Dynamic changes in plant cell polarity require early endosomal components

The plant hormone auxin is directionally transported, accumulates locally, and triggers various developmental responses. Direction of auxin transport depends on tissue polarity, but it can change flexibly in response to environmental and developmental stimuli. Dynamic relocation of the PIN family of auxin transport proteins underlies this flexible regulation of auxin transport. We found that early endosomal proteins play important roles in PIN repolarization mediating multiple developmental pathways.

HFSP Career Development Award holder Hirokazu Tanaka and colleagues
authored on Mon, 22 July 2013

Auxin is a unique plant hormone which is actively and directionally transported in plant tissues. Transported auxin accumulates locally in the plant body and triggers a multitude of responses including organ formation and patterning. Therefore, regulation of the directional auxin transport is very important for many aspects of plant development. The PIN family of auxin transporters is known to localize at specific sides of cells and export auxin from the cells enabling the directional transport of auxin in the tissues. Interestingly, the polar localization of PIN proteins changes dynamically in response to developmental and environmental stimuli. PIN proteins are rapidly shuttling between the plasma membrane and intracellular compartments by endocytic recycling, potentially allowing dynamic changes of the polar localization in response to developmental and environmental cues. However, mechanisms regulating PIN localization are not fully understood. In particular, information on the molecular components involved in early endocytic trafficking of PIN proteins remains scarce. The extent to which the early endosomal trafficking events are important for polar localization of proteins and thus to polarized development was not known.

Figure: Early endosomal components, BEN1 ARF GEF and BEN2/VPS45, are required for repolarization of PIN1-GFP (green) during lateral root organogenesis.

In this study, we have characterized early endosomal trafficking of PIN proteins using two Arabidopsis mutants,ben1 and ben2, which as we had shown previously, are defective in endosomal trafficking as well as pharmacological interference of trafficking at the trans-Golgi network / early endosome. Molecular cloning of the BEN2 gene revealed that the ben2 was a weak allele of the AtVPS45 gene encoding a Sec1/Munc18 family protein which regulates SNARE-dependent membrane fusion. We also found that VPS45/BEN2 resides in the early endocytic route and this localization was abolished by the ben2 mutation. The mutant phenotypes suggested that VPS45/BEN2 is involved in the fusion of endocytic cargoes to the TGN/EE, whereas another TGN/EE-localized ARF,GEF component BEN1,functions in the transport of cargoes from there. Taken together, the results showed that these two molecular components are important for early endosomal trafficking.

Furthermore, we showed that these components play crucial roles in polar PIN localization and their dynamic repolarization which underly auxin activity gradient formation and auxin-dependent developmental processes such as embryonic patterning and lateral root development. These findings highlight the important roles of early endosomal components for cell polarity and auxin-dependent regulation of plant architecture.

Reference

Cell polarity and patterning by PIN trafficking through early endosomal compartments in Arabidopsis thaliana.Tanaka, H., Kitakura, S., Rakusova, H., Uemura, T., Feraru, M.I., De Ricke, R., Robert, S., Kakimoto, T., Friml, J.PLOS Genetics. 2013; 9, e1003540. DOI: 10.1371/journal.pgen.1003540.

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