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

Meristems and plant architecture

From maize and rice plants with solid stalks and abundant grains to gigantic tomatoes, producing plants suited to agriculture implies understanding the laws that govern plant architecture. The major determinants of this architecture and candidate genes have been identified in model species.

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

. © INRA
© INRA

For maize, a branched structure is not an asset – a solid stalk and large ears are much more advantageous. Thousands of years were necessary for maize plants to evolve into maize as we know it today from its ancestor, teosinte, which had a bushy shape and tiny ears (1). People domesticated this plant by gradually selecting for random, spontaneous mutations. It was only recently that the genes modified by this selection were identified after the fact. The same phenomenon applies to the architecture of rice and wheat plants, tomato fruit size, etc.

Examples of cultivated plants with modified architecture

  • Maize:less branched than its ancestor, teosinte: identification of relevant QTLs (1) in the early 2000s.
  • Semi-dwarf grain varieties that are more resistant to lodging with higher yields: higher harvest index (2). Identification of dwarf genes in wheat in the early 2000s.
  • Tomato size: from the size of a marble to a grapefruit. Identification of a major QTL in 2000.
  • Increase in the number of grains on the rice plant. Identification of major QTLs in 2005 and 2013.
  • Increase in the number of grains on maize ears. Identification of QTLs in the early 2000s.

(1) QTL: quantitative trait locus. See glossary for more information.
(2) Harvest index: the ration of harvested grain to total shoot dry matter. This innovation is often referred to as the "green revolution" and won Norman Borlaug the Nobel Prize in 1970.

The identification of genes that have a positive effect on the architecture of cultivated plants led the way to plant breeding techniques that are more easily used for various species. Current research on rice aims to produce a “super rice”, with reduced ramification and higher grain yield, by altering how characterised candidate genes are regulated.

Architecture and phyllotaxis, the foundation

 

Parts of a plant, with root and shoot sections. © INRA, Alain Gallien, 2013
Parts of a plant, with root and shoot sections © INRA, Alain Gallien, 2013

The architecture of a plant reproduces characteristic patterns in the aerial part. One basic pattern is that of an internode and node with one or several leaves. The location of the leaves along the stem – known as phyllotaxis (2) – is not random but follows geometric laws specific to each species and to each stage of development. However, this basic architecture can be modified to enable the plant to adapt to the local environment.

Any new organ (leaves, flowers, secondary stems) comes from the growth and differentiation of cells into organs by specialised regions called meristems.

The meristem: the building block of plant growth

The meristem is a group of undifferentiated cells that can divide indefinitely. This is one of the characteristics that distinguish plants from animals. In plants, the growth and production of organs is continual and indeterminate, whereas it is determinate in animals.

There are several types of meristems: apical meristems (at the stem top), axillary meristems (located at the leaf axils and which cause ramification, or branching) and root meristems (3). Apical meristems give rise to the growth of the main stem and its leaves. It is dominant over axillary meristems, which only produce branches when the apical meristem is weakened, a phenomenon called apical dominance. It is controlled by a balance between two main and antagonistic hormones: auxin, which inhibits ramification, and cytokines, which promote ramification (4).

Anatomy of a meristem

Shoot meristems are made up of several layers or cellular regions that all have a different fate. Some will become leaves, flowers or buds, while others will form the vascular tissue of the stem. Currently, it is thought that the fates of meristem cells are determined by their location rather than by their lineage.

Major genes under hormone and mechanical control

Following studies of mutants, several dozen controlling genes and gene families have been identified as having a major role on plant growth and development (see text box). Scientists have long known that the expression of these genes is controlled by hormone balances. More recent studies have shown that the pressure growing cells exert on each other also have an effect on how these genes are expressed. These mechanical forces regulate the speed and direction of growth and play a determinant role in organ shape.

Researchers are now working to figure out how these different mechanisms – both hormonal and mechanical ¬– interact and better understand the networks of genes involved to have a broader view of plant development (see next section).

(1) To find out more, read this article (French only).
(2) Phyllotaxis: from the Greek φύλλον, or leaf, and taxis, τάξις, the position of organs on a stem or trunk.
(3) Trees, for example, have a layer of meristematic cells called cambium that govern the thickness of the trunk.
(4) Recent research has demonstrated the involvement of another family of hormones in the branching process called strigolactones. Read the article.  Reference: Gomez-Roldan, V. et al.2008. Strigolactone inhibition of shoot branching. Nature 455:189-194.

Several major genes and gene families

Expression of the SHOOT MERISTEMLESS (STM) gene appears to be essential for the formation and maintenance of the shoot apical meristem (SAM). In mutants of this gene, the SAM is not able to develop. Another gene, the WUSCHEL (WUS) gene, is required for the functional structuring of the SAM and maintains its cells in an undifferentiated state. The mutant wus looks somewhat “tangled” due to the anarchic appearance of leaf buds. The WUS gene interferes with CLAVATA (CLV) genes, which are responsible for cell signalling and influence the number of SAM cells. WUS activates the transcription of CLV3 but is silenced by the CLV1 gene. The STM and WUS genes encode the transcription factors for homeodomain protein products, known for regulating most development processes.

Find out more

Read the article in the journal of the French Academy of Agriculture Meristems and plant architecture (with English summary) by Jean-François Morot-Gaudry and Patrick Laufs. 22 pages, 22 May 2014.

Contact: Patrick Laufs, Patrick.Laufs@versailles.inra.fr, UMR1318 Institut Jean-Pierre Bourgin, Centre Versailles-Grignon, Plant Breeding and Genetics Division, Science and Process Engineering of Agricultural Products