Secretory carrier membrane proteins regulate aquaporin trafficking in Arabidopsis

Aquaporins belonging to the plasma membrane intrinsic protein (PIP) subfamily are channel proteins that control water flow across cells, allowing plants to rapidly adjust hydraulic conductivity and thereby sustain growth, gas exchange, and recovery from drought. We show that secretory carrier membrane proteins (SCAMP) regulate the abundance of the aquaporins and thereby water transport in Arabidopsis root cells. SCAMPs are evolutionarily conserved multi-spanning transmembrane proteins. In animal cells, they function in secretion, endocytosis and autophagy. Knowledge on their role in plants is restricted to localization and trafficking experiments in heterologous systems and few genetic perturbation experiments. Here, we analysed all five members of the Arabidopsis SCAMP family. We identified conserved tyrosine motifs assisting in transport to the plasma membrane and N-terminally located NPF motifs that are required for internalization. SCAMPs dimerize both at the plasma membrane and endosomes, and dimerization is required for their internalization. Functionally, several PIPs were identified as common targets of multiple SCAMP isoforms. Triple and quintuple scamp mutants show mild developmental delay under standard growth conditions, but they are less sensitive to drought-induced leaf wilting. This observation cannot be explained by altered stomatal dynamics or densities, nor by differential soil water content. However, scamp mutant root protoplasts contain less PIPs and swell less under hypotonic conditions compared to wild type, indicating reduced PIP plasma membrane levels. In conclusion, our research identifies the SCAMP membrane trafficking proteins as regulators of PIP abundance at the plasma membrane in root cells. We propose that the reduced PIP levels in the scamp mutants act as a priming mechanism, allowing them to respond better to conditions of reduced water availability.