Coordinated cell polarization in developing tissues is a recurrent theme in multicellular organisms. In plants, a directional distribution of the plant hormone auxin is at the core of many developmental programs. A feedback regulation of auxin on the polarized localization of PIN auxin transporters in individual cells has been proposed as a self-organizing mechanism for coordinated tissue polarization, but the molecular mechanisms linking auxin signalling to PIN-dependent auxin transport remain unknown. We used a microarray-based approach to find regulators of the auxin-induced PIN relocation in Arabidopsis thaliana root, and identified a subset of a family of phosphatidylinositol transfer proteins (PITPs), the PATELLINs (PATLs). Here, we show that PATLs are expressed in partially overlapping cell types in different tissues going through mitosis or initiating differentiation programs. PATLs are plasma membrane-associated proteins accumulated in Arabidopsis embryos, primary roots, lateral root primordia and developing stomata. Higher order patl mutants display reduced PIN1 repolarization in response to auxin, shorter root apical meristem, and drastic defects in embryo and seedling development. This suggests that PATLs play a redundant and crucial role in polarity and patterning in Arabidopsis.

In plants, vesicular trafficking is crucial for the response and survival to environmental challenges. The active trafficking of vesicles is essential to maintain cell homeostasis during salt stress. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are regulatory proteins of vesicular trafficking. They mediate membrane fusion and guarantee cargo delivery to the correct cellular compartments. SNAREs from the Qbc subfamily are the best-characterized plasma membrane SNAREs, where they control exocytosis during cell division and defense response. The Solanum lycopersicum gene SlSNAP33.2 encodes a Qbc-SNARE protein and is induced under salt stress conditions. SlSNAP33.2 localizes on the plasma membrane of root cells of Arabidopsis thaliana. In order to study its role in endocytosis and salt stress response, we overexpressed the SlSNAP33.2 cDNA in a tomato cultivar. Constitutive overexpression promoted endocytosis along with the accumulation of sodium (Na⁺) in the vacuoles. It also protected the plant from cell damage by decreasing the accumulation of hydrogen peroxide (H₂O₂) in the cytoplasm of stressed root cells. Subsequently, the higher level of SlSNAP33.2 conferred tolerance to salt stress in tomato plants. The analysis of physiological and biochemical parameters such as relative water content, the efficiency of the photosystem II, performance index, chlorophyll, and MDA contents showed that tomato plants overexpressing SlSNAP33.2 displayed a better performance under salt stress than wild type plants. These results reveal a role for SlSNAP33.2 in the endocytosis pathway involved in plant response to salt stress. This research shows that SlSNAP33.2 can be an effective tool for the genetic improvement of crop plants.

Lateral roots (LRs) increase the contact area of the root with the rhizosphere and thereby improve water and nutrient uptake from the soil. LRs are generated either via a developmentally controlled mechanism or through induction by external stimuli, such as water and nutrient availability. Auxin regulates LR organogenesis via transcriptional activation by an auxin complex receptor. Endocytic trafficking to the vacuole positively regulates LR organogenesis independently of the auxin complex receptor in Arabidopsis (Arabidopsis thaliana). Here, we demonstrate that phosphatidylinositol 4-phosphate (PI4P) biosynthesis regulated by the phosphatidylinositol 4-kinases PI4KIIIβ1 and PI4KIIIβ2 is essential for the LR organogenesis driven by endocytic trafficking to the vacuole. Stimulation with Sortin2, a biomodulator that promotes protein targeting to the vacuole, altered PI4P abundance at both the plasma membrane and endosomal compartments, a process dependent on PI4K activity. These findings suggest that endocytic trafficking to the vacuole regulated by the enzymatic activities of PI4KIIIβ1 and PI4KIIIβ2 participates in a mechanism independent of the auxin complex receptor that regulates LR organogenesis in Arabidopsis. Surprisingly, loss-of-function of PI4KIIIβ1 and PI4KIIIβ2 induced both LR primordium formation and endocytic trafficking toward the vacuole. This LR primordium induction was alleviated by exogenous PI4P, suggesting that PI4KIIIβ1 and PI4KIIIβ2 activity constitutively negatively regulates LR primordium formation. Overall, this research demonstrates a dual role of PI4KIIIβ1 and PI4KIIIβ2 in LR primordium formation in Arabidopsis.

The plant endomembrane system is an extensively connected functional unit for exchanging material between compartments. Secretory and endocytic pathways allow dynamic trafficking of proteins, lipids, and other molecules, regulating a myriad of biological processes. Chemical genetics—the use of compounds to perturb biological processes in a fast, tunable, and transient manner—provides elegant tools for investigating this system. Here, we review how chemical genetics has helped to elucidate different aspects of membrane trafficking. We discuss different strategies for uncovering the modes of action of such compounds and their use in unraveling membrane trafficking regulators. We also discuss how the bioactive chemicals that are currently used as probes to interrogate endomembrane trafficking were discovered and analyze the results regarding membrane trafficking and pathway crosstalk. The integration of different expertises and the rational implementation of chemical genetic strategies will improve the identification of molecular mechanisms that drive intracellular trafficking and our understanding of how trafficking interfaces with plant physiology and development.