Posttranslational modification and activity regulation of PILS putative auxin carriers

The plant hormone auxin is an essential determinant of the plant architecture. Multiple developmental processes including embryogenesis, organ formation, vascular development and tropisms require defined spatial distribution of auxin, therefore a tight control of auxin homeostasis is necessary for the normal growth and development of plants. The distribution of auxin is regulated in plants both by auxin metabolism (biosynthesis, conjugation and degradation) and cellular auxin transport. Several classes of transporters have been implicated in auxin transport and proteins of the PIN-FORMED (PIN) family are of particular importance as their polar distribution determines the direction of auxin flow. Recently we have identified a new class of putative auxin transporters, PIN-FORMED-LIKES (PILS) on the basis of their similarity in protein topology with PIN proteins. PILS proteins are required for auxin-dependent regulation of plant growth by determining the cellular sensitivity to auxin. PILS are localized to the endoplasmic reticulum (ER) and regulate intracellular auxin accumulation. Therefore, intracellular compartmentalization by PILS seems to be a mean to regulate cellular auxin levels, however the mechanism by which PILS activity is modulated remains elusive. This project aims to increase our understanding of the regulation of PILS putative auxin carriers. We will particularly address the posttranslational control of PILS2 and its cellular function at the ER. Our preliminary data depicted in planta phosphorylation sites in PILS2. We aim to investigate in detail the impact of phospho-mimic or -deficiency at the identified phosphorylation sites in PILS2 on its protein abundance and/or activity as putative auxin transporter in planta. The proposed work will provide the essential foundation for the understanding of the regulation of PILS putative auxin carriers and will increase our knowledge on intracellular auxin transport and metabolism at the ER.