Gravitropism and Lateral Root Emergence are Dependent on the Trans-Golgi Network Protein TNO1.

Rahul Roy,Diane C Bassham

doi:10.3389/fpls.2015.00969

Abstract

The trans-Golgi network (TGN) is a dynamic organelle that functions as a relay station for receiving endocytosed cargo, directing secretory cargo, and trafficking to the vacuole. TGN-localized SYP41-interacting protein (TNO1) is a large, TGN-localized, coiled-coil protein that associates with the membrane fusion protein SYP41, a target SNARE, and is required for efficient protein trafficking to the vacuole. Here, we show that a tno1 mutant has auxin transport-related defects. Mutant roots have delayed lateral root emergence, decreased gravitropic bending of plant organs and increased sensitivity to the auxin analog 2,4-dichlorophenoxyacetic acid and the natural auxin 3-indoleacetic acid. Auxin asymmetry at the tips of elongating stage II lateral roots was reduced in the tno1 mutant, suggesting a role for TNO1 in cellular auxin transport during lateral root emergence. During gravistimulation, tno1 roots exhibited delayed auxin transport from the columella to the basal epidermal cells. Endocytosis to the TGN was unaffected in the mutant, indicating that bulk endocytic defects are not responsible for the observed phenotypes. Together these studies demonstrate a role for TNO1 in mediating auxin responses during root development and gravistimulation, potentially through trafficking of auxin transport proteins.

Highlights

The trans-Golgi network (TGN) is a highly dynamic tubulo-vesicular organelle that matures from the two or three trans-most cisternae of the Golgi (Staehelin and Kang, 2008) and is crucial for endocytic, secretory and vacuolar trafficking routes in plant cells
The SYP4 family of sensitive factor attachment protein receptors (SNAREs) is thought to be involved in directional auxin transport (Uemura et al, 2012), we tested whether the SYP41-interacting protein TGN-localized SYP41-interacting protein (TNO1) may be required for such processes
Since the major source of auxin in the root in the first 10 days after germination is transport from the leaves (Hobbie and Estelle, 1995; Ljung et al, 2001), one possible reason for this defect may be that auxin flux is reduced in tno1, leading to suboptimal auxin levels in the root and impairing lateral root (LR) emergence

Summary

Introduction

The trans-Golgi network (TGN) is a highly dynamic tubulo-vesicular organelle that matures from the two or three trans-most cisternae of the Golgi (Staehelin and Kang, 2008) and is crucial for endocytic, secretory and vacuolar trafficking routes in plant cells. The TGN plays a crucial role in trafficking of biosynthetic traffic to the vacuole (Reyes et al, 2011) It directs secretory cargo, including plasma membrane proteins and cell wall polysaccharides, to the cell surface, potentially via mobile secretory vesicle clusters that fuse with the plasma membrane (Toyooka et al, 2009; Gendre et al, 2014). The position of the TGN at TNO1 is required for gravitropism and LR emergence the junction of the endocytic, vacuolar, and secretory pathways renders it important in regulating transport of key molecules and mediating cellular responses to the environment (Park and Jurgens, 2011; Reyes et al, 2011; Contento and Bassham, 2012)

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