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Series of photographs for the book -The Art of acclimating plants, the garden of the Villa Thuret - by Catherine Ducatillion and Landy Blanc-Chabaud, published by Editions Quae. © INRA, SLAGMULDER Christian

Plants also feel, move and communicate!

Internal communication: hormones

The importance of hormones to plant physiology has long been known, but only more recently has the complexity of the hormonal networks involved been understood, allowing plants to make integrated responses to different environmental signals.

By Pascale Mollier, translated by Vicky Hawken
Updated on 09/05/2014
Published on 07/30/2014

Early stages in root ramification.  These microscopic studies show, in green, a clump of small cells giving rise to a secondary root, and in pink, the territory of expression of one of the aquaporins studied by the scientists.  This highly precise territory of expression can be explained by the production of auxin at the tip of the secondary root.. © INRA, D.-T Luu, BPMP
Early stages in root ramification. These microscopic studies show, in green, a clump of small cells giving rise to a secondary root, and in pink, the territory of expression of one of the aquaporins studied by the scientists. This highly precise territory of expression can be explained by the production of auxin at the tip of the secondary root. © INRA, D.-T Luu, BPMP

In plants, hormones play a central role in the coordination of development and adaptation to an environment.

They are numerous, and more are still being discovered: apart from the major families already known (auxin, cytokinins, etc.), some small peptides have recently been added to the list (1).

One of their distinguishing features by comparison with their animal counterparts is that a hormone generally exerts several activities, and reciprocally, certain actions (for example, elongation, germination, etc.) are controlled by several hormones and result from integration of this hormonal balance.

One hormone can act in several ways

Auxin, the first plant hormone to be described, is omnipresent and interacts with all the others.  It is involved in numerous functions: the differentiation of leaves and sap-bearing vessels, stem elongation, the positive gravitropism of roots and the negative gravitropism of stems, etc.

Some molecular mechanisms are understood in detail: for example, auxin favours cell elongation by acidifying cell walls, which in turn triggers the breakdown of ionic bonds and hence a greater flexibility of the walls that allows the cells to extend under the effect of turgor pressure (2).

An accumulation of functions is also seen for other families of known hormones: cytokinins, gibberellins, abscissic acid, ethylene, etc.

Thus the metabolism of plants seems to be controlled by a limited number of internal signals with multiple functions.

Several hormones for the same effect

The auxin/cytokinin balance is a good example; it controls the phenomenon of apical dominance.  Thus the apex of a plant, its principal source of auxin, inhibits the initiation of underlying axillary buds.  By stimulating the synthesis of strigolactone and repressing that of cytokinin, auxin inhibits the development of axillary buds.  This "decapitation" of the plant apex will cause a marked reduction in auxin and thus stimulate the initiation of branching to give a bushy habit to the plant, a phenomenon well known to gardeners.

Another, analogous example is the abscissic acid/gibberillin balance in the control of germination.

Hormonal signals are modulated not only by global hormone levels but also by those in target organs, their transport by other proteins, and the sensitivity of receptors that can vary from one organ to another, which can sometimes give rise to contrary effects (for example, the same concentration of auxin can both inhibit root growth and stimulate that of stems).

The integration of complex signals

Each plant cell receives and integrates numerous hormonal signals before responding to the combination of these signals.  It is very difficult to study this integration because each hormone regulates the metabolism of one or several others.

One example is the integration of signals by the whole plant in terms of the different roles of abscissic acid.  This hormone allows the closure of stomata, which constitutes both a response to water stress and a defence against the entry of pathogens, mainly bacteria.   However, within deeper tissues, abscissic acid inhibits the defensive reactions induced by jasmonate and salicylic acid against pathogens that are already present. This apparent contradiction occurs at the scale of the whole plant: the aim is to favour protection against water stress, a crucial condition to ensure a plant's survival, to the detriment of defences against pre-existing local infections.

Each hormonal balance is only one link in a network of complex interactions that govern the life of a plant.  Modelling and computer simulation will clearly be necessary to gain a global understanding of the molecular data that have been accumulated on different hormones.
 
(1) They are involved in the self-regulation of nodulation in the context of nitrogen-fixing symbioses.  Publication in 2009.

(2)  Plant cells are filled with water that is under pressure.  This pressure, one hundred times stronger than in animal cells, results from an osmotic phenomenon, or in other words the diffusion of less concentrated water  (outside the cell) towards more concentrated water (inside the cell).

Contact(s)
Scientific contact(s):

Associated Division(s):
Plant Biology and Breeding, Science for Food and Bioproduct Engineering
Associated Centre(s):
Versailles-Grignon

Hormones in mosses

One of the principal issues addressed by scientific research at present is to understand the diversity of the effects of plant hormones and identify their functional origins.

By demonstrating that mosses (non-vascular plants) are able to synthesise a hormone in the strigolactone family, INRA researchers have advanced in their knowledge of the different roles of these substances during evolution. It appears that the primary function of these hormones is the elongation of absorption and anchoring organs (the equivalent of roots), while functions affecting relationships with soil symbiotic fungi develop later.  This work has also made it possible to demonstrate the role of strigolactones in a phenomenon that recalls that already known in bacteria as "quorum sensing"(1): when the density of individuals increases, their size decreases because the extension of moss filaments is inhibited.  Thus the community regulates the behaviour of its individuals in a synchronised manner.

(1) Quorum sensing: detection of a "quorum", or the number of individuals necessary to trigger a response.