1. Polar transport of auxins: intracellular trafficking of the auxin transporter PIN1

Plants, like other living organisms, need hormones to achieve harmonious growth. Among the main plant hormones are auxins, which are involved in regulating many physiological processes: they promote cell growth and differentiation (and thus longitudinal growth), stimulate fruit development and ripening, flowering, fruit abscission processes, and tropisms (plant responses to light and gravity). Furthermore, auxins allow plants to adapt their growth to continuous environmental changes thanks to a regulated differential distribution—an essential feature of many adaptive processes.

A striking characteristic of auxins is their strong polarity in transport across the plant. This transport is basipetal in stems (from the apex downward) and acropetal in roots (toward the root tip). This polar transport is a unique, specific process of plants and the auxin phytohormone. Directional auxin movement plays a crucial role in growth and development regulation in plants and in maintaining global polarity.

Polar auxin transport occurs thanks to the asymmetric (polar) distribution of PIN family auxin transporters. In particular, PIN1 is exclusively localized in the basal domain of the plasma membrane, allowing auxin to exit only from one end of the cell. A key question is how polarity is established in PIN1’s subcellular localization. Its localization depends on vesicular trafficking processes, likely mediated by sorting or targeting signals in its cytosolic domain. These signals may interact with the medium (mu) subunit of adaptor protein (AP) complexes for inclusion in clathrin-coated vesicles involved in intracellular trafficking. In fact, PIN1 endocytosis has been shown to be clathrin-dependent. Auxin inhibits clathrin-mediated endocytosis in plant cells, increasing transporter concentration in the plasma membrane when excess auxin needs to be transported. This effect is mediated by the auxin-binding protein ABP1, which facilitates clathrin recruitment to the plasma membrane and promotes endocytosis.

One of the group’s objectives is to investigate the mechanism responsible for ABP1-dependent stimulation of clathrin-mediated endocytosis. Another key objective is to elucidate the signals in the cytosolic domain of PIN1 involved in its intracellular trafficking, distinguishing between those that mediate transport from the ER to the Golgi, from the trans-Golgi network (TGN) to the plasma membrane, its endocytosis, or recycling from endosomes back to the basal plasma membrane domain.
 

2. The secretory pathway in plant cells: structural and functional peculiarities of the p24 protein family

It has been estimated that around 18% of the plant genome is dedicated to maintaining the endomembrane compartments involved in protein trafficking (including the secretory pathway), the highest proportion found among eukaryotes. Besides their role in cellular maintenance, the biosynthetic/secretory pathway is involved in many plant-specific processes such as hormonal signaling, development, tropisms, defense against pathogens, or response to abiotic stress.

All eukaryotes have a secretory pathway, and most of the molecules involved in the secretion process are fairly conserved. However, the organization of the secretory pathway differs significantly in plants compared to animals and yeasts. In particular, the most characteristic morphological trait is the absence of an intermediate compartment between the endoplasmic reticulum (ER) and the Golgi apparatus.

The p24 protein family comprises putative receptors for soluble cargo in the secretory pathway, possibly involved in quality control during protein trafficking, the organization of ER exit sites, or the biogenesis and maintenance of Golgi apparatus structure. However, their functions in plants are essentially unknown.

The peculiarities of the early secretory pathway (ER-Golgi) in plant cells suggest the involvement of plant-specific molecules in the regulation of protein trafficking and compartment organization. In this regard, p24 proteins may have undergone evolutionary adaptations, potentially related to functional adaptations. They could transport different types of cargo in the secretory pathway, including GPI-anchored proteins, contributing to the maintenance and organization of the different compartments. Additionally, some of them may have specific functions in different tissues.

Our current project proposes a functional analysis of p24 proteins in Arabidopsis to elucidate the stages of the biosynthetic/secretory pathway that require p24 protein function and/or their potential role in the organization/morphology of secretory compartments. Moreover, identifying cargo proteins recognized by p24s in plants should provide valuable insight into their specific functions in plant cells.