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The phytohormone ethylene has numerous effects on plant growth and development. Its immediate precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), is a non-proteinogenic amino acid produced by ACC SYNTHASE (ACS). ACC is often used to induce ethylene responses. Here, we demonstrate that ACC exhibits ethylene-independent signaling inArabidopsis thalianareproduction. By analyzing anacsoctuple mutant with reduced seed set, we find that ACC signaling in ovular sporophytic tissue is involved in pollen tube attraction, and promotes secretion of the pollen tube chemoattractant LURE1.2. ACC activates Ca²⁺-containing ion currents via GLUTAMATE RECEPTOR-LIKE (GLR) channels in root protoplasts. In COS-7 cells expressing mossPpGLR1, ACC induces the highest cytosolic Ca²⁺elevation compared to all twenty proteinogenic amino acids. In ovules, ACC stimulates transient Ca²⁺elevation, and Ca²⁺influx in octuple mutant ovules rescues LURE1.2 secretion. These findings uncover a novel ACC function and provide insights for unraveling new physiological implications of ACC in plants.

The sorting of eukaryotic proteins to various organellar destinations requires receptors that recognize cargo protein targeting signals and facilitate transport into the organelle. One such receptor is the peroxinPEX5, which recruits cytosolic cargo carrying a peroxisome‐targeting signal (PTS) type 1 (PTS1) for delivery into the peroxisomal lumen (matrix). In plants and mammals,PEX5 is also indirectly required for peroxisomal import of proteins carrying aPTS2 signal becausePEX5 binds thePTS2 receptor, bringing the associatedPTS2 cargo to the peroxisome along withPTS1 cargo. DespitePEX5 being thePTS1 cargo receptor, previously identified Arabidopsispex5mutants display either impairment of bothPTS1 andPTS2 import or defects only inPTS2 import. Here, we report the first Arabidopsispex5mutant with an exclusivePTS1 import defect. In addition to markedly diminishedGFP‐PTS1 import and decreased pex5‐2 protein accumulation, thispex5‐2mutant shows typical peroxisome‐related defects, including inefficient β‐oxidation and reduced growth. Growth at reduced or elevated temperatures ameliorated or exacerbatedpex5‐2peroxisome‐related defects, respectively, without markedly changing pex5‐2 protein levels. In contrast to the diminishedPTS1 import,PTS2 processing was only slightly impaired andPTS2‐GFPimport appeared normal inpex5‐2. This finding suggests that even minor peroxisomal localization of thePTS1 proteinDEG15, thePTS2‐processing protease, is sufficient to maintain robustPTS2 processing.

Catabolism of fatty acids stored in oil bodies is essential for seed germination and seedling development in Arabidopsis. This fatty acid breakdown occurs in peroxisomes, organelles that sequester oxidative reactions. Import of peroxisomal enzymes is facilitated by peroxins includingPEX5, a receptor that delivers cargo proteins from the cytosol to the peroxisomal matrix. After cargo delivery, a complex of thePEX1 andPEX6ATPases and thePEX26 tail‐anchored membrane protein removes ubiquitinatedPEX5 from the peroxisomal membrane. We identified Arabidopsispex6andpex26mutants by screening for inefficient seedling β‐oxidation phenotypes. The mutants displayed distinct defects in growth, response to a peroxisomally metabolized auxin precursor, and peroxisomal protein import. The lowPEX5 levels in these mutants were increased by treatment with a proteasome inhibitor or by combiningpex26with peroxisome‐associated ubiquitination machinery mutants, suggesting that ubiquitinatedPEX5 is degraded by the proteasome when the function ofPEX6 orPEX26 is reduced. Combiningpex26with mutations that increasePEX5 levels either worsened or improvedpex26physiological and molecular defects, depending on the introduced lesion. Moreover, elevatingPEX5 levels via a35S:PEX5transgene exacerbatedpex26defects and ameliorated the defects of only a subset ofpex6alleles, implying that decreasedPEX5 is not the sole molecular deficiency in these mutants. We found peroxisomes clustered around persisting oil bodies inpex6andpex26seedlings, suggesting a role for peroxisomal retrotranslocation machinery in oil body utilization. The disparate phenotypes of thesepexalleles may reflect unanticipated functions of the peroxisomalATPase complex.