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A new hormone in mosses

INRA researchers in Versailles-Grignon have evidenced the synthesis of hormones belonging to the strigolactone family in the moss Physcomitrella patens, a non-vascular plant whose morphology is similar to that of the first plants present on Earth. In vascular plants (endowed with vessels which transport sap), strigolactones are known to induce germination of the seeds of parasitic plants, favour some plant-fungi relations and inhibit ramification. In mosses, they intervene in communication between individuals by repressing their spread as a function of population density. This work opens the way to new applications in the field of agronomy.

Sous-bois de  montagne dans le Parc Naturel Régional du Massif des Bauges (Savoie. © INRA, MEURET Michel
Updated on 05/22/2015
Published on 05/18/2012

Plants need to integrate numerous factors (both endogenous and environmental) in order to coordinate the differentiation and development of their different parts. This coordination is based on the action of signalling molecules, or hormones, which act at very low concentrations and ensure communication between cells and between organs. These molecules include strigolactones – produced by plant roots and are released into the soil – which are known to repress the initiation of axillary buds situated in the leaf axils and to signal the presence of a host root to different organisms.
They thus favour the development of reciprocal beneficial relationships between plants and fungi (or endomycorrhizal symbioses). These symbioses concern more than 80% of terrestrial plants and are very ancient. They probably made a considerable contribution to colonisation of the terrestrial sphere by plants, more than 450 million years ago.
The strigolactones are also involved in the germination of the seeds of parasitic plants (Striga, Orobanche) close to the roots of plants they will then infest. These parasitic plants cause devastation in Africa and are inducing increasing damage in our latitudes (a notable example being broom rape on rapeseed crops).
INRA researchers in Versailles-Grignon, and their colleagues in other countries, have evidenced the synthesis of strigolactones in the Physcomitrella patens moss, a non-vascular plant. These substances are released into the environment and repress the elongation and ramification of moss filaments. In practice, when the density of individuals increases, the diameter of plants diminishes. The scientists have shown that strigolactones intervene in chemical communication between plants by restricting their spread. This mechanism is reminiscent of the system regulating population density that is described in bacteria, or quorum-sensing (read definition below).

In the longer term, studies in mosses will enable a clearer understanding of the evolution of strigolactone functioning in terrestrial plants, so as to address questions concerning the evolution of hormonal signalling pathways and better understand their action at a cellular level.
Although the control of plant architecture is an essential approach in order to increase plant productivity and improve crop quality, these studies will also contribute to understanding the origins of the ramification of flowering plants, to improving resistance against parasitic plants and to encouraging the initiation of symbiosis between fungi and plants under limiting conditions, an essential relationship in terms of nutrients and water uptake by plants.

Reference

Proust H et al. 2011. Strigolactones regulate protonema branching and act as a quorum sensing-like signal in the moss Physcomitrella patens. Development 138: 1531.

Quorum sensing

For the past fifteen years, bacteria have been considered as organisms that are capable of communicating and changing their behaviour relative to their cellular density via a process called quorum-sensing or QS. This inter-cellular communication is mediated by chemical signals that are synthesised by the bacteria and then secreted or disseminated into the external environment, and most of them belong to a family of lactones, the N-acyl-homoserine-lactones. The bacteria are thus able to detect the type and concentration of the signal, which triggers a synchronised response from the community via genetic and regulatory mechanisms.

Among the functions regulated by QS, mention should be made of the production of antifungals, antibiotics, organisation into biofilms and the expression and propagation of pathogenic potential.