in a turn‐off intensiometric response (K_d≈ 9 µM). This was further reinforced by X‐ray crystal structures of apo and nitrate‐bound NitrOFF, which revealed a large‐scale conformational rearrangement changing the relative positioning of the domains by 68.4°. This dramatic difference was triggered by the formation of a long helix at the engineered linker connecting the two domains, peeling the β7 strand off the EGFP and thus extinguishing the fluorescence upon nitrate binding. Finally, we highlighted the utility of NitrOFF to monitor exogenous nitrate uptake and modulation in the human embryonic kidney (HEK) 293 cell line.