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Abstract The chloroplast chaperone CLPC1 aids to select, unfold, and deliver hundreds of proteins to the CLP protease for degradation. Through in vivo CLPC1, trapping we previously identified dozens of proteins that are (potential) substrate adaptors or substrates for the CLP chaperone–protease system. In this study, we show that two of these highly trapped proteins, DUF760-1 and DUF760-2, are substrates for the CLP protease in Arabidopsis (Arabidopsis thaliana). Loss-of-function mutants and transgenic plants were created for phenotyping, protein expression, and localization using immunoblotting and confocal microscopy. In planta BiFC, cycloheximide chase assays, and yeast 2-hybrid analyses were conducted to determine protein interactions and protein half-life. Both DUF760 proteins directly interacted with the N-domain of CLPC1 and both were highly enriched in clpc1-1 and clpr2-1 mutants. Accordingly, in vivo cycloheximide chase assays demonstrated that both DUF760 proteins are degraded by the CLP protease. The half-life of DUF760-1 was 4 to 6 h, whereas DUF760-2 was highly unstable and difficult to detect unless CLP proteolysis was inhibited. Null mutants for DUF760-1 and DUF760-2 showed weak but differential pigment phenotypes and differential sensitivity to protein translation inhibitors. This study demonstrates that DUF760-1 and DUF760-2 are substrates of the CLP chaperone–protease system and excellent candidates for the determination of CLP substrate degrons. 

Summary Vascular bundles transport water and photosynthate to all organs, and increased bundle number contributes to crop lodging resistance. However, the regulation of vascular bundle formation is poorly understood in the Arabidopsis stem.We report a novel semi‐dominant mutant with high vascular activity,hva‐d, showing increased vascular bundle number and enhanced cambium proliferation in the stem. The activation of a C2H2 zinc finger transcription factor,AT5G27880/HVA, is responsible for thehva‐dphenotype. Genetic, biochemical, and fluorescent microscopic analyses were used to dissect the functions of HVA.HVA functions as a repressor and interacts with TOPLESS via the conserved Ethylene‐responsive element binding factor‐associated Amphiphilic Repression motif. In contrast to the HVA activation line, knockout ofHVAfunction with a CRISPR‐Cas9 approach or expression of HVA fused with an activation domain VP16 (HVA‐VP16) resulted in fewer vascular bundles. Further, HVA directly regulates the expression of the auxin transport efflux facilitatorPIN1, as a result affecting auxin accumulation. Genetics analysis demonstrated that PIN1 is epistatic to HVA in controlling bundle number.This research identifies HVA as a positive regulator of vascular initiation through negatively modulating auxin transport and sheds new light on the mechanism of bundle formation in the stem.