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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

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

The mysterious fractal structure of Romanesco broccoli – as well as that of big tomatoes or maize and wheat with large ears – is a question of shapes and architecture. This report examines how plants acquire their forms.

From maize and rice plants with solid stalks and abundant grains to large tomatoes, producing plants suited to agriculture implies understanding the natural laws that govern plant development and architecture. These mechanisms can be studied at different levels, such as the entire plant, organs, cells and molecular networks.

The study of genomes and the development of mutants have made it possible to characterise numerous genes involved in plant growth. “You might even say that all of a cell’s genes are involved in growth in some way or another because growth means an expansion of the cell wall and cell volume, and therefore metabolic mobilisation and synthesis of various components, proteins, DNA, the wall, etc. That said, there are genes that lead the process and coordinate groups of effectors to create an entire hierarchy that we are nowhere close to recreating,” explains Jan Traas, a specialist in plant development at INRA.

What are the signals that induce activity in these genes? What are the mechanisms at a cellular level? How is growth coordinated across the thousands of cells in an organ?

Scientists have long understood the role that hormones play in growth, namely auxin, which coordinates cell growth in specialized regions called meristems. They are just starting to grasp how auxin acts at the organ level – such as how it induces leaf formation – and how it acts at the cellular level to extend the cell wall.

More recently, pioneering research by Jan Traas and his teams have opened the door to a new direction of study by showing that another mechanism involving mechanical forces between meristematic cells exists. The mechanical pressure these cells exert on one another as they grow guides the direction of growth and therefore organ shape.

This research offers a glimpse of a complete system in which the expression of certain genes sets off the growth process, producing mechanical constraints which in turn influence other genes to regulate growth.