Vascular System Formation: an Auxin-Dependent Physiological Process Involved in Plant Organogenesis

The vascular system is fundamental for land plants, ensuring the long-distance transport of water, nutrients, and signaling molecules. In Arabidopsis thaliana, vascular patterning is tightly regulated by the hormone auxin, which, according to the canalization hypothesis, promotes the formation of self-organizing transport channels via feedback regulation of its own polar transport. Vascular differentiation typically occurs where elevated auxin levels intersect with responsive cell populations. It is thought that the small yet functionally active apoplastic fraction of Auxin Binding Protein 1 (ABP1) act in this extracellular acidic environment as a receptor mediating some auxin-driven processes such as canalization. Here, we present preliminary data on the effects of two synthetic urea derivatives: N, N’-bis-(2,3-methylenedioxyphenyl) urea (2,3-MDPU) and 1,3-di(benzo[d]oxazol-5-yl) urea (5-BDPU) on vascular development in cotyledons of Arabidopsis thaliana. Seedlings were grown in vitro under various concentrations of the compounds alone, in mixture with auxin or in single sequential culture of both types of compounds (urea derivative and auxin). When seedlings are cultured in the presence of the compounds alone the vascular pattern is very similar to that of hormone free (HF) condition. In the simultaneous presence of auxin plus the urea derivatives, the results suggest that these molecules reduce auxin responsiveness and may alter vascular patterning. By contrast, in the sequential treatments it seems that the last one is responsible for the result. In addition, molecular docking studies indicate that both 2,3-MDPU and 5-BDPU can potentially bind to the ABP1 receptor in the absence of auxin, with distinct binding energies. Furthermore, enzymatic assays show that these synthetic urea derivatives inhibit cytokinin oxidase/dehydrogenase (CKX) activity. Future researches will include: • a molecular expression analysis of some genes related with vascular differentiation and auxin transport (including the ones that behave as cytokinin-responsive genes). • an analysis of the endogenous cytokinin content in the presence of the urea derivatives.