Phosphate starvation-induced CORNICHON HOMOLOG 5 as endoplasmic reticulum cargo receptor for PHT1 transporters in Arabidopsis

Phosphate (Pi) is essential for plant growth and is acquired by the plasma membrane (PM) PHOSPHATE TRANSPORTER 1 proteins (PHT1s). Enhancing the abundance of PHT1s at the cell surface under Pi limitation thus ensures plant productivity and sustainable agriculture. CORNICHON HOMOLOG proteins (CNIHs) are conserved eukaryotic cargo receptors that mediate the selective endoplasmic reticulum (ER) export of membrane proteins. We identified the Arabidopsis thaliana CNIH5 ( AtCNIH5 ) as a Pi starvation-inducible gene preferentially expressed in the outer root cell layers above the meristem. At CNIH5 co-localizes to the At SAR1A/ At SEC16A/ At SEC24A-labeled ER exit sites and interacts with At PHT1;1 and PHOSPHATE TRANSPORTER TRAFFIC FACILITATOR1 ( At PHF1). Loss of AtCNIH5 confers reduced shoot Pi levels and decreased At PHT1s but increased At PHF1. The root hair and the epidermis within the transition/elongation zone of the cnih5 mutant display a lower PM targeting efficiency of At PHT1;1. Moreover, dysfunctional At CNIH5 exerts a suppressive effect on the growth of phf1 and alleviates Pi toxicity in the Pi overaccumulator pho2 . However, the in vivo protein–protein interaction and degradation assays indicated that At CNIH5 is not a direct target of At PHO2. Our findings unveil that low Pi-responsive At CNIH5 interplays with At PHF1 to promote the PM targeting of At PHT1s in a cell type-dependent manner.