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Romanesco broccoli grows in a geometric shape called a fractal. The central structure is a meristem that produces small primordial meristems that induce new meristems, which gives rise to the fractal shape. © INRA, LAUFS Patrick

Shaping up: how plants take form

A closer look at auxin gradients

The circulation of auxin in the plant and its gradient concentration in certain regions determines areas of growth and differentiation of plant organs.

By Pascale Mollier, translated by Teri Jones-Villeneuve
Updated on 02/10/2016
Published on 12/15/2015

  Early stages of root branching. Microscopy studies indicate a mass of small cells resulting in a secondary root (in green) and the region of expression of one of the aquaporins (in pink) examined by researchers. This very precise region of expression can be explained by the production of auxin at the point of the secondary root. © INRA, D.-T Luu, BPMP,
Early stages of root branching. Microscopy studies indicate a mass of small cells resulting in a secondary root (in green) and the region of expression of one of the aquaporins (in pink) examined by researchers. This very precise region of expression can be explained by the production of auxin at the point of the secondary root © INRA, D.-T Luu, BPMP,

 

Auxin is a hormone involved in organ formation, cell elongation and gravitropism (the ability of roots to grow downwards). It is synthesised at the tips of the floral stem and transported cell to cell all the way to the roots. This polar transport occurs via membrane transporters that determine the release of auxin from cells. Depending on whether these transporters are located on the basal or lateral surface of the cell, auxin will follow a “vertical” or “lateral” pathway. The vertical flow of auxin determines the growth of the primary root, while a lateral flow induces lateral root formation.

Diagram of lateral root formation. © INRA
Diagram of lateral root formation © INRA

How lateral roots form

When a lateral root begins to extend from the primary root – a model studied by researchers in the model plant Arabidopsis thaliana (a distant relative of rapeseed) – one type of auxin transporters change polarity. These transporters move from the basal surface to the lateral surface of the cell.

The polar localisation of these transporters is maintained by a continual cycle of endocytosis and exocytosis

Transit of transporters in the cell. © INRA
Transit of transporters in the cell © INRA

Diagram key:
• 1 –Endocytosis: The transporter is enveloped in a membrane vesicle
• 2 –The transporter moves towards the E1 endosome
• 3 –The transporter moves towards the E2 sorting endosome
• 4 –Exocytosis: the transporter is released at the lateral surface of the cell (role of the retromer complex). Green dotted arrow: role currently attributed to the retromer complex: transport of proteins from the endosomes towards the trans-Golgi network (TGN)

During endocytosis, a membrane vesicle forms around the transporter and pulls it inside the cell. It then reappears at a different side of the cell via a reverse process called exocytosis. Between the two, it is carried to its destination inside of endosomes, organelles (1) involved in sorting and transporting proteins. INRA researchers have identified a protein required in this repolarising mechanism. This protein is part of the retromer complex, which was previously known for carrying out another function in the cell, transporting proteins between the endosomes and the trans-Golgi network. Researchers are now focusing on the other proteins in the retromer complex, which appear to play various key roles in several plant development processes.

(1) Organelle: cell compartment enclosed by a membrane

Findings that can be extrapolated to the animal kingdom

Retromer complex proteins are highly conserved in plants and animals.

The emergence of this complex’s new role in the polarisation of development hormone transporters, a mechanism that conditions the formation of organs, has led to studies of it in animals. This role was recently confirmed by other research teams for a protein transporter involved both in organ formation and cell proliferation in nematodes, fruit flies (Drosophila) and people.

This research has provided a new view of the molecular mechanisms that control cellular polarity necessary in the development of multicellular organisms. They underline the major role of the endocytosis-exocytosis cycle in the process of plant growth and development.

Contact(s)
Scientific contact(s):

Associated Division(s):
Plant Biology and Breeding
Associated Centre(s):
Auvergne-Rhône-Alpes

Reference

Jaillais Y, Santambrogio M, Rozier F., Fobis-Loisy I., Miège C. et Thierry Gaude T., 2007. The retromer protein VPS29 links cell polarity and organ initiation in plants. Cell 130, 1057-1070.